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This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to the compilers (GCC) version 4.8.3. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See section `Introduction' in GNU Compiler Collection (GCC) Internals.
1. Programming Languages Supported by GCC You can compile C or C++ programs. 2. Language Standards Supported by GCC Language standards supported by GCC. 3. GCC Command Options Command options supported by `gcc'. 4. C Implementation-defined behavior How GCC implements the ISO C specification. 6. Extensions to the C Language Family GNU extensions to the C language family. 5. C++ Implementation-defined behavior How GCC implements the ISO C++ specification. 7. Extensions to the C++ Language GNU extensions to the C++ language. 8. GNU Objective-C features GNU Objective-C runtime features. 9. Binary Compatibility 10. gcov---a Test Coverage Programgcov---a test coverage program.11. Known Causes of Trouble with GCC If you have trouble using GCC. 12. Reporting Bugs How, why and where to report bugs. 13. How To Get Help with GCC How to find suppliers of support for GCC. 14. Contributing to GCC Development How to contribute to testing and developing GCC.
Funding Free Software How to help assure funding for free software. The GNU Project and GNU/Linux
GNU General Public License GNU General Public License says how you can copy and share GCC. GNU Free Documentation License How you can copy and share this manual. Contributors to GCC People who have contributed to GCC.
Option Index Index to command line options. Keyword Index Index of concepts and symbol names.
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GCC stands for "GNU Compiler Collection". GCC is an integrated distribution of compilers for several major programming languages. These languages currently include C, C++, Objective-C, Objective-C++, Java, Fortran, Ada, and Go.
The abbreviation GCC has multiple meanings in common use. The current official meaning is "GNU Compiler Collection", which refers generically to the complete suite of tools. The name historically stood for "GNU C Compiler", and this usage is still common when the emphasis is on compiling C programs. Finally, the name is also used when speaking of the language-independent component of GCC: code shared among the compilers for all supported languages.
The language-independent component of GCC includes the majority of the optimizers, as well as the "back ends" that generate machine code for various processors.
The part of a compiler that is specific to a particular language is called the "front end". In addition to the front ends that are integrated components of GCC, there are several other front ends that are maintained separately. These support languages such as Pascal, Mercury, and COBOL. To use these, they must be built together with GCC proper.
Most of the compilers for languages other than C have their own names. The C++ compiler is G++, the Ada compiler is GNAT, and so on. When we talk about compiling one of those languages, we might refer to that compiler by its own name, or as GCC. Either is correct.
Historically, compilers for many languages, including C++ and Fortran, have been implemented as "preprocessors" which emit another high level language such as C. None of the compilers included in GCC are implemented this way; they all generate machine code directly. This sort of preprocessor should not be confused with the C preprocessor, which is an integral feature of the C, C++, Objective-C and Objective-C++ languages.
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For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions.
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GCC supports three versions of the C standard, although support for the most recent version is not yet complete.
The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options `-ansi', `-std=c90' or `-std=iso9899:1990'; to obtain all the diagnostics required by the standard, you should also specify `-pedantic' (or `-pedantic-errors' if you want them to be errors rather than warnings). See section Options Controlling C Dialect.
Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.
An amendment to the 1990 standard was published in 1995. This
amendment added digraphs and __STDC_VERSION__ to the language,
but otherwise concerned the library. This amendment is commonly known
as AMD1; the amended standard is sometimes known as C94 or
C95. To select this standard in GCC, use the option
`-std=iso9899:199409' (with, as for other standard versions,
`-pedantic' to receive all required diagnostics).
A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-4.7/c99status.html for details. To select this standard, use `-std=c99' or `-std=iso9899:1999'. (While in development, drafts of this standard version were referred to as C9X.)
Errors in the 1999 ISO C standard were corrected in three Technical Corrigenda published in 2001, 2004 and 2007. GCC does not support the uncorrected version.
A fourth version of the C standard, known as C11, was published in 2011 as ISO/IEC 9899:2011. GCC has limited incomplete support for parts of this standard, enabled with `-std=c11' or `-std=iso9899:2011'. (While in development, drafts of this standard version were referred to as C1X.)
By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See section Extensions to the C Language Family. Use of the `-std' options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with `-std=gnu90' (for C90 with GNU extensions), `-std=gnu99' (for C99 with GNU extensions) or `-std=gnu11' (for C11 with GNU extensions). The default, if no C language dialect options are given, is `-std=gnu90'; this will change to `-std=gnu99' or `-std=gnu11' in some future release when the C99 or C11 support is complete. Some features that are part of the C99 standard are accepted as extensions in C90 mode, and some features that are part of the C11 standard are accepted as extensions in C90 and C99 modes.
The ISO C standard defines (in clause 4) two classes of conforming
implementation. A conforming hosted implementation supports the
whole standard including all the library facilities; a conforming
freestanding implementation is only required to provide certain
library facilities: those in <float.h>, <limits.h>,
<stdarg.h>, and <stddef.h>; since AMD1, also those in
<iso646.h>; since C99, also those in <stdbool.h> and
<stdint.h>; and since C11, also those in <stdalign.h>
and <stdnoreturn.h>. In addition, complex types, added in C99, are not
required for freestanding implementations. The standard also defines
two environments for programs, a freestanding environment,
required of all implementations and which may not have library
facilities beyond those required of freestanding implementations,
where the handling of program startup and termination are
implementation-defined, and a hosted environment, which is not
required, in which all the library facilities are provided and startup
is through a function int main (void) or int main (int,
char *[]). An OS kernel would be a freestanding environment; a
program using the facilities of an operating system would normally be
in a hosted implementation.
GCC aims towards being usable as a conforming freestanding
implementation, or as the compiler for a conforming hosted
implementation. By default, it will act as the compiler for a hosted
implementation, defining __STDC_HOSTED__ as 1 and
presuming that when the names of ISO C functions are used, they have
the semantics defined in the standard. To make it act as a conforming
freestanding implementation for a freestanding environment, use the
option `-ffreestanding'; it will then define
__STDC_HOSTED__ to 0 and not make assumptions about the
meanings of function names from the standard library, with exceptions
noted below. To build an OS kernel, you may well still need to make
your own arrangements for linking and startup.
See section Options Controlling C Dialect.
GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). See section Standard Libraries.
Most of the compiler support routines used by GCC are present in
`libgcc', but there are a few exceptions. GCC requires the
freestanding environment provide memcpy, memmove,
memset and memcmp.
Finally, if __builtin_trap is used, and the target does
not implement the trap pattern, then GCC will emit a call
to abort.
For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html
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GCC supports the original ISO C++ standard (1998) and contains experimental support for the second ISO C++ standard (2011).
The original ISO C++ standard was published as the ISO standard (ISO/IEC
14882:1998) and amended by a Technical Corrigenda published in 2003
(ISO/IEC 14882:2003). These standards are referred to as C++98 and
C++03, respectively. GCC implements the majority of C++98 (export
is a notable exception) and most of the changes in C++03. To select
this standard in GCC, use one of the options `-ansi',
`-std=c++98', or `-std=c++03'; to obtain all the diagnostics
required by the standard, you should also specify `-pedantic' (or
`-pedantic-errors' if you want them to be errors rather than
warnings).
A revised ISO C++ standard was published in 2011 as ISO/IEC 14882:2011, and is referred to as C++11; before its publication it was commonly referred to as C++0x. C++11 contains several changes to the C++ language, most of which have been implemented in an experimental C++11 mode in GCC. For information regarding the C++11 features available in the experimental C++11 mode, see http://gcc.gnu.org/projects/@/cxx0x.html. To select this standard in GCC, use the option `-std=c++11'; to obtain all the diagnostics required by the standard, you should also specify `-pedantic' (or `-pedantic-errors' if you want them to be errors rather than warnings).
More information about the C++ standards is available on the ISO C++ committee's web site at http://www.open-std.org/@/jtc1/@/sc22/@/wg21/.
By default, GCC provides some extensions to the C++ language; See section Options Controlling C++ Dialect. Use of the `-std' option listed above will disable these extensions. You may also select an extended version of the C++ language explicitly with `-std=gnu++98' (for C++98 with GNU extensions) or `-std=gnu++11' (for C++11 with GNU extensions). The default, if no C++ language dialect options are given, is `-std=gnu++98'.
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GCC supports "traditional" Objective-C (also known as "Objective-C 1.0") and contains support for the Objective-C exception and synchronization syntax. It has also support for a number of "Objective-C 2.0" language extensions, including properties, fast enumeration (only for Objective-C), method attributes and the @optional and @required keywords in protocols. GCC supports Objective-C++ and features available in Objective-C are also available in Objective-C++.
GCC by default uses the GNU Objective-C runtime library, which is part of GCC and is not the same as the Apple/NeXT Objective-C runtime library used on Apple systems. There are a number of differences documented in this manual. The options `-fgnu-runtime' and `-fnext-runtime' allow you to switch between producing output that works with the GNU Objective-C runtime library and output that works with the Apple/NeXT Objective-C runtime library.
There is no formal written standard for Objective-C or Objective-C++. The authoritative manual on traditional Objective-C (1.0) is "Object-Oriented Programming and the Objective-C Language", available at a number of web sites:
The Objective-C exception and synchronization syntax (that is, the keywords @try, @throw, @catch, @finally and @synchronized) is supported by GCC and is enabled with the option `-fobjc-exceptions'. The syntax is briefly documented in this manual and in the Objective-C 2.0 manuals from Apple.
The Objective-C 2.0 language extensions and features are automatically enabled; they include properties (via the @property, @synthesize and @dynamic keywords), fast enumeration (not available in Objective-C++), attributes for methods (such as deprecated, noreturn, sentinel, format), the unused attribute for method arguments, the @package keyword for instance variables and the @optional and @required keywords in protocols. You can disable all these Objective-C 2.0 language extensions with the option `-fobjc-std=objc1', which causes the compiler to recognize the same Objective-C language syntax recognized by GCC 4.0, and to produce an error if one of the new features is used.
GCC has currently no support for non-fragile instance variables.
The authoritative manual on Objective-C 2.0 is available from Apple:
For more information concerning the history of Objective-C that is available online, see http://gcc.gnu.org/readings.html
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The Go language continues to evolve as of this writing; see the current language specifications. At present there are no specific versions of Go, and there is no way to describe the language supported by GCC in terms of a specific version. In general GCC tracks the evolving specification closely, and any given release will support the language as of the date that the release was frozen.
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See section `About This Guide' in GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.
See section `Standards' in The GNU Fortran Compiler, for details of standards supported by GNU Fortran.
See section `Compatibility with the Java Platform' in GNU gcj,
for details of compatibility between gcj and the Java Platform.
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When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the `-c' option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
See section Compiling C++ Programs, for a summary of special options for compiling C++ programs.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter options
may not be grouped: `-dv' is very different from `-d
-v'.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify `-L' more than once, the directories are searched in the order specified. Also, the placement of the `-l' option is significant.
Many options have long names starting with `-f' or with `-W'---for example, `-fmove-loop-invariants', `-Wformat' and so on. Most of these have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. This manual documents only one of these two forms, whichever one is not the default.
See section Option Index, for an index to GCC's options.
3.1 Option Summary Brief list of all options, without explanations. 3.2 Options Controlling the Kind of Output Controlling the kind of output: an executable, object files, assembler files, or preprocessed source. 3.3 Compiling C++ Programs Compiling C++ programs. 3.4 Options Controlling C Dialect Controlling the variant of C language compiled. 3.5 Options Controlling C++ Dialect Variations on C++. 3.6 Options Controlling Objective-C and Objective-C++ Dialects Variations on Objective-C and Objective-C++. 3.7 Options to Control Diagnostic Messages Formatting Controlling how diagnostics should be formatted. 3.8 Options to Request or Suppress Warnings How picky should the compiler be? 3.9 Options for Debugging Your Program or GCC Symbol tables, measurements, and debugging dumps. 3.10 Options That Control Optimization How much optimization? 3.11 Options Controlling the Preprocessor Controlling header files and macro definitions. Also, getting dependency information for Make. 3.12 Passing Options to the Assembler Passing options to the assembler. 3.13 Options for Linking Specifying libraries and so on. 3.14 Options for Directory Search Where to find header files and libraries. Where to find the compiler executable files. 3.15 Specifying subprocesses and the switches to pass to them How to pass switches to sub-processes. 3.16 Specifying Target Machine and Compiler Version Running a cross-compiler, or an old version of GCC. 3.17 Hardware Models and Configurations Specifying minor hardware or convention variations, such as 68010 vs 68020. 3.18 Options for Code Generation Conventions Specifying conventions for function calls, data layout and register usage. 3.19 Environment Variables Affecting GCC Env vars that affect GCC. 3.20 Using Precompiled Headers Compiling a header once, and using it many times.
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Here is a summary of all the options, grouped by type. Explanations are in the following sections.
-c -S -E -o file |
-ansi -std=standard |
-fabi-version=n |
-fconstant-string-class=class-name |
-fmessage-length=n |
-fsyntax-only -fmax-errors=n |
{-Wbad-function-cast -Wmissing-declarations
|
-dletters |
-falign-functions[=n |
-Aquestion |
-Wa,option |
object-file-name |
-Bprefix |
H8/300 Options
-mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300 |
M32C Options
-mcpu=cpu |
RL78 Options
-msim -mmul=none -mmul=g13 -mmul=rl78 |
RX Options
{-m64bit-doubles -m32bit-doubles -fpu -nofpu
|
SH Options
{-m1 -m2 -m2e
|
-fcall-saved-reg |
3.2 Options Controlling the Kind of Output Controlling the kind of output: an executable, object files, assembler files, or preprocessed source. 3.4 Options Controlling C Dialect Controlling the variant of C language compiled. 3.5 Options Controlling C++ Dialect Variations on C++. 3.6 Options Controlling Objective-C and Objective-C++ Dialects Variations on Objective-C and Objective-C++. 3.7 Options to Control Diagnostic Messages Formatting Controlling how diagnostics should be formatted. 3.8 Options to Request or Suppress Warnings How picky should the compiler be? 3.9 Options for Debugging Your Program or GCC Symbol tables, measurements, and debugging dumps. 3.10 Options That Control Optimization How much optimization? 3.11 Options Controlling the Preprocessor Controlling header files and macro definitions. Also, getting dependency information for Make. 3.12 Passing Options to the Assembler Passing options to the assembler. 3.13 Options for Linking Specifying libraries and so on. 3.14 Options for Directory Search Where to find header files and libraries. Where to find the compiler executable files. 3.15 Specifying subprocesses and the switches to pass to them How to pass switches to sub-processes. 3.16 Specifying Target Machine and Compiler Version Running a cross-compiler, or an old version of GCC.
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Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
file.i
file.ii
file.m
file.mi
file.mm
file.M
file.mii
file.h
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
file.mm
file.M
file.mii
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
file.f
file.for
file.ftn
file.F
file.FOR
file.fpp
file.FPP
file.FTN
file.f90
file.f95
file.f03
file.f08
file.F90
file.F95
file.F03
file.F08
file.go
file.ads
file.adb
file.s
file.S
file.sx
other
You can specify the input language explicitly with the `-x' option:
-x language
c c-header cpp-output c++ c++-header c++-cpp-output objective-c objective-c-header objective-c-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output assembler assembler-with-cpp ada f77 f77-cpp-input f95 f95-cpp-input go java |
-x none
-pass-exit-codes
gcc program will exit with the code of 1 if any
phase of the compiler returns a non-success return code. If you specify
`-pass-exit-codes', the gcc program will instead return with
numerically highest error produced by any phase that returned an error
indication. The C, C++, and Fortran frontends return 4, if an internal
compiler error is encountered.
If you only want some of the stages of compilation, you can use
`-x' (or filename suffixes) to tell gcc where to start, and
one of the options `-c', `-S', or `-E' to say where
gcc is to stop. Note that some combinations (for example,
`-x cpp-output -E') instruct gcc to do nothing at all.
-c
By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S
By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'.
Input files that don't require compilation are ignored.
-E
Input files that don't require preprocessing are ignored.
-o file
If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `source.suffix' in `source.o', its assembler file in `source.s', a precompiled header file in `source.suffix.gch', and all preprocessed C source on standard output.
-v
-###
./-_.
This is useful for shell scripts to capture the driver-generated command lines.
-pipe
--help
gcc. If the `-v' option is also specified
then `--help' will also be passed on to the various processes
invoked by gcc, so that they can display the command-line options
they accept. If the `-Wextra' option has also been specified
(prior to the `--help' option), then command-line options that
have no documentation associated with them will also be displayed.
--target-help
--help={class|[^]qualifier}[,...]
These are the supported qualifiers:
Thus for example to display all the undocumented target-specific switches supported by the compiler the following can be used:
--help=target,undocumented |
The sense of a qualifier can be inverted by prefixing it with the `^' character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumented |
The argument to `--help=' should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output by so much that there is nothing to display. One case where it does work however is when one of the classes is target. So for example to display all the target-specific optimization options the following can be used:
--help=target,optimizers |
The `--help=' option can be repeated on the command line. Each successive use will display its requested class of options, skipping those that have already been displayed.
If the `-Q' option appears on the command line before the `--help=' option, then the descriptive text displayed by `--help=' is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the `--help=' option is used).
The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at `-O2' by using:
-Q -O2 --help=optimizers |
Alternatively you can discover which binary optimizations are enabled by `-O3' by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled |
-no-canonical-prefixes
--version
-wrapper
gcc -c t.c -wrapper gdb,--args |
This will invoke all subprograms of gcc under
`gdb --args', thus the invocation of cc1 will be
`gdb --args cc1 ...'.
-fplugin=name.so
-fplugin-arg-name-key=value
-fdump-ada-spec[-slim]
-fdump-go-spec=file
const,
type, var, and func declarations which may be a
useful way to start writing a Go interface to code written in some
other language.
@file
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.
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C++ source files conventionally use one of the suffixes `.C',
`.cc', `.cpp', `.CPP', `.c++', `.cp', or
`.cxx'; C++ header files often use `.hh', `.hpp',
`.H', or (for shared template code) `.tcc'; and
preprocessed C++ files use the suffix `.ii'. GCC recognizes
files with these names and compiles them as C++ programs even if you
call the compiler the same way as for compiling C programs (usually
with the name gcc).
However, the use of gcc does not add the C++ library.
g++ is a program that calls GCC and treats `.c',
`.h' and `.i' files as C++ source files instead of C source
files unless `-x' is used, and automatically specifies linking
against the C++ library. This program is also useful when
precompiling a C header file with a `.h' extension for use in C++
compilations. On many systems, g++ is also installed with
the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See section Options Controlling C Dialect, for explanations of options for languages related to C. See section Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.
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The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi
This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the asm and typeof keywords, and
predefined macros such as unix and vax that identify the
type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler,
it disables recognition of C++ style `//' comments as well as
the inline keyword.
The alternate keywords __asm__, __extension__,
__inline__ and __typeof__ continue to work despite
`-ansi'. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with `-ansi'. Alternate predefined macros
such as __unix__ and __vax__ are also available, with or
without `-ansi'.
The `-ansi' option does not cause non-ISO programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. See section 3.8 Options to Request or Suppress Warnings.
The macro __STRICT_ANSI__ is predefined when the `-ansi'
option is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions that would normally be built in but do not have semantics
defined by ISO C (such as alloca and ffs) are not built-in
functions when `-ansi' is used. See section Other built-in functions provided by GCC, for details of the functions
affected.
-std=
The compiler can accept several base standards, such as `c90' or
`c++98', and GNU dialects of those standards, such as
`gnu90' or `gnu++98'. By specifying a base standard, the
compiler will accept all programs following that standard and those
using GNU extensions that do not contradict it. For example,
`-std=c90' turns off certain features of GCC that are
incompatible with ISO C90, such as the asm and typeof
keywords, but not other GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of a ?:
expression. On the other hand, by specifying a GNU dialect of a
standard, all features the compiler support are enabled, even when
those features change the meaning of the base standard and some
strict-conforming programs may be rejected. The particular standard
is used by `-pedantic' to identify which features are GNU
extensions given that version of the standard. For example
`-std=gnu90 -pedantic' would warn about C++ style `//'
comments, while `-std=gnu99 -pedantic' would not.
A value for this option must be provided; possible values are
-fgnu89-inline
inline functions when in C99 mode.
See section An Inline Function is As Fast As a Macro. This option
is accepted and ignored by GCC versions 4.1.3 up to but not including
4.3. In GCC versions 4.3 and later it changes the behavior of GCC in
C99 mode. Using this option is roughly equivalent to adding the
gnu_inline function attribute to all inline functions
(see section 6.30 Declaring Attributes of Functions).
The option `-fno-gnu89-inline' explicitly tells GCC to use the
C99 semantics for inline when in C99 or gnu99 mode (i.e., it
specifies the default behavior). This option was first supported in
GCC 4.3. This option is not supported in `-std=c90' or
`-std=gnu90' mode.
The preprocessor macros __GNUC_GNU_INLINE__ and
__GNUC_STDC_INLINE__ may be used to check which semantics are
in effect for inline functions. See section `Common Predefined Macros' in The C Preprocessor.
-aux-info filename
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (`I', `N' for new or `O' for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (`C' or `F', respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functions
Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++.
-fno-asm
asm, inline or typeof as a
keyword, so that code can use these words as identifiers. You can use
the keywords __asm__, __inline__ and __typeof__
instead. `-ansi' implies `-fno-asm'.
In C++, this switch only affects the typeof keyword, since
asm and inline are standard keywords. You may want to
use the `-fno-gnu-keywords' flag instead, which has the same
effect. In C99 mode (`-std=c99' or `-std=gnu99'), this
switch only affects the asm and typeof keywords, since
inline is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to alloca may become single
instructions which adjust the stack directly, and calls to memcpy
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library. In addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For example,
warnings are given with `-Wformat' for bad calls to
printf, when printf is built in, and strlen is
known not to modify global memory.
With the `-fno-builtin-function' option only the built-in function function is disabled. function must not begin with `__builtin_'. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding `-fbuiltin-function' option; if you wish to enable built-in functions selectively when using `-fno-builtin' or `-ffreestanding', you may define macros such as:
#define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s)) |
-fhosted
Assert that compilation takes place in a hosted environment. This implies
`-fbuiltin'. A hosted environment is one in which the
entire standard library is available, and in which main has a return
type of int. Examples are nearly everything except a kernel.
This is equivalent to `-fno-freestanding'.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This
implies `-fno-builtin'. A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at main. The most obvious example is an OS kernel.
This is equivalent to `-fno-hosted'.
See section Language Standards Supported by GCC, for details of freestanding and hosted environments.
-fopenmp
#pragma omp in C/C++ and
!$omp in Fortran. When `-fopenmp' is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v3.0 http://www.openmp.org/. This option
implies `-pthread', and thus is only supported on targets that
have support for `-pthread'.
-fgnu-tm
For more information on GCC's support for transactional memory, See section `The GNU Transactional Memory Library' in GNU Transactional Memory Library.
Note that the transactional memory feature is not supported with non-call exceptions (`-fnon-call-exceptions').
-fms-extensions
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
|
Some cases of unnamed fields in structures and unions are only accepted with this option. See section Unnamed struct/union fields within structs/unions, for details.
-fplan9-extensions
This enables `-fms-extensions', permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. See section Unnamed struct/union fields within structs/unions, for details. This is only supported for C, not C++.
-trigraphs
-no-integrated-cpp
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
-fcond-mismatch
-flax-vector-conversions
-funsigned-char
char be unsigned, like unsigned char.
Each kind of machine has a default for what char should
be. It is either like unsigned char by default or like
signed char by default.
Ideally, a portable program should always use signed char or
unsigned char when it depends on the signedness of an object.
But many programs have been written to use plain char and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type char is always a distinct type from each of
signed char or unsigned char, even though its behavior
is always just like one of those two.
-fsigned-char
char be signed, like signed char.
Note that this is equivalent to `-fno-unsigned-char', which is the negative form of `-funsigned-char'. Likewise, the option `-fno-signed-char' is equivalent to `-funsigned-char'.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
signed or unsigned. By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as int are signed types.
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This section describes the command-line options that are only meaningful
for C++ programs; but you can also use most of the GNU compiler options
regardless of what language your program is in. For example, you
might compile a file firstClass.C like this:
g++ -g -frepo -O -c firstClass.C |
In this example, only `-frepo' is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
The default is version 2.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
See also `-Wabi'.
-fno-access-control
-fcheck-new
operator new is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
operator new will only return 0 if it is declared
`throw()', in which case the compiler will always check the
return value even without this option. In all other cases, when
operator new has a non-empty exception specification, memory
exhaustion is signalled by throwing std::bad_alloc. See also
`new (nothrow)'.
-fconserve-space
main() has
completed, you may have an object that is being destroyed twice because
two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.
-fconstexpr-depth=n
-fdeduce-init-list
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
|
This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++.
-ffriend-injection
This option is for compatibility, and may be removed in a future release of G++.
-fno-elide-constructors
-fno-enforce-eh-specs
-ffor-scope
-fno-for-scope
The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
typeof as a keyword, so that code can use this
word as an identifier. You can use the keyword __typeof__ instead.
`-ansi' implies `-fno-gnu-keywords'.
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines
-fms-extensions
-fno-nonansi-builtins
ffs, alloca, _exit,
index, bzero, conjf, and other related functions.
-fnothrow-opt
throw() exception specification as though it were a
noexcept specification to reduce or eliminate the text size
overhead relative to a function with no exception specification. If
the function has local variables of types with non-trivial
destructors, the exception specification will actually make the
function smaller because the EH cleanups for those variables can be
optimized away. The semantic effect is that an exception thrown out of
a function with such an exception specification will result in a call
to terminate rather than unexpected.
-fno-operator-names
and, bitand,
bitor, compl, not, or and xor as
synonyms as keywords.
-fno-optional-diags
-fpermissive
-fno-pretty-templates
void f(T) [with T = int]
rather than void f(int)) so that it's clear which template is
involved. When an error message refers to a specialization of a class
template, the compiler will omit any template arguments that match
the default template arguments for that template. If either of these
behaviors make it harder to understand the error message rather than
easier, using `-fno-pretty-templates' will disable them.
-frepo
-fno-rtti
void * or to
unambiguous base classes.
-fstats
-fstrict-enums
-ftemplate-depth=n
-fno-threadsafe-statics
-fuse-cxa-atexit
__cxa_atexit function rather than the atexit function.
This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
__cxa_atexit.
-fno-use-cxa-get-exception-ptr
__cxa_get_exception_ptr runtime routine. This
will cause std::uncaught_exception to be incorrect, but is necessary
if the runtime routine is not available.
-fvisibility-inlines-hidden
The effect of this is that GCC may, effectively, mark inline methods with
__attribute__ ((visibility ("hidden"))) so that they do not
appear in the export table of a DSO and do not require a PLT indirection
when used within the DSO. Enabling this option can have a dramatic effect
on load and link times of a DSO as it massively reduces the size of the
dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility will have no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See section 7.5 Where's the Template?.
-fvisibility-ms-compat
The flag makes these changes to GCC's linkage model:
hidden, like
`-fvisibility=hidden'.
In new code it is better to use `-fvisibility=hidden' and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects will be different, so changing one will not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.
-fno-weak
-nostdinc++
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
-Wabi (C, Objective-C, C++ and Objective-C++ only)
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities in `-fabi-version=2' (the default) include:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
|
This is fixed in `-fabi-version=3'.
__attribute ((vector_size)) are
mangled in a non-standard way that does not allow for overloading of
functions taking vectors of different sizes.
The mangling is changed in `-fabi-version=4'.
The known incompatibilities in `-fabi-version=1' include:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
|
In this case, G++ will place B::f2 into the same byte
asA::f1; other compilers will not. You can avoid this problem
by explicitly padding A so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to
layout B identically.
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
|
In this case, G++ will not place B into the tail-padding for
A; other compilers will. You can avoid this problem by
explicitly padding A so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++ and other
compilers to layout C identically.
union U { int i : 4096; };
|
Assuming that an int does not have 4096 bits, G++ will make the
union too small by the number of bits in an int.
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
|
G++ will place the A base class of C at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
A data member of B is already at offset zero.
typename or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
|
Instantiations of these templates may be mangled incorrectly.
It also warns psABI related changes. The known psABI changes at this point include:
union U {
long double ld;
int i;
};
|
union U will always be passed in memory.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
-Wnarrowing (C++ and Objective-C++ only)
int i = { 2.2 }; // error: narrowing from double to int
|
This flag is included in `-Wall' and `-Wc++11-compat'.
With -std=c++11, `-Wno-narrowing' suppresses the diagnostic required by the standard. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE context.
-Wnoexcept (C++ and Objective-C++ only)
-Wnon-virtual-dtor (C++ and Objective-C++ only)
-Wreorder (C++ and Objective-C++ only)
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
|
The compiler will rearrange the member initializers for `i' and `j' to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by `-Wall'.
The following `-W...' options are not affected by `-Wall'.
-Weffc++ (C++ and Objective-C++ only)
operator= return a reference to *this.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
&&, ||, or ,.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use `grep -v' to filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
NULL as sentinel. When
compiling only with GCC this is a valid sentinel, as NULL is defined
to __null. Although it is a null pointer constant not a null pointer,
it is guaranteed to be of the same size as a pointer. But this use is
not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
-Wold-style-cast (C++ and Objective-C++ only)
-Woverloaded-virtual (C++ and Objective-C++ only)
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
|
the A class version of f is hidden in B, and code
like:
B* b; b->f(); |
will fail to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
-Wsign-promo (C++ and Objective-C++ only)
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
|
In this example, G++ will synthesize a default `A& operator = (const A&);', while cfront will use the user-defined `operator ='.
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(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See section Language Standards Supported by GCC, for references.)
This section describes the command-line options that are only meaningful
for Objective-C and Objective-C++ programs, but you can also use most of
the language-independent GNU compiler options.
For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m |
In this example, `-fgnu-runtime' is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., `-Wtraditional'). Similarly, Objective-C++ compilations may use C++-specific options (e.g., `-Wabi').
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
@"...". The default
class name is NXConstantString if the GNU runtime is being used, and
NSConstantString if the NeXT runtime is being used (see below). The
`-fconstant-cfstrings' option, if also present, will override the
`-fconstant-string-class' setting and cause @"..." literals
to be laid out as constant CoreFoundation strings.
-fgnu-runtime
-fnext-runtime
__NEXT_RUNTIME__ is predefined if (and only if) this option is
used.
-fno-nil-receivers
[receiver
message:arg]) in this translation unit ensure that the receiver is
not nil. This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction with
the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
-fobjc-call-cxx-cdtors
- (id) .cxx_construct instance method which will run
non-trivial default constructors on any such instance variables, in order,
and then return self. Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a
special - (void) .cxx_destruct method which will run
all such default destructors, in reverse order.
The - (id) .cxx_construct and - (void) .cxx_destruct
methods thusly generated will only operate on instance variables
declared in the current Objective-C class, and not those inherited
from superclasses. It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance
hierarchy. The - (id) .cxx_construct methods will be invoked
by the runtime immediately after a new object instance is allocated;
the - (void) .cxx_destruct methods will be invoked immediately
before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the - (id) .cxx_construct and
- (void) .cxx_destruct methods.
-fobjc-direct-dispatch
-fobjc-exceptions
@try,
@throw, @catch, @finally and
@synchronized. This option is available with both the GNU
runtime and the NeXT runtime (but not available in conjunction with
the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
-fobjc-nilcheck
-fobjc-std=objc1
-freplace-objc-classes
ld(1) not to statically link in
the resulting object file, and allow dyld(1) to load it in at
run time instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself. Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
-fzero-link
objc_getClass("...") (when the name of the class is known at
compile time) with static class references that get initialized at load time,
which improves run-time performance. Specifying the `-fzero-link' flag
suppresses this behavior and causes calls to objc_getClass("...")
to be retained. This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program execution.
The GNU runtime currently always retains calls to objc_get_class("...")
regardless of command-line options.
-gen-decls
-Wassign-intercept (Objective-C and Objective-C++ only)
-Wno-protocol (Objective-C and Objective-C++ only)
-Wselector (Objective-C and Objective-C++ only)
@selector(...)
expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the `-fsyntax-only' option is
being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
id or Class. When this flag
is off (which is the default behavior), the compiler will omit such warnings
if any differences found are confined to types that share the same size
and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
@selector(...) expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared before the
@selector(...) expression, either explicitly in an
@interface or @protocol declaration, or implicitly in
an @implementation section. This option always performs its
checks as soon as a @selector(...) expression is found,
while `-Wselector' only performs its checks in the final stage of
compilation. This also enforces the coding style convention
that methods and selectors must be declared before being used.
-print-objc-runtime-info
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Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n
g++ and 0 for the rest of
the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a single
line.
-fdiagnostics-show-location=once
-fdiagnostics-show-location=every-line
-fno-diagnostics-show-option
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Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
-fmax-errors=n
-w
-Werror
-Werror=
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with `-Werror=' and `-Wno-error=' as described above. (Printing of the option in the warning message can be disabled using the `-fno-diagnostics-show-option' flag.)
Note that specifying `-Werror='foo automatically implies `-W'foo. However, `-Wno-error='foo does not imply anything.
-Wfatal-errors
You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default. For further, language-specific options also refer to 3.5 Options Controlling C++ Dialect and 3.6 Options Controlling Objective-C and Objective-C++ Dialects.
When an unrecognized warning option is requested (e.g., `-Wunknown-warning'), GCC will emit a diagnostic stating that the option is not recognized. However, if the `-Wno-' form is used, the behavior is slightly different: No diagnostic will be produced for `-Wno-unknown-warning' unless other diagnostics are being produced. This allows the use of new `-Wno-' options with old compilers, but if something goes wrong, the compiler will warn that an unrecognized option was used.
-pedantic
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require `-ansi' or a `-std' option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.
`-pedantic' does not cause warning messages for use of the
alternate keywords whose names begin and end with `__'. Pedantic
warnings are also disabled in the expression that follows
__extension__. However, only system header files should use
these escape routes; application programs should avoid them.
See section 6.45 Alternate Keywords.
Some users try to use `-pedantic' to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all--only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic'. We don't have plans to support such a feature in the near future.
Where the standard specified with `-std' represents a GNU extended dialect of C, such as `gnu90' or `gnu99', there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from `-pedantic' are given where they are required by the base standard. (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
-Wall
`-Wall' turns on the following warning flags:
{-Waddress
|
Note that some warning flags are not implied by `-Wall'. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by `-Wextra' but many of them must be enabled individually.
-Wextra
{-Wclobbered
|
The option `-Wextra' also prints warning messages for the following cases:
-Wchar-subscripts
char. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
This warning is enabled by `-Wall'.
-Wcomment
-Wno-coverage-mismatch
-Wno-cpp
Suppress warning messages emitted by #warning directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
float is implicitly
promoted to double. CPUs with a 32-bit "single-precision"
floating-point unit implement float in hardware, but emulate
double in software. On such a machine, doing computations
using double values is much more expensive because of the
overhead required for software emulation.
It is easy to accidentally do computations with double because
floating-point literals are implicitly of type double. For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
|
double
because the floating-point literal is a double.
-Wformat
printf and scanf, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes (see section 6.30 Declaring Attributes of Functions), in the printf,
scanf, strftime and strfmon (an X/Open extension,
not in the C standard) families (or other target-specific families).
Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of
functions without the attribute specified are disabled by
`-ffreestanding' or `-fno-builtin'.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if `-pedantic' is used
with `-Wformat', warnings will be given about format features not
in the selected standard version (but not for strfmon formats,
since those are not in any version of the C standard). See section Options Controlling C Dialect.
Since `-Wformat' also checks for null format arguments for several functions, `-Wformat' also implies `-Wnonnull'.
`-Wformat' is included in `-Wall'. For more control over some aspects of format checking, the options `-Wformat-y2k', `-Wno-format-extra-args', `-Wno-format-zero-length', `-Wformat-nonliteral', `-Wformat-security', and `-Wformat=2' are available, but are not included in `-Wall'.
-Wformat-y2k
strftime
formats that may yield only a two-digit year.
-Wno-format-contains-nul
-Wno-format-extra-args
printf or scanf format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with `$' operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to va_arg to skip the unused arguments. However,
in the case of scanf formats, this option will suppress the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length
-Wformat-nonliteral
va_list.
-Wformat-security
printf and scanf functions where the
format string is not a string literal and there are no format arguments,
as in printf (foo);. This may be a security hole if the format
string came from untrusted input and contains `%n'. (This is
currently a subset of what `-Wformat-nonliteral' warns about, but
in future warnings may be added to `-Wformat-security' that are not
included in `-Wformat-nonliteral'.)
-Wformat=2
-Wnonnull
nonnull function attribute.
`-Wnonnull' is included in `-Wall' and `-Wformat'. It can be disabled with the `-Wno-nonnull' option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
For example, GCC will warn about i being uninitialized in the
following snippet only when `-Winit-self' has been specified:
int f()
{
int i = i;
return i;
}
|
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Wimplicit (C and Objective-C only)
-Wignored-qualifiers (C and C++ only)
const. For ISO C such a type qualifier has no effect,
since the value returned by a function is not an lvalue.
For C++, the warning is only emitted for scalar types or void.
ISO C prohibits qualified void return types on function
definitions, so such return types always receive a warning
even without this option.
This warning is also enabled by `-Wextra'.
-Wmain
-Wmissing-braces
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
|
This warning is enabled by `-Wall'.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
-Wparentheses
Also warn if a comparison like `x<=y<=z' appears; this is equivalent to `(x<=y ? 1 : 0) <= z', which is a different interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which
if statement an else branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
|
In C/C++, every else branch belongs to the innermost possible
if statement, which in this example is if (b). This is
often not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC will issue a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost if statement so there is no way the else
could belong to the enclosing if. The resulting code would
look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
|
Also warn for dangerous uses of the ?: with omitted middle operand GNU extension. When the condition in the ?: operator is a boolean expression the omitted value will be always 1. Often the user expects it to be a value computed inside the conditional expression instead.
This warning is enabled by `-Wall'.
-Wsequence-point
The C and C++ standards defines the order in which expressions in a C/C++
program are evaluated in terms of sequence points, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it. These
occur after the evaluation of a full expression (one which is not part
of a larger expression), after the evaluation of the first operand of a
&&, ||, ? : or , (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are a = a++;, a[n]
= b[n++] and a[i++] = i;. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/@/readings.html.
This warning is enabled by `-Wall' for C and C++.
-Wreturn-type
int. Also warn about any return statement with no
return-value in a function whose return-type is not void
(falling off the end of the function body is considered returning
without a value), and about a return statement with an
expression in a function whose return-type is void.
For C++, a function without return type always produces a diagnostic message, even when `-Wno-return-type' is specified. The only exceptions are `main' and functions defined in system headers.
This warning is enabled by `-Wall'.
-Wswitch
switch statement has an index of enumerated type
and lacks a case for one or more of the named codes of that
enumeration. (The presence of a default label prevents this
warning.) case labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
default label).
This warning is enabled by `-Wall'.
-Wswitch-default
switch statement does not have a default
case.
-Wswitch-enum
switch statement has an index of enumerated type
and lacks a case for one or more of the named codes of that
enumeration. case labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between `-Wswitch' and this option is that this option gives a
warning about an omitted enumeration code even if there is a
default label.
-Wsync-nand (C and C++ only)
__sync_fetch_and_nand and __sync_nand_and_fetch
built-in functions are used. These functions changed semantics in GCC 4.4.
-Wtrigraphs
-Wunused-but-set-parameter
To suppress this warning use the `unused' attribute (see section 6.36 Specifying Attributes of Variables).
This warning is also enabled by `-Wunused' together with `-Wextra'.
-Wunused-but-set-variable
To suppress this warning use the `unused' attribute (see section 6.36 Specifying Attributes of Variables).
This warning is also enabled by `-Wunused', which is enabled by `-Wall'.
-Wunused-function
-Wunused-label
To suppress this warning use the `unused' attribute (see section 6.36 Specifying Attributes of Variables).
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
-Wunused-parameter
To suppress this warning use the `unused' attribute (see section 6.36 Specifying Attributes of Variables).
-Wno-unused-result
warn_unused_result (see section 6.30 Declaring Attributes of Functions) does not use
its return value. The default is `-Wunused-result'.
-Wunused-variable
To suppress this warning use the `unused' attribute (see section 6.36 Specifying Attributes of Variables).
-Wunused-value
This warning is enabled by `-Wall'.
-Wunused
In order to get a warning about an unused function parameter, you must either specify `-Wextra -Wunused' (note that `-Wall' implies `-Wunused'), or separately specify `-Wunused-parameter'.
-Wuninitialized
setjmp call. In C++,
warn if a non-static reference or non-static `const' member
appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the `-Winit-self' option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared volatile. Because
these warnings depend on optimization, the exact variables or elements
for which there are warnings will depend on the precise optimization
options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.
-Wmaybe-uninitialized
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
|
If the value of y is always 1, 2 or 3, then x is
always initialized, but GCC doesn't know this. To suppress the
warning, the user needs to provide a default case with assert(0) or
similar code.
This option also warns when a non-volatile automatic variable might be
changed by a call to longjmp. These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because longjmp cannot
in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as noreturn. See section 6.30 Declaring Attributes of Functions.
This warning is enabled by `-Wall' or `-Wextra'.
-Wunknown-pragmas
#pragma directive is encountered that is not understood by
GCC. If this command-line option is used, warnings will even be issued
for unknown pragmas in system header files. This is not the case if
the warnings were only enabled by the `-Wall' command-line option.
-Wno-pragmas
-Wstrict-aliasing
-Wstrict-aliasing=n
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for `-Wstrict-aliasing'):
Should have very few false positives and few false
negatives. Slightly slower than levels 1 or 2 when optimization is enabled.
Takes care of the common pun+dereference pattern in the front end:
*(int*)&some_float.
If optimization is enabled, it also runs in the back end, where it deals
with multiple statement cases using flow-sensitive points-to information.
Only warns when the converted pointer is dereferenced.
Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop will require, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1
x + 1 > x; with `-fstrict-overflow', the
compiler will simplify this to 1. This level of
`-Wstrict-overflow' is enabled by `-Wall'; higher levels
are not, and must be explicitly requested.
-Wstrict-overflow=2
abs (x) >= 0. This can only be
simplified when `-fstrict-overflow' is in effect, because
abs (INT_MIN) overflows to INT_MIN, which is less than
zero. `-Wstrict-overflow' (with no level) is the same as
`-Wstrict-overflow=2'.
-Wstrict-overflow=3
x + 1 > 1 will be simplified to x > 0.
-Wstrict-overflow=4
(x * 10) / 5 will be simplified to x * 2.
-Wstrict-overflow=5
x + 2 > y will
be simplified to x + 1 >= y. This is reported only at the
highest warning level because this simplification applies to many
comparisons, so this warning level will give a very large number of
false positives.
-Wsuggest-attribute=[pure|const|noreturn]
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes
pure, const or noreturn. The compiler only warns for
functions visible in other compilation units or (in the case of pure and
const) if it cannot prove that the function returns normally. A function
returns normally if it doesn't contain an infinite loop nor returns abnormally
by throwing, calling abort() or trapping. This analysis requires option
`-fipa-pure-const', which is enabled by default at `-O' and
higher. Higher optimization levels improve the accuracy of the analysis.
-Warray-bounds
-Wno-div-by-zero
-Wsystem-headers
-Wtrampolines
A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly.
-Wfloat-equal
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you would check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
<limits.h>.
Use of these macros in user code might normally lead to spurious
warnings, however GCC's integrated preprocessor has enough context to
avoid warning in these cases.
switch statement has an operand of type long.
static function declaration follows a static one.
This construct is not accepted by some traditional C compilers.
__STDC__ to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
PARAMS and
VPARAMS. This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
-Wdeclaration-after-statement (C and Objective-C only)
-Wundef
-Wno-endif-labels
-Wshadow
-Wlarger-than=len
-Wframe-larger-than=len
alloca, variable-length arrays, or related constructs
is not included by the compiler when determining
whether or not to issue a warning.
-Wno-free-nonheap-object
-Wstack-usage=len
alloca, variable-length arrays, or related
constructs is included by the compiler when determining whether or not to
issue a warning.
The message is in keeping with the output of `-fstack-usage'.
warning: stack usage is 1120 bytes |
warning: stack usage might be 1648 bytes |
warning: stack usage might be unbounded |
-Wunsafe-loop-optimizations
-Wno-pedantic-ms-format (MinGW targets only)
printf / scanf format
width specifiers I32, I64, and I used on Windows targets
depending on the MS runtime, when you are using the options `-Wformat'
and `-pedantic' without gnu-extensions.
-Wpointer-arith
void. GNU C assigns these types a size of 1, for
convenience in calculations with void * pointers and pointers
to functions. In C++, warn also when an arithmetic operation involves
NULL. This warning is also enabled by `-pedantic'.
-Wtype-limits
-Wbad-function-cast (C and Objective-C only)
int malloc() is cast to anything *.
-Wc++-compat (C and Objective-C only)
void * to a pointer to non-void type.
-Wc++11-compat (C++ and Objective-C++ only)
-Wcast-qual
const char * is cast
to an ordinary char *.
Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting char ** to const char **
is unsafe, as in this example:
/* p is char ** value. */ const char **q = (const char **) p; /* Assignment of readonly string to const char * is OK. */ *q = "string"; /* Now char** pointer points to read-only memory. */ **p = 'b'; |
-Wcast-align
char * is cast to
an int * on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
const
char[length] so that copying the address of one into a
non-const char * pointer will get a warning. These
warnings will help you find at compile time code that can try to write
into a string constant, but only if you have been very careful about
using const in declarations and prototypes. Otherwise, it will
just be a nuisance. This is why we did not make `-Wall' request
these warnings.
When compiling C++, warn about the deprecated conversion from string
literals to char *. This warning is enabled by default for C++
programs.
-Wclobbered
-Wconversion
abs (x) when
x is double; conversions between signed and unsigned,
like unsigned ui = -1; and conversions to smaller types, like
sqrtf (M_PI). Do not warn for explicit casts like abs
((int) x) and ui = (unsigned) -1, or if the value is not
changed by the conversion like in abs (2.0). Warnings about
conversions between signed and unsigned integers can be disabled by
using `-Wno-sign-conversion'.
For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that will never use a type conversion
operator: conversions to void, the same type, a base class or a
reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless
`-Wsign-conversion' is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
NULL and non-pointer
types. `-Wconversion-null' is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
nullptr in C++11.
-Wempty-body
-Wenum-compare
-Wjump-misses-init (C, Objective-C only)
goto statement or a switch statement jumps
forward across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns about
variables that are initialized when they are declared. This warning is
only supported for C and Objective-C; in C++ this sort of branch is an
error in any case.
`-Wjump-misses-init' is included in `-Wc++-compat'. It can be disabled with the `-Wno-jump-misses-init' option.
-Wsign-compare
-Wsign-conversion
-Waddress
void func(void); if (func), and comparisons against the memory
address of a string literal, such as if (x == "abc"). Such
uses typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that the
programmer intended to use strcmp. This warning is enabled by
`-Wall'.
-Wlogical-op
-Waggregate-return
-Wno-attributes
__attribute__ is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This will not stop errors for incorrect use of supported
attributes.
-Wno-builtin-macro-redefined
__TIMESTAMP__, __TIME__,
__DATE__, __FILE__, and __BASE_FILE__.
-Wstrict-prototypes (C and Objective-C only)
-Wold-style-declaration (C and Objective-C only)
static are not the first things in a declaration. This warning
is also enabled by `-Wextra'.
-Wold-style-definition (C and Objective-C only)
-Wmissing-parameter-type (C and Objective-C only)
void foo(bar) { }
|
This warning is also enabled by `-Wextra'.
-Wmissing-prototypes (C and Objective-C only)
-Wmissing-declarations
-Wmissing-field-initializers
x.h is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
|
This option does not warn about designated initializers, so the following modification would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
|
This warning is included in `-Wextra'. To get other `-Wextra' warnings without this one, use `-Wextra -Wno-missing-field-initializers'.
-Wmissing-format-attribute
format
attributes. Note these are only possible candidates, not absolute ones.
GCC will guess that function pointers with format attributes that
are used in assignment, initialization, parameter passing or return
statements should have a corresponding format attribute in the
resulting type. I.e. the left-hand side of the assignment or
initialization, the type of the parameter variable, or the return type
of the containing function respectively should also have a format
attribute to avoid the warning.
GCC will also warn about function definitions that might be
candidates for format attributes. Again, these are only
possible candidates. GCC will guess that format attributes
might be appropriate for any function that calls a function like
vprintf or vscanf, but this might not always be the
case, and some functions for which format attributes are
appropriate may not be detected.
-Wno-multichar
-Wnormalized=<none|id|nfc|nfkc>
There are four levels of warning supported by GCC. The default is `-Wnormalized=nfc', which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. `-Wnormalized=id' suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing `-Wnormalized=none'. You would only want to do this if you were using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical
in some fonts or display methodologies, especially once formatting has
been applied. For instance \u207F, "SUPERSCRIPT LATIN SMALL
LETTER N", will display just like a regular n that has been
placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC will warn if your code is not in NFKC if you use
`-Wnormalized=nfkc'. This warning is comparable to warning
about every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment is
unable to be fixed to display these characters distinctly.
-Wno-deprecated
-Wno-deprecated-declarations
deprecated
attribute.
-Wno-overflow
-Woverride-init (C and Objective-C only)
This warning is included in `-Wextra'. To get other `-Wextra' warnings without this one, use `-Wextra -Wno-override-init'.
-Wpacked
f.x in struct bar
will be misaligned even though struct bar does not itself
have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
|
-Wpacked-bitfield-compat
packed attribute
on bit-fields of type char. This has been fixed in GCC 4.4 but
the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field a
and b in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
|
This warning is enabled by default. Use `-Wno-packed-bitfield-compat' to disable this warning.
-Wpadded
-Wredundant-decls
-Wnested-externs (C and Objective-C only)
extern declaration is encountered within a function.
-Winline
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by `-Winline' to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
The restrictions on `offsetof' may be relaxed in a future version of the C++ standard.
-Wno-int-to-pointer-cast
-Wno-pointer-to-int-cast (C and Objective-C only)
-Winvalid-pch
-Wlong-long
-Wvariadic-macros
-Wvector-operation-performance
piecewise, which means that the
scalar operation is performed on every vector element;
in parallel, which means that the vector operation is implemented
using scalars of wider type, which normally is more performance efficient;
and as a single scalar, which means that vector fits into a
scalar type.
-Wvla
-Wvolatile-register-var
-Wdisabled-optimization
-Wpointer-sign (C and Objective-C only)
-Wstack-protector
-Wno-mudflap
-Woverlength-strings
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by `-pedantic', and can be disabled with `-Wno-overlength-strings'.
-Wunsuffixed-float-constants (C and Objective-C only)
GCC will issue a warning for any floating constant that does not have
a suffix. When used together with `-Wsystem-headers' it will
warn about such constants in system header files. This can be useful
when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma
from the decimal floating-point extension to C99.
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GCC has various special options that are used for debugging either your program or GCC:
-g
On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', or `-gvms' (see below).
GCC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one debugging format.
-ggdb
-gstabs
-feliminate-unused-debug-symbols
-femit-class-debug-always
-fno-debug-types-section
-gstabs+
-gcoff
-gxcoff
-gxcoff+
-gdwarf-version
Note that with DWARF version 2 some ports require, and will always use, some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and `-fvar-tracking-assignments' for maximum benefit.
-grecord-gcc-switches
-gno-record-gcc-switches
-gstrict-dwarf
-gno-strict-dwarf
-gvms
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Level 0 produces no debug information at all. Thus, `-g0' negates `-g'.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'.
`-gdwarf-2' does not accept a concatenated debug level, because GCC used to support an option `-gdwarf' that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be changed now. Instead use an additional `-glevel' option to change the debug level for DWARF.
-gtoggle
-fdump-final-insns[=file]
.), the name
of the dump file will be determined by appending .gkd to the
compilation output file name.
-fcompare-debug[=opts]
If the equal sign is omitted, the default `-gtoggle' is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables `-fcompare-debug'. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash,
then it is used for opts, otherwise the default `-gtoggle'
is used.
`-fcompare-debug=', with the equal sign but without opts,
is equivalent to `-fno-compare-debug', which disables the dumping
of the final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during `-fcompare-debug' testing, set
GCC_COMPARE_DEBUG to say `-fcompare-debug-not-overridden',
which GCC will reject as an invalid option in any actual compilation
(rather than preprocessing, assembly or linking). To get just a
warning, setting GCC_COMPARE_DEBUG to `-w%n-fcompare-debug
not overridden' will do.
-fcompare-debug-second
.gk additional extension during the second compilation, to avoid
overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.
-feliminate-dwarf2-dups
-femit-struct-debug-baseonly
This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See `-femit-struct-debug-reduced' for a less aggressive option. See `-femit-struct-debug-detailed' for more detailed control.
This option works only with DWARF 2.
-femit-struct-debug-reduced
This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See `-femit-struct-debug-baseonly' for a more aggressive option. See `-femit-struct-debug-detailed' for more detailed control.
This option works only with DWARF 2.
-femit-struct-debug-detailed[=spec-list]
This option is a detailed version of `-femit-struct-debug-reduced' and `-femit-struct-debug-baseonly', which will serve for most needs.
A specification has the syntax
[`dir:'|`ind:'][`ord:'|`gen:'](`any'|`sys'|`base'|`none')
The optional first word limits the specification to structs that are used directly (`dir:') or used indirectly (`ind:'). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct would be legal, the use is indirect. An example is `struct one direct; struct two * indirect;'.
The optional second word limits the specification to ordinary structs (`ord:') or generic structs (`gen:'). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but `-femit-struct-debug-detailed' does not yet implement them.
The third word specifies the source files for those structs for which the compiler will emit debug information. The values `none' and `any' have the normal meaning. The value `base' means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that types declared in `foo.c' and `foo.h' will have debug information, but types declared in other header will not. The value `sys' means those types satisfying `base' or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is `-femit-struct-debug-detailed=all'.
This option works only with DWARF 2.
-fno-merge-debug-strings
-fdebug-prefix-map=old=new
-fno-dwarf2-cfi-asm
.eh_frame section
instead of using GAS .cfi_* directives.
-p
prof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-pg
gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-Q
-ftime-report
-fmem-report
-fpre-ipa-mem-report
-fpost-ipa-mem-report
-fstack-usage
static, dynamic, bounded.
The qualifier static means that the function manipulates the stack
statically: a fixed number of bytes are allocated for the frame on function
entry and released on function exit; no stack adjustments are otherwise made
in the function. The second field is this fixed number of bytes.
The qualifier dynamic means that the function manipulates the stack
dynamically: in addition to the static allocation described above, stack
adjustments are made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier bounded is also
present, the amount of these adjustments is bounded at compile time and
the second field is an upper bound of the total amount of stack used by
the function. If it is not present, the amount of these adjustments is
not bounded at compile time and the second field only represents the
bounded part.
-fprofile-arcs
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for `-fprofile-arcs' `-ftest-coverage' (when compiling) and `-lgcov' (when linking). See the documentation for those options for more details.
fork calls are detected and correctly handled (double counting
will not happen).
gcov to produce human readable
information from the `.gcno' and `.gcda' files. Refer to the
gcov documentation for further information.
With `-fprofile-arcs', for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.
-ftest-coverage
gcov code-coverage utility
(see section gcov---a Test Coverage Program) can use to
show program coverage. Each source file's note file is called
`auxname.gcno'. Refer to the `-fprofile-arcs' option
above for a description of auxname and instructions on how to
generate test coverage data. Coverage data will match the source files
more closely, if you do not optimize.
-fdbg-cnt-list
-fdbg-cnt=counter-value-list
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of debugging options that are used to explicitly disable/enable optimization passes. For compiler users, regular options for enabling/disabling passes should be used instead.
# disable ccp1 for all functions -fdisable-tree-ccp1 # disable complete unroll for function whose cgraph node uid is 1 -fenable-tree-cunroll=1 # disable gcse2 for functions at the following ranges [1,1], # [300,400], and [400,1000] # disable gcse2 for functions foo and foo2 -fdisable-rtl-gcse2=foo,foo2 # disable early inlining -fdisable-tree-einline # disable ipa inlining -fdisable-ipa-inline # enable tree full unroll -fenable-tree-unroll |
-dletters
-fdump-rtl-pass
Debug dumps can be enabled with a `-fdump-rtl' switch or some `-d' option letters. Here are the possible letters for use in pass and letters, and their meanings:
-fdump-rtl-alignments
-fdump-rtl-asmcons
-fdump-rtl-auto_inc_dec
-fdump-rtl-barriers
-fdump-rtl-bbpart
-fdump-rtl-bbro
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-bypass
-fdump-rtl-combine
-fdump-rtl-compgotos
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-cprop_hardreg
-fdump-rtl-csa
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-dce
-fdump-rtl-dbr
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-eh
-fdump-rtl-eh_ranges
-fdump-rtl-expand
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-init-regs
-fdump-rtl-initvals
-fdump-rtl-into_cfglayout
-fdump-rtl-ira
-fdump-rtl-jump
-fdump-rtl-loop2
-fdump-rtl-mach
-fdump-rtl-mode_sw
-fdump-rtl-rnreg
-fdump-rtl-outof_cfglayout
-fdump-rtl-peephole2
-fdump-rtl-postreload
-fdump-rtl-pro_and_epilogue
-fdump-rtl-regmove
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-see
-fdump-rtl-seqabstr
-fdump-rtl-shorten
-fdump-rtl-sibling
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
-fdump-rtl-sms
-fdump-rtl-stack
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-unshare
-fdump-rtl-vartrack
-fdump-rtl-vregs
-fdump-rtl-web
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
-fdump-rtl-all
-dA
-dD
-dH
-dm
-dp
-dP
-dv
-dx
-fdump-noaddr
-fdump-unnumbered
-fdump-unnumbered-links
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
-fdump-ipa-switch
-fdump-passes
-fdump-statistics-option
-fdump-tree-switch
-fdump-tree-switch-options
DECL_ASSEMBLER_NAME has been set for a given decl, use that
in the dump instead of DECL_NAME. Its primary use is ease of
use working backward from mangled names in the assembly file.
DECL_UID) for each variable.
The following tree dumps are possible:
-ftree-vectorizer-verbose=n
-frandom-seed=string
The string should be different for every file you compile.
-fsched-verbose=n
For n greater than zero, `-fsched-verbose' outputs the same information as `-fdump-rtl-sched1' and `-fdump-rtl-sched2'. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, `-fsched-verbose' also includes dependence info.
-save-temps
-save-temps=cwd
When used in combination with the `-x' command-line option, `-save-temps' is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using `-save-temps'.
If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c& gcc -save-temps -o outdir2/foo.o indir2/foo.c& |
may result in `foo.i' and `foo.o' being written to simultaneously by both compilers.
-save-temps=obj
For example:
gcc -save-temps=obj -c foo.c gcc -save-temps=obj -c bar.c -o dir/xbar.o gcc -save-temps=obj foobar.c -o dir2/yfoobar |
would create `foo.i', `foo.s', `dir/xbar.i', `dir/xbar.s', `dir2/yfoobar.i', `dir2/yfoobar.s', and `dir2/yfoobar.o'.
-time[=file]
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01 # as 0.00 0.01 |
The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 options 0.00 0.01 as options |
The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.
-fvar-tracking
It is enabled by default when compiling with optimization (`-Os', `-O', `-O2', ...), debugging information (`-g') and the debug info format supports it.
-fvar-tracking-assignments
It can be enabled even if var-tracking is disabled, in which case annotations will be created and maintained, but discarded at the end.
-fvar-tracking-assignments-toggle
-print-file-name=library
-print-multi-directory
GCC_EXEC_PREFIX.
-print-multi-lib
-print-multi-os-directory
-print-prog-name=program
-print-libgcc-file-name
This is useful when you use `-nostdlib' or `-nodefaultlibs' but you do want to link with `libgcc.a'. You can do
gcc -nostdlib files... `gcc -print-libgcc-file-name` |
-print-search-dirs
gcc will search--and don't do anything else.
This is useful when gcc prints the error message
`installation problem, cannot exec cpp0: No such file or directory'.
To resolve this you either need to put `cpp0' and the other compiler
components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing `/'.
See section 3.19 Environment Variables Affecting GCC.
-print-sysroot
-print-sysroot-headers-suffix
-dumpmachine
-dumpversion
-dumpspecs
-feliminate-unused-debug-types
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These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an `-O' level is set on the command line. Otherwise they are disabled, even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each `-O' level than those listed here. You can invoke GCC with `-Q --help=optimizers' to find out the exact set of optimizations that are enabled at each level. See section 3.2 Options Controlling the Kind of Output, for examples.
-O
-O1
With `-O', the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
`-O' turns on the following optimization flags:
{
|
`-O' also turns on `-fomit-frame-pointer' on machines where doing so does not interfere with debugging.
-O2
`-O2' turns on all optimization flags specified by `-O'. It also turns on the following optimization flags:
{-fthread-jumps
|
Please note the warning under `-fgcse' about invoking `-O2' on programs that use computed gotos.
-O3
-O0
-Os
`-Os' disables the following optimization flags:
{-falign-functions -falign-jumps -falign-loops
|
-Ofast
If you use multiple `-O' options, with or without level numbers, the last such option is the one that is effective.
Options of the form `-fflag' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one you typically will use. You can figure out the other form by either removing `no-' or adding it.
The following options control specific optimizations. They are either activated by `-O' options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
-fno-default-inline
-fno-defer-pop
Disabled at levels `-O', `-O2', `-O3', `-Os'.
-fforward-propagate
This option is enabled by default at optimization levels `-O', `-O2', `-O3', `-Os'.
-ffp-contract=style
The default is `-ffp-contract=fast'.
-fomit-frame-pointer
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro FRAME_POINTER_REQUIRED controls
whether a target machine supports this flag. See section `Register Usage' in GNU Compiler Collection (GCC) Internals.
Starting with GCC version 4.6, the default setting (when not optimizing for size) for 32-bit Linux x86 and 32-bit Darwin x86 targets has been changed to `-fomit-frame-pointer'. The default can be reverted to `-fno-omit-frame-pointer' by configuring GCC with the `--enable-frame-pointer' configure option.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-foptimize-sibling-calls
Enabled at levels `-O2', `-O3', `-Os'.
-fno-inline
always_inline attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them
with the noinline attribute.
-finline-small-functions
Enabled at level `-O2'.
-findirect-inlining
Enabled at level `-O2'.
-finline-functions
If all calls to a given function are integrated, and the function is
declared static, then the function is normally not output as
assembler code in its own right.
Enabled at level `-O3'.
-fsort-data
Enabled at level `-Os'.
-finline-functions-called-once
static functions called once for inlining into their
caller even if they are not marked inline. If a call to a given
function is integrated, then the function is not output as assembler code
in its own right.
Enabled at levels `-O1', `-O2', `-O3' and `-Os'.
-fearly-inlining
always_inline and functions whose body seems
smaller than the function call overhead early before doing
`-fprofile-generate' instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on programs
having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Enabled at levels `-O2', `-O3' and `-Os'.
-finline-limit=n
Inlining is actually controlled by a number of parameters, which may be specified individually by using `--param name=value'. The `-finline-limit=n' option sets some of these parameters as follows:
max-inline-insns-single
max-inline-insns-auto
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to `-finline-limit' that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.
-fno-keep-inline-dllexport
dllexport
attribute or declspec (See section Declaring Attributes of Functions.)
-fkeep-inline-functions
static functions that are declared inline
into the object file, even if the function has been inlined into all
of its callers. This switch does not affect functions using the
extern inline extension in GNU C90. In C++, emit any and all
inline functions into the object file.
-fkeep-static-consts
static const when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts' option.
-fmerge-constants
This option is the default for optimized compilation if the assembler and linker support it. Use `-fno-merge-constants' to inhibit this behavior.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fmerge-all-constants
This option implies `-fmerge-constants'. In addition to `-fmerge-constants' this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option will result in non-conforming behavior.
-fmodulo-sched
-fmodulo-sched-allow-regmoves
-fno-branch-count-reg
The default is `-fbranch-count-reg'.
-fno-function-cse
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is `-ffunction-cse'
-fno-zero-initialized-in-bss
This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is `-fzero-initialized-in-bss'.
-fmudflap -fmudflapth -fmudflapir
MUDFLAP_OPTIONS
environment variable. See env MUDFLAP_OPTIONS=-help a.out
for its options.
Use `-fmudflapth' instead of `-fmudflap' to compile and to link if your program is multi-threaded. Use `-fmudflapir', in addition to `-fmudflap' or `-fmudflapth', if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.
-fthread-jumps
Enabled at levels `-O2', `-O3', `-Os'.
-fsplit-wide-types
long
long on a 32-bit system, split the registers apart and allocate them
independently. This normally generates better code for those types,
but may make debugging more difficult.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fcse-follow-jumps
if statement with an
else clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels `-O2', `-O3', `-Os'.
-fcse-skip-blocks
if statement with no else clause,
`-fcse-skip-blocks' causes CSE to follow the jump around the
body of the if.
Enabled at levels `-O2', `-O3', `-Os'.
-frerun-cse-after-loop
Enabled at levels `-O2', `-O3', `-Os'.
-fgcse
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding `-fno-gcse' to the command line.
Enabled at levels `-O2', `-O3', `-Os'.
-fgcse-lm
Enabled by default when gcse is enabled.
-fgcse-sm
Not enabled at any optimization level.
-fgcse-las
Not enabled at any optimization level.
-fgcse-after-reload
-funsafe-loop-optimizations
-fcrossjumping
Enabled at levels `-O2', `-O3', `-Os'.
-fauto-inc-dec
-fdce
-fdse
-fif-conversion
if-conversion2.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fif-conversion2
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fdelete-null-pointer-checks
Note however that in some environments this assumption is not true. Use `-fno-delete-null-pointer-checks' to disable this optimization for programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all levels. Otherwise it is enabled at all levels: `-O0', `-O1', `-O2', `-O3', `-Os'. Passes that use the information are enabled independently at different optimization levels.
-fdevirtualize
-findirect-inlining) and interprocedural constant
propagation (`-fipa-cp').
Enabled at levels `-O2', `-O3', `-Os'.
-fexpensive-optimizations
Enabled at levels `-O2', `-O3', `-Os'.
-free
Enabled for x86 at levels `-O2', `-O3'.
-foptimize-register-move
-fregmove
Note `-fregmove' and `-foptimize-register-move' are the same optimization.
Enabled at levels `-O2', `-O3', `-Os'.
-fira-algorithm=algorithm
-fira-region=region
-fira-loop-pressure
This option is enabled at level `-O3' for some targets.
-fno-ira-share-save-slots
-fno-ira-share-spill-slots
-fira-verbose=n
-fdelayed-branch
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fschedule-insns
Enabled at levels `-O2', `-O3'.
-fschedule-insns2
Enabled at levels `-O2', `-O3', `-Os'.
-fno-sched-interblock
-fno-sched-spec
-fsched-pressure
-fsched-spec-load
-fsched-spec-load-dangerous
-fsched-stalled-insns
-fsched-stalled-insns=n
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
-fsched2-use-superblocks
This only makes sense when scheduling after register allocation, i.e. with `-fschedule-insns2' or at `-O2' or higher.
-fsched-group-heuristic
-fsched-critical-path-heuristic
-fsched-spec-insn-heuristic
-fsched-rank-heuristic
-fsched-last-insn-heuristic
-fsched-dep-count-heuristic
-freschedule-modulo-scheduled-loops
-fselective-scheduling
-fselective-scheduling2
-fsel-sched-pipelining
-fsel-sched-pipelining-outer-loops
-fshrink-wrap
-fcaller-saves
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Enabled at levels `-O2', `-O3', `-Os'.
-fcombine-stack-adjustments
Enabled by default at `-O1' and higher.
-fconserve-stack
-ftree-reassoc
-ftree-pre
-ftree-forwprop
-ftree-fre
-ftree-phiprop
-ftree-copy-prop
-fipa-pure-const
-fipa-reference
-fipa-pta
-fipa-profile
cold, noreturn, static constructors or destructors) are identified. Cold
functions and loop less parts of functions executed once are then optimized for
size.
Enabled by default at `-O' and higher.
-fipa-cp
-fipa-cp-clone
-fipa-matrix-reorg
-ftree-sink
-ftree-bit-ccp
-ftree-ccp
-ftree-switch-conversion
-ftree-tail-merge
-ftree-dce
-ftree-builtin-call-dce
errno but are otherwise side-effect free. This flag is
enabled by default at `-O2' and higher if `-Os' is not also
specified.
-ftree-dominator-opts
-ftree-dse
-ftree-ch
-ftree-loop-optimize
-ftree-loop-linear
-floop-interchange
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
|
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
|
N is larger than the caches,
because in Fortran, the elements of an array are stored in memory
contiguously by column, and the original loop iterates over rows,
potentially creating at each access a cache miss. This optimization
applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured
with `--with-ppl' and `--with-cloog' to enable the
Graphite loop transformation infrastructure.
-floop-strip-mine
DO I = 1, N A(I) = A(I) + C ENDDO |
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
|
-floop-block
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
|
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
|
M is larger than the caches,
because the innermost loop will iterate over a smaller amount of data
which can be kept in the caches. This optimization applies to all the
languages supported by GCC and is not limited to Fortran. To use this
code transformation, GCC has to be configured with `--with-ppl'
and `--with-cloog' to enable the Graphite loop transformation
infrastructure.
-fgraphite-identity
-floop-flatten
-floop-parallelize-all
-fcheck-data-deps
-ftree-loop-if-convert
-ftree-loop-if-convert-stores
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
|
for (i = 0; i < N; i++) A[i] = cond ? expr : A[i]; |
-ftree-loop-distribution
DO I = 1, N A(I) = B(I) + C D(I) = E(I) * F ENDDO |
DO I = 1, N A(I) = B(I) + C ENDDO DO I = 1, N D(I) = E(I) * F ENDDO |
-ftree-loop-distribute-patterns
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N A(I) = 0 B(I) = A(I) + I ENDDO |
DO I = 1, N A(I) = 0 ENDDO DO I = 1, N B(I) = A(I) + I ENDDO |
-ftree-loop-im
-ftree-loop-ivcanon
-fivopts
-ftree-parallelize-loops=n
-ftree-pta
-ftree-sra
-ftree-copyrename
-ftree-ter
-ftree-vectorize
-ftree-slp-vectorize
-ftree-vect-loop-version
-fvect-cost-model
-ftree-vrp
-ftracer
-funroll-loops
-funroll-all-loops
-fsplit-ivs-in-unroller
Combination of `-fweb' and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
-fpartial-inlining
Enabled at level `-O2'.
-fpredictive-commoning
This option is enabled at level `-O3'.
-fprefetch-loop-arrays
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.
Disabled at level `-Os'.
-fno-peephole
-fno-peephole2
`-fpeephole' is enabled by default. `-fpeephole2' enabled at levels `-O2', `-O3', `-Os'.
-fno-guess-branch-probability
GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (`-fprofile-arcs'). These heuristics are based on the control flow graph. If some branch probabilities are specified by `__builtin_expect', then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the `__builtin_expect' info into account. The interactions between the heuristics and `__builtin_expect' can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of `__builtin_expect' are easier to understand.
The default is `-fguess-branch-probability' at levels `-O', `-O2', `-O3', `-Os'.
-freorder-blocks
Enabled at levels `-O2', `-O3'.
-freorder-blocks-and-partition
This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.
-freorder-functions
.text.hot for most frequently executed functions and
.text.unlikely for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See `-fprofile-arcs' for details.
Enabled at levels `-O2', `-O3', `-Os'.
-fstrict-aliasing
unsigned int can alias an int, but not a
void* or a double. A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
|
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
|
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
|
The `-fstrict-aliasing' option is enabled at levels `-O2', `-O3', `-Os'.
-fstrict-overflow
i + 10 > i will always be true for
signed i. This assumption is only valid if signed overflow is
undefined, as the expression is false if i + 10 overflows when
using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers will
overflow must be written carefully to not actually involve overflow.
This option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset to that
pointer does not produce a pointer to the same object, the addition is
undefined. This permits the compiler to conclude that p + u >
p is always true for a pointer p and unsigned integer
u. This assumption is only valid because pointer wraparound is
undefined, as the expression is false if p + u overflows using
twos complement arithmetic.
See also the `-fwrapv' option. Using `-fwrapv' means that integer signed overflow is fully defined: it wraps. When `-fwrapv' is used, there is no difference between `-fstrict-overflow' and `-fno-strict-overflow' for integers. With `-fwrapv' certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with `-fwrapv', but not otherwise.
The `-fstrict-overflow' option is enabled at levels `-O2', `-O3', `-Os'.
-falign-functions
-falign-functions=n
`-fno-align-functions' and `-falign-functions=1' are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
-falign-labels
-falign-labels=n
`-fno-align-labels' and `-falign-labels=1' are equivalent and mean that labels will not be aligned.
If `-falign-loops' or `-falign-jumps' are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be `1', meaning no alignment.
Enabled at levels `-O2', `-O3'.
-falign-loops
-falign-loops=n
`-fno-align-loops' and `-falign-loops=1' are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
-falign-jumps
-falign-jumps=n
`-fno-align-jumps' and `-falign-jumps=1' are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
-funit-at-a-time
Enabled by default.
-fno-toplevel-reorder
asm
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static variables
will not be removed. This option is intended to support existing code
that relies on a particular ordering. For new code, it is better to
use attributes.
Enabled at level `-O0'. When disabled explicitly, it also implies `-fno-section-anchors', which is otherwise enabled at `-O0' on some targets.
-fweb
Enabled by default with `-funroll-loops'.
-fwhole-program
main
and those merged by attribute externally_visible become static functions
and in effect are optimized more aggressively by interprocedural optimizers. If gold is used as the linker plugin, externally_visible attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file, explicit externally_visible attributes are still necessary.
While this option is equivalent to proper use of the static keyword for
programs consisting of a single file, in combination with option
`-flto' this flag can be used to
compile many smaller scale programs since the functions and variables become
local for the whole combined compilation unit, not for the single source file
itself.
This option implies `-fwhole-file' for Fortran programs.
-flto[=n]
To use the link-time optimizer, `-flto' needs to be specified at compile time and during the final link. For example:
gcc -c -O2 -flto foo.c gcc -c -O2 -flto bar.c gcc -o myprog -flto -O2 foo.o bar.o |
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside `foo.o' and `bar.o'. The final invocation reads the GIMPLE bytecode from `foo.o' and `bar.o', merges the two files into a single internal image, and compiles the result as usual. Since both `foo.o' and `bar.o' are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in `bar.o' into functions in `foo.o' and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c |
The above generates bytecode for `foo.c' and `bar.c', merges them together into a single GIMPLE representation and optimizes them as usual to produce `myprog'.
The only important thing to keep in mind is that to enable link-time optimizations the `-flto' flag needs to be passed to both the compile and the link commands.
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see `-fuse-linker-plugin') passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, `-fwhole-program' should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.
Note that when a file is compiled with `-flto', the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code. This means that object files with LTO information can be linked as normal object files; if `-flto' is not passed to the linker, no interprocedural optimizations are applied.
Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link time. For instance,
gcc -c -O0 -flto foo.c gcc -c -O0 -flto bar.c gcc -o myprog -flto -O3 foo.o bar.o |
This produces individual object files with unoptimized assembler code, but the resulting binary `myprog' is optimized at `-O3'. If, instead, the final binary is generated without `-flto', then `myprog' is not optimized.
When producing the final binary with `-flto', GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link stage. Currently, the following options are saved into the GIMPLE bytecode files: `-fPIC', `-fcommon' and all the `-m' target flags.
At link time, these options are read in and reapplied. Note that the current implementation makes no attempt to recognize conflicting values for these options. If different files have conflicting option values (e.g., one file is compiled with `-fPIC' and another isn't), the compiler simply uses the last value read from the bytecode files. It is recommended, then, that you compile all the files participating in the same link with the same options.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages. This requires support in the language front end. Currently, the C, C++ and Fortran front ends are capable of emitting GIMPLE bytecodes, so something like this should work:
gcc -c -flto foo.c g++ -c -flto bar.cc gfortran -c -flto baz.f90 g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran |
Notice that the final link is done with g++ to get the C++
runtime libraries and `-lgfortran' is added to get the Fortran
runtime libraries. In general, when mixing languages in LTO mode, you
should use the same link command options as when mixing languages in a
regular (non-LTO) compilation; all you need to add is `-flto' to
all the compile and link commands.
If object files containing GIMPLE bytecode are stored in a library archive, say `libfoo.a', it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To enable this feature, use the flag `-fuse-linker-plugin' at link time:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo |
With the linker plugin enabled, the linker extracts the needed GIMPLE files from `libfoo.a' and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside `libfoo.a' are extracted and linked as usual, but they do not participate in the LTO optimization process.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine `-flto' and `-fwhole-program' to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of `-fwhole-program' is not needed when linker plugin is active (see `-fuse-linker-plugin').
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC will not work with an older/newer version of GCC.
Link-time optimization does not work well with generation of debugging information. Combining `-flto' with `-g' is currently experimental and expected to produce wrong results.
If you specify the optional n, the optimization and code
generation done at link time is executed in parallel using n
parallel jobs by utilizing an installed make program. The
environment variable MAKE may be used to override the program
used. The default value for n is 1.
You can also specify `-flto=jobserver' to use GNU make's
job server mode to determine the number of parallel jobs. This
is useful when the Makefile calling GCC is already executing in parallel.
You must prepend a `+' to the command recipe in the parent Makefile
for this to work. This option likely only works if MAKE is
GNU make.
This option is disabled by default
-flto-partition=alg
1to1 to specify a partitioning mirroring
the original source files or balanced to specify partitioning
into equally sized chunks (whenever possible). Specifying none
as an algorithm disables partitioning and streaming completely. The
default value is balanced.
-flto-compression-level=n
-flto-report
Disabled by default.
-fuse-linker-plugin
This option enables the extraction of object files with GIMPLE bytecode out
of library archives. This improves the quality of optimization by exposing
more code to the link-time optimizer. This information specifies what
symbols can be accessed externally (by non-LTO object or during dynamic
linking). Resulting code quality improvements on binaries (and shared
libraries that use hidden visibility) are similar to -fwhole-program.
See `-flto' for a description of the effect of this flag and how to
use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
`-fno-fat-lto-objects' improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc).
The default is `-ffat-lto-objects' but this default is intended to change in future releases when linker plugin enabled environments become more common.
-fcompare-elim
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fcprop-registers
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fprofile-correction
-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects only the profile data generated by `-fprofile-generate', `-ftest-coverage', `-fprofile-arcs' and used by `-fprofile-use' and `-fbranch-probabilities' and its related options. Both absolute and relative paths can be used. By default, GCC will use the current directory as path, thus the profile data file will appear in the same directory as the object file.
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use `-fprofile-generate' both when compiling and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
If path is specified, GCC will look at the path to find the profile feedback data files. See `-fprofile-dir'.
-fprofile-use
-fprofile-use=path
The following options are enabled: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using `-Wcoverage-mismatch'. Note this may result in poorly optimized code.
If path is specified, GCC will look at the path to find the profile feedback data files. See `-fprofile-dir'.
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
-ffloat-store
This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a double is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use `-ffloat-store' for such programs, after modifying
them to store all pertinent intermediate computations into variables.
-fexcess-precision=style
float and double types and the processor does not
support operations rounding to those types. By default,
`-fexcess-precision=fast' is in effect; this means that
operations are carried out in the precision of the registers and that
it is unpredictable when rounding to the types specified in the source
code takes place. When compiling C, if
`-fexcess-precision=standard' is specified then excess
precision will follow the rules specified in ISO C99; in particular,
both casts and assignments cause values to be rounded to their
semantic types (whereas `-ffloat-store' only affects
assignments). This option is enabled by default for C if a strict
conformance option such as `-std=c99' is used.
`-fexcess-precision=standard' is not implemented for languages other than C, and has no effect if `-funsafe-math-optimizations' or `-ffast-math' is specified. On the x86, it also has no effect if `-mfpmath=sse' or `-mfpmath=sse+387' is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.
-ffast-math
This option causes the preprocessor macro __FAST_MATH__ to be defined.
This option is not turned on by any `-O' option besides `-Ofast' since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
-fno-math-errno
This option is not turned on by any `-O' option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is `-fmath-errno'.
On Darwin systems, the math library never sets errno. There is
therefore no reason for the compiler to consider the possibility that
it might, and `-fno-math-errno' is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.
This option is not turned on by any `-O' option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables `-fno-signed-zeros', `-fno-trapping-math', `-fassociative-math' and `-freciprocal-math'.
The default is `-fno-unsafe-math-optimizations'.
-fassociative-math
Allow re-association of operands in series of floating-point operations.
This violates the ISO C and C++ language standard by possibly changing
computation result. NOTE: re-ordering may change the sign of zero as
well as ignore NaNs and inhibit or create underflow or overflow (and
thus cannot be used on code that relies on rounding behavior like
(x + 2**52) - 2**52. May also reorder floating-point comparisons
and thus may not be used when ordered comparisons are required.
This option requires that both `-fno-signed-zeros' and
`-fno-trapping-math' be in effect. Moreover, it doesn't make
much sense with `-frounding-math'. For Fortran the option
is automatically enabled when both `-fno-signed-zeros' and
`-fno-trapping-math' are in effect.
The default is `-fno-associative-math'.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example x / y
can be replaced with x * (1/y), which is useful if (1/y)
is subject to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.
The default is `-fno-reciprocal-math'.
-ffinite-math-only
This option is not turned on by any `-O' option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is `-fno-finite-math-only'.
-fno-signed-zeros
The default is `-fsigned-zeros'.
-fno-trapping-math
This option should never be turned on by any `-O' option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is `-ftrapping-math'.
-frounding-math
The default is `-fno-rounding-math'.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's FENV_ACCESS pragma. This command-line option
will be used to specify the default state for FENV_ACCESS.
-fsignaling-nans
This option causes the preprocessor macro __SUPPORT_SNAN__ to
be defined.
The default is `-fno-signaling-nans'.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
-fsingle-precision-constant
-fcx-limited-range
NaN
+ I*NaN, with an attempt to rescue the situation in that case. The
default is `-fno-cx-limited-range', but is enabled by
`-ffast-math'.
This option controls the default setting of the ISO C99
CX_LIMITED_RANGE pragma. Nevertheless, the option applies to
all languages.
-fcx-fortran-rules
NaN
+ I*NaN, with an attempt to rescue the situation in that case.
The default is `-fno-cx-fortran-rules'.
The following options control optimizations that may improve performance, but are not enabled by any `-O' options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
gcc), you can compile it a second time using
`-fbranch-probabilities', to improve optimizations based on
the number of times each branch was taken. When the program
compiled with `-fprofile-arcs' exits it saves arc execution
counts to a file called `sourcename.gcda' for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.
With `-fbranch-probabilities', GCC puts a `REG_BR_PROB' note on each `JUMP_INSN' and `CALL_INSN'. These can be used to improve optimization. Currently, they are only used in one place: in `reorg.c', instead of guessing which path a branch is most likely to take, the `REG_BR_PROB' values are used to exactly determine which path is taken more often.
-fprofile-values
With `-fbranch-probabilities', it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled with `-fprofile-generate' and `-fprofile-use'.
-fvpt
With `-fbranch-probabilities', it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operation using the knowledge about the value of the denominator.
-frename-registers
Enabled by default with `-funroll-loops' and `-fpeel-loops'.
-ftracer
Enabled with `-fprofile-use'.
-funroll-loops
Enabled with `-fprofile-use'.
-funroll-all-loops
-fpeel-loops
Enabled with `-fprofile-use'.
-fmove-loop-invariants
-funswitch-loops
-ffunction-sections
-fdata-sections
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use gprof on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and `-g'.
-fbranch-target-load-optimize
-fbranch-target-load-optimize2
-fbtr-bb-exclusive
-fstack-protector
-fstack-protector-all
-fsection-anchors
For example, the implementation of the following function foo:
static int a, b, c;
int foo (void) { return a + b + c; }
|
would usually calculate the addresses of all three variables, but if you compile it with `-fsection-anchors', it will access the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
|
Not all targets support this option.
--param name=value
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are given in the following table:
predictable-branch-outcome
max-crossjump-edges
min-crossjump-insns
max-grow-copy-bb-insns
max-goto-duplication-insns
max-delay-slot-insn-search
max-delay-slot-live-search
max-gcse-memory
max-gcse-insertion-ratio
max-pending-list-length
max-modulo-backtrack-attempts
max-inline-insns-single
max-inline-insns-auto
large-function-insns
large-function-growth
large-unit-insns
inline-unit-growth
ipcp-unit-growth
large-stack-frame
large-stack-frame-growth
max-inline-insns-recursive
max-inline-insns-recursive-auto
For functions declared inline `--param max-inline-insns-recursive' is taken into account. For function not declared inline, recursive inlining happens only when `-finline-functions' (included in `-O3') is enabled and `--param max-inline-insns-recursive-auto' is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
For functions declared inline `--param max-inline-recursive-depth' is taken into account. For function not declared inline, recursive inlining happens only when `-finline-functions' (included in `-O3') is enabled and `--param max-inline-recursive-depth-auto' is used. The default value is 8.
min-inline-recursive-probability
When profile feedback is available (see `-fprofile-generate') the actual recursion depth can be guessed from probability that function will recurse via given call expression. This parameter limits inlining only to call expression whose probability exceeds given threshold (in percents). The default value is 10.
early-inlining-insns
max-early-inliner-iterations
max-early-inliner-iterations
comdat-sharing-probability
comdat-sharing-probability
min-vect-loop-bound
gcse-cost-distance-ratio
gcse-unrestricted-cost
max-hoist-depth
max-tail-merge-comparisons
max-tail-merge-iterations
max-unrolled-insns
max-average-unrolled-insns
max-unroll-times
max-peeled-insns
max-peel-times
max-completely-peeled-insns
max-completely-peel-times
max-completely-peel-loop-nest-depth
max-unswitch-insns
max-unswitch-level
lim-expensive
iv-consider-all-candidates-bound
iv-max-considered-uses
iv-always-prune-cand-set-bound
scev-max-expr-size
scev-max-expr-complexity
omega-max-vars
omega-max-geqs
omega-max-eqs
omega-max-wild-cards
omega-hash-table-size
omega-max-keys
omega-eliminate-redundant-constraints
vect-max-version-for-alignment-checks
vect-max-version-for-alias-checks
max-iterations-to-track
The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop tries to evaluate.
hot-bb-count-fraction
hot-bb-frequency-fraction
max-predicted-iterations
align-threshold
Select fraction of the maximal frequency of executions of basic block in function given basic block will get aligned.
align-loop-iterations
A loop expected to iterate at lest the selected number of iterations will get aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.
The `tracer-dynamic-coverage-feedback' is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.
tracer-max-code-growth
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to `tracer-dynamic-coverage' two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers. The default is 10.
max-cse-insns
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
RAM >= 1GB. If getrlimit is available, the notion of "RAM" is
the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If
GCC is not able to calculate RAM on a particular platform, the lower
bound of 30% is used. Setting this parameter and
`ggc-min-heapsize' to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be useful for
debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by `ggc-min-expand'% beyond `ggc-min-heapsize'. Again, tuning this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and `ggc-min-expand' to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
max-cselib-memory-locations
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.
The `reorder-block-duplicate-feedback' is used only when profile feedback is available and may be set to higher values than `reorder-block-duplicate' since information about the hot spots is more accurate.
max-sched-ready-insns
max-sched-region-blocks
max-pipeline-region-blocks
max-sched-region-insns
max-pipeline-region-insns
min-spec-prob
max-sched-extend-regions-iters
max-sched-insn-conflict-delay
sched-spec-prob-cutoff
sched-mem-true-dep-cost
selsched-max-lookahead
selsched-max-sched-times
selsched-max-insns-to-rename
sms-min-sc
max-last-value-rtl
integer-share-limit
min-virtual-mappings
virtual-mappings-ratio
ssp-buffer-size
max-jump-thread-duplication-stmts
max-fields-for-field-sensitive
prefetch-latency
simultaneous-prefetches
l1-cache-line-size
l1-cache-size
l2-cache-size
min-insn-to-prefetch-ratio
prefetch-min-insn-to-mem-ratio
use-canonical-types
switch-conversion-max-branch-ratio
max-partial-antic-length
sccvn-max-scc-size
ira-max-loops-num
ira-max-conflict-table-size
ira-loop-reserved-regs
loop-invariant-max-bbs-in-loop
loop-max-datarefs-for-datadeps
max-vartrack-size
max-vartrack-expr-depth
min-nondebug-insn-uid
ipa-sra-ptr-growth-factor
tm-max-aggregate-size
graphite-max-nb-scop-params
graphite-max-bbs-per-function
loop-block-tile-size
ipa-cp-value-list-size
lto-partitions
lto-minpartition
cxx-max-namespaces-for-diagnostic-help
sink-frequency-threshold
max-stores-to-sink
allow-load-data-races
allow-store-data-races
allow-packed-load-data-races
allow-packed-store-data-races
case-values-threshold
tree-reassoc-width
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These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the `-E' option, nothing is done except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor output to be unsuitable for actual compilation.
-Wp,option
-Xpreprocessor option
If you want to pass an option that takes an argument, you must use `-Xpreprocessor' twice, once for the option and once for the argument.
-D name
1.
-D name=definition
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh,
`-D'name(args...)=definition'' works.
`-D' and `-U' options are processed in the order they are given on the command line. All `-imacros file' and `-include file' options are processed after all `-D' and `-U' options.
-U name
-undef
-I dir
=, then the = will be replaced
by the sysroot prefix; see `--sysroot' and `-isysroot'.
-o file
cpp. gcc has a
different interpretation of a second non-option argument, so you must
use `-o' to specify the output file.
-Wall
#if expressions. Note that many of the
preprocessor's warnings are on by default and have no options to
control them.
-Wcomment
-Wcomments
-Wtrigraphs
This option is implied by `-Wall'. If `-Wall' is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other `-Wall' warnings, use `-trigraphs -Wall -Wno-trigraphs'.
-Wtraditional
-Wundef
-Wunused-macros
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning #endif |
-Wendif-labels
#if FOO ... #else FOO ... #endif FOO |
The second and third FOO should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
-Wsystem-headers
-w
-pedantic
-pedantic-errors
-M
make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of all
the included files, including those coming from `-include' or
`-imacros' command line options.
Unless specified explicitly (with `-MT' or `-MQ'), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using `\'-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as
`-dM'. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
`-MF', or use an environment variable like
DEPENDENCIES_OUTPUT (see section 3.19 Environment Variables Affecting GCC). Debug output
will still be sent to the regular output stream as normal.
Passing `-M' to the driver implies `-E', and suppresses warnings with an implicit `-w'.
-MM
This implies that the choice of angle brackets or double quotes in an `#include' directive does not in itself determine whether that header will appear in `-MM' dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.
-MF file
When used with the driver options `-MD' or `-MMD', `-MF' overrides the default dependency output file.
-MG
#include directive without prepending any path. `-MG'
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
-MP
make gives if you remove header
files without updating the `Makefile' to match.
This is typical output:
test.o: test.c test.h test.h: |
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as `.c', and appends the platform's usual object suffix. The result is the target.
An `-MT' option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to `-MT', or use multiple `-MT' options.
For example, `-MT '$(objpfx)foo.o'' might give
$(objpfx)foo.o: foo.c |
-MQ target
Same as `-MT', but it quotes any characters which are special to Make. `-MQ '$(objpfx)foo.o'' gives
$$(objpfx)foo.o: foo.c |
The default target is automatically quoted, as if it were given with `-MQ'.
-MD
If `-MD' is used in conjunction with `-E', any `-o' switch is understood to specify the dependency output file (see section -MF), but if used without `-E', each `-o' is understood to specify a target object file.
Since `-E' is not implied, `-MD' can be used to generate a dependency output file as a side-effect of the compilation process.
-MMD
-fpch-deps
-fpch-preprocess
#pragma,
#pragma GCC pch_preprocess "filename" in the output to mark
the place where the precompiled header was found, and its filename.
When `-fpreprocessed' is in use, GCC recognizes this #pragma
and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by `-save-temps'.
You should not write this #pragma in your own code, but it is
safe to edit the filename if the PCH file is available in a different
location. The filename may be absolute or it may be relative to GCC's
current directory.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Note: Previous versions of cpp accepted a `-lang' option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the `-l' option.
-std=standard
-ansi
standard may be one of:
c90
c89
iso9899:1990
The `-ansi' option is equivalent to `-std=c90'.
iso9899:199409
iso9899:1999
c99
iso9899:199x
c9x
iso9899:2011
c11
c1x
gnu90
gnu89
gnu99
gnu9x
gnu11
gnu1x
c++98
gnu++98
-I-
#include "file"; they are not searched for
#include <file>. If additional directories are
specified with `-I' options after the `-I-', those
directories are searched for all `#include' directives.
In addition, `-I-' inhibits the use of the directory of the current
file directory as the first search directory for #include
"file".
This option has been deprecated.
-nostdinc
-nostdinc++
-include file
#include "file" appeared as the first
line of the primary source file. However, the first directory searched
for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it
is searched for in the remainder of the #include "..." search
chain as normal.
If multiple `-include' options are given, the files are included in the order they appear on the command line.
-imacros file
All files specified by `-imacros' are processed before all files specified by `-include'.
-idirafter dir
=, then the = will be replaced
by the sysroot prefix; see `--sysroot' and `-isysroot'.
-iprefix prefix
-iwithprefix dir
-iwithprefixbefore dir
-isysroot dir
-imultilib dir
-isystem dir
=, then the = will be replaced
by the sysroot prefix; see `--sysroot' and `-isysroot'.
-iquote dir
#include "file"; they are not searched for
#include <file>, before all directories specified by
`-I' and before the standard system directories.
If dir begins with =, then the = will be replaced
by the sysroot prefix; see `--sysroot' and `-isysroot'.
-fdirectives-only
The option's behavior depends on the `-E' and `-fpreprocessed' options.
With `-E', preprocessing is limited to the handling of directives
such as #define, #ifdef, and #error. Other
preprocessor operations, such as macro expansion and trigraph
conversion are not performed. In addition, the `-dD' option is
implicitly enabled.
With `-fpreprocessed', predefinition of command line and most
builtin macros is disabled. Macros such as __LINE__, which are
contextually dependent, are handled normally. This enables compilation of
files previously preprocessed with -E -fdirectives-only.
With both `-E' and `-fpreprocessed', the rules for
`-fpreprocessed' take precedence. This enables full preprocessing of
files previously preprocessed with -E -fdirectives-only.
-fdollars-in-identifiers
-fextended-identifiers
-fpreprocessed
`-fpreprocessed' is implicit if the input file has one of the extensions `.i', `.ii' or `.mi'. These are the extensions that GCC uses for preprocessed files created by `-save-temps'.
-ftabstop=width
-fdebug-cpp
{`P':`/file/path';`F':`/includer/path';`L':line_num;`C':col_num;`S':system_header_p;`M':map_address;`E':macro_expansion_p,`loc':location}
|
When used without `-E', this option has no effect.
-ftrack-macro-expansion[=level]
-fexec-charset=charset
iconv library routine.
-fwide-exec-charset=charset
wchar_t. As with
`-fexec-charset', charset can be any encoding supported
by the system's iconv library routine; however, you will have
problems with encodings that do not fit exactly in wchar_t.
-finput-charset=charset
iconv library routine.
-fworking-directory
#line directives are emitted whatsoever.
-fno-show-column
dejagnu.
-A predicate=answer
-A -predicate=answer
-dCHARS
touch foo.h; cpp -dM foo.h |
will show all the predefined macros.
If you use `-dM' without the `-E' option, `-dM' is interpreted as a synonym for `-fdump-rtl-mach'. See section 3.9 Options for Debugging Your Program or GCC.
-P
-C
You should be prepared for side effects when using `-C'; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a `#'.
-CC
In addition to the side-effects of the `-C' option, the `-CC' option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The `-CC' option is generally used to support lint comments.
-traditional-cpp
-trigraphs
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
|
-remap
--help
--target-help
-v
-H
-version
--version
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You can pass options to the assembler.
-Wa,option
-Xassembler option
If you want to pass an option that takes an argument, you must use `-Xassembler' twice, once for the option and once for the argument.
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These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
object-file-name
-c
-S
-E
-llibrary
-l library
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, `foo.o -lz bar.o' searches library `z' after file `foo.o' but before `bar.o'. If `bar.o' refers to functions in `z', those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with `-L'.
Normally the files found this way are library files--archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an `-l' option and specifying a file name is that `-l' surrounds library with `lib' and `.a' and searches several directories.
-lobjc
-nostartfiles
-nodefaultlibs
-static-libgcc
or -shared-libgcc, will be ignored.
The standard startup files are used normally, unless `-nostartfiles'
is used. The compiler may generate calls to memcmp,
memset, memcpy and memmove.
These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
-nostdlib
-static-libgcc or -shared-libgcc, will be ignored.
The compiler may generate calls to memcmp, memset,
memcpy and memmove.
These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
One of the standard libraries bypassed by `-nostdlib' and
`-nodefaultlibs' is `libgcc.a', a library of internal subroutines
which GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
(See section `Interfacing to GCC Output' in GNU Compiler Collection (GCC) Internals,
for more discussion of `libgcc.a'.)
In most cases, you need `libgcc.a' even when you want to avoid
other standard libraries. In other words, when you specify `-nostdlib'
or `-nodefaultlibs' you should usually specify `-lgcc' as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example, `__main', used to ensure C++
constructors will be called; see section `collect2' in GNU Compiler Collection (GCC) Internals.)
-pie
-rdynamic
dlopen or to allow obtaining backtraces
from within a program.
-s
-static
-shared
-shared-libgcc
-static-libgcc
There are several situations in which an application should use the shared `libgcc' instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared `libgcc'.
Therefore, the G++ and GCJ drivers automatically add `-shared-libgcc' whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared `libgcc'. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option `--eh-frame-hdr', it will link the shared version of `libgcc' into shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of `libgcc', linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option `-shared-libgcc', such that it is linked with the shared `libgcc'.
-static-libstdc++
g++ program is used to link a C++ program, it will
normally automatically link against `libstdc++'. If
`libstdc++' is available as a shared library, and the
`-static' option is not used, then this will link against the
shared version of `libstdc++'. That is normally fine. However, it
is sometimes useful to freeze the version of `libstdc++' used by
the program without going all the way to a fully static link. The
`-static-libstdc++' option directs the g++ driver to
link `libstdc++' statically, without necessarily linking other
libraries statically.
-symbolic
-T script
-Xlinker option
If you want to pass an option that takes a separate argument, you must use `-Xlinker' twice, once for the option and once for the argument. For example, to pass `-assert definitions', you must write `-Xlinker -assert -Xlinker definitions'. It does not work to write `-Xlinker "-assert definitions"', because this passes the entire string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the `option=value' syntax than as separate arguments. For example, you can specify `-Xlinker -Map=output.map' rather than `-Xlinker -Map -Xlinker output.map'. Other linkers may not support this syntax for command-line options.
-Wl,option
-u symbol
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These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
If a standard system include directory, or a directory specified with `-isystem', is also specified with `-I', the `-I' option will be ignored. The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the `-nostdinc' and/or `-isystem' options.
-iplugindir=dir
-iquotedir
-Ldir
-Bprefix
The compiler driver program runs one or more of the subprograms `cpp', `cc1', `as' and `ld'. It tries prefix as a prefix for each program it tries to run, both with and without `machine/version/' (see section 3.16 Specifying Target Machine and Compiler Version).
For each subprogram to be run, the compiler driver first tries the
`-B' prefix, if any. If that name is not found, or if `-B'
was not specified, the driver tries two standard prefixes,
`/usr/lib/gcc/' and `/usr/local/lib/gcc/'. If neither of
those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your
PATH environment variable.
The compiler will check to see if the path provided by the `-B' refers to a directory, and if necessary it will add a directory separator character at the end of the path.
`-B' prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L' options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into `-isystem' options for the preprocessor. In this case, the compiler appends `include' to the prefix.
The runtime support file `libgcc.a' can also be searched for using the `-B' prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.
Another way to specify a prefix much like the `-B' prefix is to use
the environment variable GCC_EXEC_PREFIX. See section 3.19 Environment Variables Affecting GCC.
As a special kludge, if the path provided by `-B' is `[dir/]stageN/', where N is a number in the range 0 to 9, then it will be replaced by `[dir/]include'. This is to help with boot-strapping the compiler.
-specs=file
--sysroot=dir
If you use both this option and the `-isysroot' option, then the `--sysroot' option will apply to libraries, but the `-isysroot' option will apply to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of `--sysroot' will still work, but the library aspect will not.
-I-
If additional directories are specified with `-I' options after the `-I-', these directories are searched for all `#include' directives. (Ordinarily all `-I' directories are used this way.)
In addition, the `-I-' option inhibits the use of the current directory (where the current input file came from) as the first search directory for `#include "file"'. There is no way to override this effect of `-I-'. With `-I.' you can specify searching the directory that was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.
`-I-' does not inhibit the use of the standard system directories for header files. Thus, `-I-' and `-nostdinc' are independent.
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gcc is a driver program. It performs its job by invoking a
sequence of other programs to do the work of compiling, assembling and
linking. GCC interprets its command-line parameters and uses these to
deduce which programs it should invoke, and which command-line options
it ought to place on their command lines. This behavior is controlled
by spec strings. In most cases there is one spec string for each
program that GCC can invoke, but a few programs have multiple spec
strings to control their behavior. The spec strings built into GCC can
be overridden by using the `-specs=' command-line switch to specify
a spec file.
Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line, which can be one of the following:
%command
%include <file>
%include_noerr <file>
%rename old_name new_name
*[spec_name]:
[suffix]:
.ZZ: z-compile -input %i |
This says that any input file whose name ends in `.ZZ' should be passed to the program `z-compile', which should be invoked with the command-line switch `-input' and with the result of performing the `%i' substitution. (See below.)
As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:
@language
.ZZ: @c++ |
Says that .ZZ files are, in fact, C++ source files.
#name
name compiler not installed on this system. |
GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.
GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.
asm Options to pass to the assembler
asm_final Options to pass to the assembler post-processor
cpp Options to pass to the C preprocessor
cc1 Options to pass to the C compiler
cc1plus Options to pass to the C++ compiler
endfile Object files to include at the end of the link
link Options to pass to the linker
lib Libraries to include on the command line to the linker
libgcc Decides which GCC support library to pass to the linker
linker Sets the name of the linker
predefines Defines to be passed to the C preprocessor
signed_char Defines to pass to CPP to say whether |
Here is a small example of a spec file:
%rename lib old_lib *lib: --start-group -lgcc -lc -leval1 --end-group %(old_lib) |
This example renames the spec called `lib' to `old_lib' and then overrides the previous definition of `lib' with a new one. The new definition adds in some extra command-line options before including the text of the old definition.
Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain `%'-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.
Here is a table of all defined `%'-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.
%%
%i
%b
%B
%d
%gsuffix
%usuffix
%Usuffix
%jsuffix
HOST_BIT_BUCKET, if any, and if it is
writable, and if save-temps is off; otherwise, substitute the name
of a temporary file, just like `%u'. This temporary file is not
meant for communication between processes, but rather as a junk
disposal mechanism.
%|suffix
%msuffix
X}'
construct: see for example `f/lang-specs.h'.
%.SUFFIX
%w
%o
%O
%p
cpp.
%P
%I
GCC_EXEC_PREFIX),
`-isysroot' (made from TARGET_SYSTEM_ROOT),
`-isystem' (made from COMPILER_PATH and `-B' options)
and `-imultilib' as necessary.
%s
%T
%estr
%(name)
%x{option}
%X
%Y
%Z
%a
asm spec. This is used to compute the
switches to be passed to the assembler.
%A
asm_final spec. This is a spec string for
passing switches to an assembler post-processor, if such a program is
needed.
%l
link spec. This is the spec for computing the
command line passed to the linker. Typically it will make use of the
`%L %G %S %D and %E' sequences.
%D
%L
lib spec. This is a spec string for deciding which
libraries should be included on the command line to the linker.
%G
libgcc spec. This is a spec string for deciding
which GCC support library should be included on the command line to the linker.
%S
startfile spec. This is a spec for deciding which
object files should be the first ones passed to the linker. Typically
this might be a file named `crt0.o'.
%E
endfile spec. This is a spec string that specifies
the last object files that will be passed to the linker.
%C
cpp spec. This is used to construct the arguments
to be passed to the C preprocessor.
%1
cc1 spec. This is used to construct the options to be
passed to the actual C compiler (`cc1').
%2
cc1plus spec. This is used to construct the options to be
passed to the actual C++ compiler (`cc1plus').
%*
%<S
-S from the command line. Note--this
command is position dependent. `%' commands in the spec string
before this one will see -S, `%' commands in the spec string
after this one will not.
%:function(args)
The following built-in spec functions are provided:
getenvgetenv spec function takes two arguments: an environment
variable name and a string. If the environment variable is not
defined, a fatal error is issued. Otherwise, the return value is the
value of the environment variable concatenated with the string. For
example, if TOPDIR is defined as `/path/to/top', then:
%:getenv(TOPDIR /include) |
expands to `/path/to/top/include'.
if-existsif-exists spec function takes one argument, an absolute
pathname to a file. If the file exists, if-exists returns the
pathname. Here is a small example of its usage:
*startfile: crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s |
if-exists-elseif-exists-else spec function is similar to the if-exists
spec function, except that it takes two arguments. The first argument is
an absolute pathname to a file. If the file exists, if-exists-else
returns the pathname. If it does not exist, it returns the second argument.
This way, if-exists-else can be used to select one file or another,
based on the existence of the first. Here is a small example of its usage:
*startfile: crt0%O%s %:if-exists(crti%O%s) \ %:if-exists-else(crtbeginT%O%s crtbegin%O%s) |
replace-outfilereplace-outfile spec function takes two arguments. It looks for the
first argument in the outfiles array and replaces it with the second argument. Here
is a small example of its usage:
%{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)}
|
remove-outfileremove-outfile spec function takes one argument. It looks for the
first argument in the outfiles array and removes it. Here is a small example
its usage:
%:remove-outfile(-lm) |
pass-through-libspass-through-libs spec function takes any number of arguments. It
finds any `-l' options and any non-options ending in ".a" (which it
assumes are the names of linker input library archive files) and returns a
result containing all the found arguments each prepended by
`-plugin-opt=-pass-through=' and joined by spaces. This list is
intended to be passed to the LTO linker plugin.
%:pass-through-libs(%G %L %G) |
print-asm-headerprint-asm-header function takes no arguments and simply
prints a banner like:
Assembler options ================= Use "-Wa,OPTION" to pass "OPTION" to the assembler. |
It is used to separate compiler options from assembler options in the `--target-help' output.
%{S}
-S switch, if that switch was given to GCC.
If that switch was not specified, this substitutes nothing. Note that
the leading dash is omitted when specifying this option, and it is
automatically inserted if the substitution is performed. Thus the spec
string `%{foo}' would match the command-line option `-foo'
and would output the command-line option `-foo'.
%W{S}
S} but mark last argument supplied within as a file to be
deleted on failure.
%{S*}
-S, but which also take an argument. This is used for
switches like `-o', `-D', `-I', etc.
GCC considers `-o foo' as being
one switch whose names starts with `o'. %{o*} would substitute this
text, including the space. Thus two arguments would be generated.
%{S*&T*}
S*}, but preserve order of S and T options
(the order of S and T in the spec is not significant).
There can be any number of ampersand-separated variables; for each the
wild card is optional. Useful for CPP as `%{D*&U*&A*}'.
%{S:X}
X, if the `-S' switch was given to GCC.
%{!S:X}
X, if the `-S' switch was not given to GCC.
%{S*:X}
X if one or more switches whose names start with
-S are specified to GCC. Normally X is substituted only
once, no matter how many such switches appeared. However, if %*
appears somewhere in X, then X will be substituted once
for each matching switch, with the %* replaced by the part of
that switch that matched the *.
%{.S:X}
X, if processing a file with suffix S.
%{!.S:X}
X, if not processing a file with suffix S.
%{,S:X}
X, if processing a file for language S.
%{!,S:X}
X, if not processing a file for language S.
%{S|P:X}
X if either -S or -P was given to
GCC. This may be combined with `!', `.', `,', and
* sequences as well, although they have a stronger binding than
the `|'. If %* appears in X, all of the
alternatives must be starred, and only the first matching alternative
is substituted.
For example, a spec string like this:
%{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}
|
will output the following command-line options from the following input command-line options:
fred.c -foo -baz jim.d -bar -boggle -d fred.c -foo -baz -boggle -d jim.d -bar -baz -boggle |
%{S:X; T:Y; :D}
If S was given to GCC, substitutes X; else if T was
given to GCC, substitutes Y; else substitutes D. There can
be as many clauses as you need. This may be combined with .,
,, !, |, and * as needed.
The conditional text X in a %{S:X} or similar
construct may contain other nested `%' constructs or spaces, or
even newlines. They are processed as usual, as described above.
Trailing white space in X is ignored. White space may also
appear anywhere on the left side of the colon in these constructs,
except between . or * and the corresponding word.
The `-O', `-f', `-m', and `-W' switches are
handled specifically in these constructs. If another value of
`-O' or the negated form of a `-f', `-m', or
`-W' switch is found later in the command line, the earlier
switch value is ignored, except with {S*} where S is
just one letter, which passes all matching options.
The character `|' at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if `-pipe' is specified.
It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).
GCC also knows implicitly that arguments starting in `-l' are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.
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The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or
machine-gcc-version to run a version other than the
one that was installed last.
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Each target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations--for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.
3.17.1 H8/300 Options 3.17.2 M32C Options 3.17.3 RL78 Options 3.17.4 RX Options 3.17.5 SH Options
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These `-m' options are defined for the H8/300 implementations:
-mrelax
ld and the H8/300' in Using ld, for a fuller description.
-mh
-ms
-mn
-ms2600
-mexr
-mno-exr
-mint32
int data 32 bits by default.
-malign-300
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-mcpu=name
-msim
-memregs=number
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-msim
-mmul=none
-mmul=g13
-mmul=rl78
none, which uses software multiplication functions.
The g13 option is for the hardware multiply/divide peripheral
only on the RL78/G13 targets. The rl78 option is for the
standard hardware multiplication defined in the RL78 software manual.
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These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
double data type be 64 bits (`-m64bit-doubles')
or 32 bits (`-m32bit-doubles') in size. The default is
`-m32bit-doubles'. Note RX floating-point hardware only
works on 32-bit values, which is why the default is
`-m32bit-doubles'.
-fpu
-nofpu
Floating-point instructions will only be generated for 32-bit floating-point values however, so if the `-m64bit-doubles' option is in use then the FPU hardware will not be used for doubles.
Note If the `-fpu' option is enabled then `-funsafe-math-optimizations' is also enabled automatically. This is because the RX FPU instructions are themselves unsafe.
-mcpu=name
The only difference between RX600 and RX610 is that the
RX610 does not support the MVTIPL instruction.
The RX200 and RX100 series do not have a hardware floating-point unit and the `-nofpu' is enabled by default when this type is selected.
-mbig-endian-data
-mlittle-endian-data
-msmall-data-limit=N
r13) is reserved for use pointing to this
area, so it is no longer available for use by the compiler. This
could result in slower and/or larger code if variables which once
could have been held in the reserved register are now pushed onto the
stack.
Note, common variables (variables that have not been initialized) and constants are not placed into the small data area as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (`-O2' etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the `-mpid' option for a description of how the actual register to hold the small data area pointer is chosen.
-msim
-mno-sim
-mas100-syntax
-mno-as100-syntax
-mmax-constant-size=N
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed.
-mrelax
-mint-register=N
r13 will be reserved for the exclusive use
of fast interrupt handlers. A value of 2 reserves r13 and
r12. A value of 3 reserves r13, r12 and
r11, and a value of 4 reserves r13 through r10.
A value of 0, the default, does not reserve any registers.
-msave-acc-in-interrupts
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
-mpid
-mno-pid
Note, using this feature reserves a register, usually r13, for
the constant data base address. This can result in slower and/or
larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address
depends upon whether the `-msmall-data-limit' and/or the
`-mint-register' command-line options are enabled. Starting
with register r13 and proceeding downwards, registers are
allocated first to satisfy the requirements of `-mint-register',
then `-mpid' and finally `-msmall-data-limit'. Thus it
is possible for the small data area register to be r8 if both
`-mint-register=4' and `-mpid' are specified on the
command line.
By default this feature is not enabled. The default can be restored via the `-mno-pid' command-line option.
Note: The generic GCC command-line option `-ffixed-reg'
has special significance to the RX port when used with the
interrupt function attribute. This attribute indicates a
function intended to process fast interrupts. GCC will will ensure
that it only uses the registers r10, r11, r12
and/or r13 and only provided that the normal use of the
corresponding registers have been restricted via the
`-ffixed-reg' or `-mint-register' command-line
options.
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These `-m' options are defined for the SH implementations:
-m1
-m2
-m2e
-m2a-nofpu
-m2a-single-only
-m2a-single
-m2a
-m3
-m3e
-m4-nofpu
-m4-single-only
-m4-single
-m4
-m4a-nofpu
-m4a-single-only
-m4a-single
-m4a
-m4al
-mb
-ml
-mdalign
-mrelax
-mbigtable
switch tables. The default is to use
16-bit offsets.
-mbitops
-mfmovd
fmovd. Check `-mdalign' for
alignment constraints.
-mhitachi
-mrenesas
-mno-renesas
-mnomacsave
MAC register as call-clobbered, even if
`-mhitachi' is given.
-mieee
-minline-ic_invalidate
-misize
-mpadstruct
-msoft-atomic
sh-*-linux*.
For details on the atomic built-in functions see 6.52 Built-in functions for memory model aware atomic operations.
-mspace
-mprefergot
-musermode
sh-*-linux*.
-multcost=number
-mdiv=strategy
-maccumulate-outgoing-args
-mdivsi3_libfunc=name
-mfixed-range=register-range
-madjust-unroll
-mindexed-addressing
-mgettrcost=number
-mpt-fixed
-minvalid-symbols
-mbranch-cost=num
-mcbranchdi
cbranchdi4 instruction pattern.
-mcmpeqdi
cmpeqdi_t instruction pattern even when `-mcbranchdi'
is in effect.
-mfused-madd
fmac instruction (floating-point
multiply-accumulate) if the processor type supports it. Enabling this
option might generate code that produces different numeric floating-point
results compared to strict IEEE 754 arithmetic.
-mpretend-cmove
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These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one that is not the default. You can figure out the other form by either removing `no-' or adding it.
-fbounds-check
-ftrapv
-fwrapv
-fexceptions
-fnon-call-exceptions
SIGALRM.
-funwind-tables
-fasynchronous-unwind-tables
-fpcc-struct-return
struct and union values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the `-fpcc-struct-return' switch is not binary compatible with code compiled with the `-freg-struct-return' switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
struct and union values in registers when possible.
This is more efficient for small structures than
`-fpcc-struct-return'.
If you specify neither `-fpcc-struct-return' nor `-freg-struct-return', GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to `-fpcc-struct-return', except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the `-freg-struct-return' switch is not binary compatible with code compiled with the `-fpcc-struct-return' switch. Use it to conform to a non-default application binary interface.
-fshort-enums
enum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type
will be equivalent to the smallest integer type that has enough room.
Warning: the `-fshort-enums' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-double
double as for float.
Warning: the `-fshort-double' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Warning: the `-fshort-wchar' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fno-common
extern) in two different compilations,
you will get a multiple-definition error when you link them.
In this case, you must compile with `-fcommon' instead.
Compiling with `-fno-common' is useful on targets for which
it provides better performance, or if you wish to verify that the
program will work on other systems that always treat uninitialized
variable declarations this way.
-fno-ident
-finhibit-size-directive
.size assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling `crtstuff.c'; you should not need to use it
for anything else.
-fverbose-asm
`-fno-verbose-asm', the default, causes the extra information to be omitted and is useful when comparing two assembler files.
-frecord-gcc-switches
-fpic
Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros __pic__ and __PIC__
are defined to 1.
-fPIC
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros __pic__ and __PIC__
are defined to 2.
-fpie
-fPIE
`-fpie' and `-fPIE' both define the macros
__pie__ and __PIE__. The macros have the value 1
for `-fpie' and 2 for `-fPIE'.
-fno-jump-tables
-ffixed-reg
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the REGISTER_NAMES
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Warning: the `-fpack-struct' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
-finstrument-functions
__builtin_return_address does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
|
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use `extern inline' in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute no_instrument_function, in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see
the description of -finstrument-functions). If the file that
contains a function definition matches with one of file, then
that function is not instrumented. The match is done on substrings:
if the file parameter is a substring of the file name, it is
considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys |
will exclude any inline function defined in files whose pathnames
contain /bits/stl or include/sys.
If, for some reason, you want to include letter ',' in one of
sym, write '\,'. For example,
-finstrument-functions-exclude-file-list='\,\,tmp'
(note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list,
but this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-visible
name, such as vector<int> blah(const vector<int> &), not the
internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The
match is done on substrings: if the sym parameter is a substring
of the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
-fstack-check
Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking,
specific means use the best checking method and is equivalent
to bare `-fstack-check'.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:
Note that old-style stack checking is also the fallback method for
specific if no target support has been added in the compiler.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
For instance, if the stack starts at absolute address `0x80000000' and grows downwards, you can use the flags `-fstack-limit-symbol=__stack_limit' and `-Wl,--defsym,__stack_limit=0x7ffe0000' to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.
-fsplit-stack
When code compiled with `-fsplit-stack' calls code compiled without `-fsplit-stack', there may not be much stack space available for the latter code to run. If compiling all code, including library code, with `-fsplit-stack' is not an option, then the linker can fix up these calls so that the code compiled without `-fsplit-stack' always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.
-fleading-underscore
Warning: the `-fleading-underscore' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.
-ftls-model=model
global-dynamic,
local-dynamic, initial-exec or local-exec.
The default without `-fpic' is initial-exec; with
`-fpic' the default is global-dynamic.
-fvisibility=default|internal|hidden|protected
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared object.
protected and internal are pretty useless in real-world
usage so the only other commonly used option will be hidden.
The default if `-fvisibility' isn't specified is
default, i.e., make every
symbol public--this causes the same behavior as previous versions of
GCC.
A good explanation of the benefits offered by ensuring ELF
symbols have the correct visibility is given by "How To Write
Shared Libraries" by Ulrich Drepper (which can be found at
http://people.redhat.com/~drepper/)---however a superior
solution made possible by this option to marking things hidden when
the default is public is to make the default hidden and mark things
public. This is the norm with DLL's on Windows and with `-fvisibility=hidden'
and __attribute__ ((visibility("default"))) instead of
__declspec(dllexport) you get almost identical semantics with
identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find `#pragma GCC visibility' of use. This works by you enclosing the declarations you wish to set visibility for with (for example) `#pragma GCC visibility push(hidden)' and `#pragma GCC visibility pop'. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads--making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say `#pragma GCC visibility push(default)' before including any such headers.
`extern' declarations are not affected by `-fvisibility', so a lot of code can be recompiled with `-fvisibility=hidden' with no modifications. However, this means that calls to `extern' functions with no explicit visibility will use the PLT, so it is more effective to use `__attribute ((visibility))' and/or `#pragma GCC visibility' to tell the compiler which `extern' declarations should be treated as hidden.
Note that `-fvisibility' does affect C++ vague linkage entities. This means that, for instance, an exception class that will be thrown between DSOs must be explicitly marked with default visibility so that the `type_info' nodes will be unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at http://gcc.gnu.org/@/wiki/@/Visibility.
-fstrict-volatile-bitfields
If this option is disabled, the compiler will use the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that will access bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.
If the target requires strict alignment, and honoring the field
type would require violating this alignment, a warning is issued.
If the field has packed attribute, the access is done without
honoring the field type. If the field doesn't have packed
attribute, the access is done honoring the field type. In both cases,
GCC assumes that the user knows something about the target hardware
that it is unaware of.
The default value of this option is determined by the application binary interface for the target processor.
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This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see section 3.14 Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See section `Controlling the Compilation Driver `gcc'' in GNU Compiler Collection (GCC) Internals.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
LC_CTYPE and LC_MESSAGES if it has been configured to do
so. These locale categories can be set to any value supported by your
installation. A typical value is `en_GB.UTF-8' for English in the United
Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries in
a string; this is needed for some multibyte encodings that contain quote
and escape characters that would otherwise be interpreted as a string
end or escape.
The LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
TMPDIR
TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.
GCC_COMPARE_DEBUG
GCC_COMPARE_DEBUG is nearly equivalent to passing
`-fcompare-debug' to the compiler driver. See the documentation
of this option for more details.
GCC_EXEC_PREFIX
GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out
an appropriate prefix to use based on the pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
`prefix/lib/gcc/' where prefix is the prefix to
the installed compiler. In many cases prefix is the value
of prefix when you ran the `configure' script.
Other prefixes specified with `-B' take precedence over this prefix.
This prefix is also used for finding files such as `crt0.o' that are used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with `/usr/local/lib/gcc'
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with `-Bfoo/', GCC will search
`foo/bar' where it would normally search `/usr/local/lib/bar'.
These alternate directories are searched first; the standard directories
come next. If a standard directory begins with the configured
prefix then the value of prefix is replaced by
GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native compiler,
GCC tries the directories thus specified when searching for special
linker files, if it can't find them using GCC_EXEC_PREFIX. Linking
using GCC also uses these directories when searching for ordinary
libraries for the `-l' option (but directories specified with
`-L' come first).
LANG
LANG are recognized:
If LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale to
recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
PATH, in which to look for header files.
The special character, PATH_SEPARATOR, is target-dependent and
determined at GCC build time. For Microsoft Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with `-I', but after any paths given with `-I'
options on the command line. This environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with `-isystem', but after any paths given with `-isystem' options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at the
beginning or end of a path. For instance, if the value of
CPATH is :/special/include, that has the same
effect as `-I. -I/special/include'.
DEPENDENCIES_OUTPUT
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
`file target', in which case the rules are written to
file file using target as the target name.
In other words, this environment variable is equivalent to combining the options `-MM' and `-MF' (see section 3.11 Options Controlling the Preprocessor), with an optional `-MT' switch too.
SUNPRO_DEPENDENCIES
DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies
`-M' rather than `-MM'. However, the dependence on the
main input file is omitted.
See section 3.11 Options Controlling the Preprocessor.
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Often large projects have many header files that are included in every source file. The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project. To make builds faster, GCC allows users to `precompile' a header file; then, if builds can use the precompiled header file they will be much faster.
To create a precompiled header file, simply compile it as you would any
other file, if necessary using the `-x' option to make the driver
treat it as a C or C++ header file. You will probably want to use a
tool like make to keep the precompiled header up-to-date when
the headers it contains change.
A precompiled header file will be searched for when #include is
seen in the compilation. As it searches for the included file
(see section `Search Path' in The C Preprocessor) the
compiler looks for a precompiled header in each directory just before it
looks for the include file in that directory. The name searched for is
the name specified in the #include with `.gch' appended. If
the precompiled header file can't be used, it is ignored.
For instance, if you have #include "all.h", and you have
`all.h.gch' in the same directory as `all.h', then the
precompiled header file will be used if possible, and the original
header will be used otherwise.
Alternatively, you might decide to put the precompiled header file in a
directory and use `-I' to ensure that directory is searched
before (or instead of) the directory containing the original header.
Then, if you want to check that the precompiled header file is always
used, you can put a file of the same name as the original header in this
directory containing an #error command.
This also works with `-include'. So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and `-include' the precompiled header. If the header files have guards against multiple inclusion, they will be skipped because they've already been included (in the precompiled header).
If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like `all.h.gch', and put each precompiled header in the directory, perhaps using `-o'. It doesn't matter what you call the files in the directory, every precompiled header in the directory will be considered. The first precompiled header encountered in the directory that is valid for this compilation will be used; they're searched in no particular order.
There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.
A precompiled header file can be used only when these conditions apply:
#include.
The `-D' option is one way to define a macro before a
precompiled header is included; using a #define can also do it.
There are also some options that define macros implicitly, like
`-O' and `-Wdeprecated'; the same rule applies to macros
defined this way.
-fexceptions |
{-fmessage-length= -fpreprocessed -fsched-interblock
|
For all of these except the last, the compiler will automatically ignore the precompiled header if the conditions aren't met. If you find an option combination that doesn't work and doesn't cause the precompiled header to be ignored, please consider filing a bug report, see 12. Reporting Bugs.
If you do use differing options when generating and using the precompiled header, the actual behavior will be a mixture of the behavior for the options. For instance, if you use `-g' to generate the precompiled header but not when using it, you may or may not get debugging information for routines in the precompiled header.
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A conforming implementation of ISO C is required to document its choice of behavior in each of the areas that are designated "implementation defined". The following lists all such areas, along with the section numbers from the ISO/IEC 9899:1990 and ISO/IEC 9899:1999 standards. Some areas are only implementation-defined in one version of the standard.
Some choices depend on the externally determined ABI for the platform (including standard character encodings) which GCC follows; these are listed as "determined by ABI" below. See section Binary Compatibility, and http://gcc.gnu.org/readings.html. Some choices are documented in the preprocessor manual. See section `Implementation-defined behavior' in The C Preprocessor. Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details.
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Diagnostics consist of all the output sent to stderr by GCC.
See section `Implementation-defined behavior' in The C Preprocessor.
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The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
See section `Implementation-defined behavior' in The C Preprocessor.
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See section `Implementation-defined behavior' in The C Preprocessor.
For internal names, all characters are significant. For external names, the number of significant characters are defined by the linker; for almost all targets, all characters are significant.
This is a property of the linker. C99 requires that case distinctions are always significant in identifiers with external linkage and systems without this property are not supported by GCC.
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Determined by ABI.
Determined by ABI.
Determined by ABI.
char object into which has been stored any
character other than a member of the basic execution character set
(C90 6.1.2.5, C99 6.2.5).
Determined by ABI.
signed char or unsigned char has the same
range, representation, and behavior as "plain" char (C90
6.1.2.5, C90 6.2.1.1, C99 6.2.5, C99 6.3.1.1).
Determined by ABI. The options `-funsigned-char' and `-fsigned-char' change the default. See section Options Controlling C Dialect.
Determined by ABI.
See section `Implementation-defined behavior' in The C Preprocessor.
See section `Implementation-defined behavior' in The C Preprocessor.
See section `Implementation-defined behavior' in The C Preprocessor.
See section `Implementation-defined behavior' in The C Preprocessor.
See section `Implementation-defined behavior' in The C Preprocessor.
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GCC does not support any extended integer types.
GCC supports only two's complement integer types, and all bit patterns are ordinary values.
GCC does not support any extended integer types.
For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type; no signal is raised.
Bitwise operators act on the representation of the value including both the sign and value bits, where the sign bit is considered immediately above the highest-value value bit. Signed `>>' acts on negative numbers by sign extension.
GCC does not use the latitude given in C99 only to treat certain aspects of signed `<<' as undefined, but this is subject to change.
GCC always follows the C99 requirement that the result of division is truncated towards zero.
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<math.h> and <complex.h> that return floating-point
results (C90 and C99 5.2.4.2.2).
The accuracy is unknown.
FLT_ROUNDS
(C90 and C99 5.2.4.2.2).
GCC does not use such values.
FLT_EVAL_METHOD (C99 5.2.4.2.2).
GCC does not use such values.
C99 Annex F is followed.
C99 Annex F is followed.
C99 Annex F is followed.
FP_CONTRACT pragma (C99 6.5).
Expressions are currently only contracted if `-funsafe-math-optimizations' or `-ffast-math' are used. This is subject to change.
FENV_ACCESS pragma (C99 7.6.1).
This pragma is not implemented, but the default is to "off" unless `-frounding-math' is used in which case it is "on".
This is dependent on the implementation of the C library, and is not defined by GCC itself.
FP_CONTRACT pragma (C99 7.12.2).
This pragma is not implemented. Expressions are currently only contracted if `-funsafe-math-optimizations' or `-ffast-math' are used. This is subject to change.
This is dependent on the implementation of the C library, and is not defined by GCC itself.
This is dependent on the implementation of the C library, and is not defined by GCC itself.
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A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends(2) if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.
A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.
When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in C99 6.5.6/8.
The value is as specified in the standard and the type is determined by the ABI.
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register
storage-class specifier are effective (C90 6.5.1, C99 6.7.1).
The register specifier affects code generation only in these ways:
register
storage-class specifier; if register is specified, the variable
may have a shorter lifespan than the code would indicate and may never
be placed in memory.
setjmp doesn't save the registers in
all circumstances. In those cases, GCC doesn't allocate any variables
in registers unless they are marked register.
GCC will not inline any functions if the `-fno-inline' option is used or if `-O0' is used. Otherwise, GCC may still be unable to inline a function for many reasons; the `-Winline' option may be used to determine if a function has not been inlined and why not.
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The relevant bytes of the representation of the object are treated as an object of the type used for the access. See Type-punning. This may be a trap representation.
int bit-field is treated as a
signed int bit-field or as an unsigned int bit-field
(C90 6.5.2, C90 6.5.2.1, C99 6.7.2, C99 6.7.2.1).
By default it is treated as signed int but this may be changed
by the `-funsigned-bitfields' option.
_Bool, signed int,
and unsigned int (C99 6.7.2.1).
No other types are permitted in strictly conforming mode.
Determined by ABI.
Determined by ABI.
Determined by ABI.
Normally, the type is unsigned int if there are no negative
values in the enumeration, otherwise int. If
`-fshort-enums' is specified, then if there are negative values
it is the first of signed char, short and int
that can represent all the values, otherwise it is the first of
unsigned char, unsigned short and unsigned int
that can represent all the values.
On some targets, `-fshort-enums' is the default; this is determined by the ABI.
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Such an object is normally accessed by pointers and used for accessing hardware. In most expressions, it is intuitively obvious what is a read and what is a write. For example
volatile int *dst = somevalue; volatile int *src = someothervalue; *dst = *src; |
will cause a read of the volatile object pointed to by src and store the
value into the volatile object pointed to by dst. There is no
guarantee that these reads and writes are atomic, especially for objects
larger than int.
However, if the volatile storage is not being modified, and the value of the volatile storage is not used, then the situation is less obvious. For example
volatile int *src = somevalue; *src; |
According to the C standard, such an expression is an rvalue whose type
is the unqualified version of its original type, i.e. int. Whether
GCC interprets this as a read of the volatile object being pointed to or
only as a request to evaluate the expression for its side-effects depends
on this type.
If it is a scalar type, or on most targets an aggregate type whose only member object is of a scalar type, or a union type whose member objects are of scalar types, the expression is interpreted by GCC as a read of the volatile object; in the other cases, the expression is only evaluated for its side-effects.
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GCC is only limited by available memory.
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case values in a switch
statement (C90 6.6.4.2).
GCC is only limited by available memory.
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See section `Implementation-defined behavior' in The C Preprocessor, for details of these aspects of implementation-defined behavior.
#include directive are combined into a header
name (C90 6.8.2, C99 6.10.2).
#include processing (C90 6.8.2, C99
6.10.2).
STDC #pragma
directive (C90 6.8.6, C99 6.10.6).
See section `Pragmas' in The C Preprocessor, for details of pragmas accepted by GCC on all targets. See section Pragmas Accepted by GCC, for details of target-specific pragmas.
__DATE__ and __TIME__ when
respectively, the date and time of translation are not available (C90
6.8.8, C99 6.10.8).
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The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
NULL expands
(C90 7.1.6, C99 7.17).
In <stddef.h>, NULL expands to ((void *)0). GCC
does not provide the other headers which define NULL and some
library implementations may use other definitions in those headers.
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<float.h>, <limits.h>, and <stdint.h>
(C90 and C99 5.2.4.2, C99 7.18.2, C99 7.18.3).
Determined by ABI.
Determined by ABI.
sizeof operator (C90
6.3.3.4, C99 6.5.3.4).
Determined by ABI.
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The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
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A conforming implementation of ISO C++ is required to document its choice of behavior in each of the areas that are designated "implementation defined". The following lists all such areas, along with the section numbers from the ISO/IEC 14822:1998 and ISO/IEC 14822:2003 standards. Some areas are only implementation-defined in one version of the standard.
Some choices depend on the externally determined ABI for the platform (including standard character encodings) which GCC follows; these are listed as "determined by ABI" below. See section Binary Compatibility, and http://gcc.gnu.org/readings.html. Some choices are documented in the preprocessor manual. See section `Implementation-defined behavior' in The C Preprocessor. Some choices are documented in the corresponding document for the C language. See section 4. C Implementation-defined behavior. Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details.
5.1 Conditionally-supported behavior 5.2 Exception handling
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Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support (C++0x 1.4).
Such argument passing is not supported.
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The stack is not unwound before std::terminate is called.
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GNU C provides several language features not found in ISO standard C.
(The `-pedantic' option directs GCC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GCC.
These extensions are available in C and Objective-C. Most of them are also available in C++. See section Extensions to the C++ Language, for extensions that apply only to C++.
Some features that are in ISO C99 but not C90 or C++ are also, as extensions, accepted by GCC in C90 mode and in C++.
(With them you can define "built-in" functions.)
6.1 Statements and Declarations in Expressions Putting statements and declarations inside expressions. 6.2 Locally Declared Labels Labels local to a block. 6.3 Labels as Values Getting pointers to labels, and computed gotos. 6.4 Nested Functions As in Algol and Pascal, lexical scoping of functions. 6.5 Constructing Function Calls Dispatching a call to another function. 6.6 Referring to a Type with typeoftypeof: referring to the type of an expression.6.7 Conditionals with Omitted Operands Omitting the middle operand of a `?:' expression. 6.9 Double-Word Integers Double-word integers--- long long int.6.8 128-bits integers 128-bit integers--- __int128.6.10 Complex Numbers Data types for complex numbers. 6.11 Additional Floating Types 6.12 Half-Precision Floating Point 6.13 Decimal Floating Types 6.14 Hex Floats Hexadecimal floating-point constants. 6.15 Fixed-Point Types 6.16 Named Address Spaces Named address spaces. 6.17 Arrays of Length Zero Zero-length arrays. 6.19 Arrays of Variable Length Arrays whose length is computed at run time. 6.18 Structures With No Members Structures with no members. 6.20 Macros with a Variable Number of Arguments. Macros with a variable number of arguments. 6.21 Slightly Looser Rules for Escaped Newlines Slightly looser rules for escaped newlines. 6.22 Non-Lvalue Arrays May Have Subscripts Any array can be subscripted, even if not an lvalue. 6.23 Arithmetic on void- and Function-PointersArithmetic on void-pointers and function pointers.6.24 Non-Constant Initializers Non-constant initializers. 6.25 Compound Literals Compound literals give structures, unions or arrays as values. 6.26 Designated Initializers Labeling elements of initializers. 6.28 Cast to a Union Type Casting to union type from any member of the union. 6.27 Case Ranges `case 1 ... 9' and such. 6.29 Mixed Declarations and Code Mixing declarations and code. 6.30 Declaring Attributes of Functions Declaring that functions have no side effects, or that they can never return. 6.31 Attribute Syntax Formal syntax for attributes. 6.32 Prototypes and Old-Style Function Definitions Prototype declarations and old-style definitions. 6.33 C++ Style Comments C++ comments are recognized. 6.34 Dollar Signs in Identifier Names Dollar sign is allowed in identifiers. 6.35 The Character ESC in Constants `\e' stands for the character ESC. 6.36 Specifying Attributes of Variables Specifying attributes of variables. 6.37 Specifying Attributes of Types Specifying attributes of types. 6.38 Inquiring on Alignment of Types or Variables Inquiring about the alignment of a type or variable. 6.39 An Inline Function is As Fast As a Macro Defining inline functions (as fast as macros). 6.40 When is a Volatile Object Accessed? What constitutes an access to a volatile object. 6.41 Assembler Instructions with C Expression Operands Assembler instructions with C expressions as operands.
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A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
|
is a valid (though slightly more complex than necessary) expression
for the absolute value of foo ().
The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct. (If you use some other kind of statement
last within the braces, the construct has type void, and thus
effectively no value.)
This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b)) |
But this definition computes either a or b twice, with bad
results if the operand has side effects. In GNU C, if you know the
type of the operands (here taken as int), you can define
the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
|
Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.
If you don't know the type of the operand, you can still do this, but you
must use typeof (see section 6.6 Referring to a Type with typeof).
In G++, the result value of a statement expression undergoes array and
function pointer decay, and is returned by value to the enclosing
expression. For instance, if A is a class, then
A a;
({a;}).Foo ()
|
will construct a temporary A object to hold the result of the
statement expression, and that will be used to invoke Foo.
Therefore the this pointer observed by Foo will not be the
address of a.
Any temporaries created within a statement within a statement expression will be destroyed at the statement's end. This makes statement expressions inside macros slightly different from function calls. In the latter case temporaries introduced during argument evaluation will be destroyed at the end of the statement that includes the function call. In the statement expression case they will be destroyed during the statement expression. For instance,
#define macro(a) ({__typeof__(a) b = (a); b + 3; })
template<typename T> T function(T a) { T b = a; return b + 3; }
void foo ()
{
macro (X ());
function (X ());
}
|
will have different places where temporaries are destroyed. For the
macro case, the temporary X will be destroyed just after
the initialization of b. In the function case that
temporary will be destroyed when the function returns.
These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)
Jumping into a statement expression with goto or using a
switch statement outside the statement expression with a
case or default label inside the statement expression is
not permitted. Jumping into a statement expression with a computed
goto (see section 6.3 Labels as Values) yields undefined behavior.
Jumping out of a statement expression is permitted, but if the
statement expression is part of a larger expression then it is
unspecified which other subexpressions of that expression have been
evaluated except where the language definition requires certain
subexpressions to be evaluated before or after the statement
expression. In any case, as with a function call the evaluation of a
statement expression is not interleaved with the evaluation of other
parts of the containing expression. For example,
foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();
|
will call foo and bar1 and will not call baz but
may or may not call bar2. If bar2 is called, it will be
called after foo and before bar1
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GCC allows you to declare local labels in any nested block
scope. A local label is just like an ordinary label, but you can
only reference it (with a goto statement, or by taking its
address) within the block in which it was declared.
A local label declaration looks like this:
__label__ label; |
or
__label__ label1, label2, /* ... */; |
Local label declarations must come at the beginning of the block, before any ordinary declarations or statements.
The label declaration defines the label name, but does not define
the label itself. You must do this in the usual way, with
label:, within the statements of the statement expression.
The local label feature is useful for complex macros. If a macro
contains nested loops, a goto can be useful for breaking out of
them. However, an ordinary label whose scope is the whole function
cannot be used: if the macro can be expanded several times in one
function, the label will be multiply defined in that function. A
local label avoids this problem. For example:
#define SEARCH(value, array, target) \
do { \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ (value) = i; goto found; } \
(value) = -1; \
found:; \
} while (0)
|
This could also be written using a statement-expression:
#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
|
Local label declarations also make the labels they declare visible to nested functions, if there are any. See section 6.4 Nested Functions, for details.
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You can get the address of a label defined in the current function
(or a containing function) with the unary operator `&&'. The
value has type void *. This value is a constant and can be used
wherever a constant of that type is valid. For example:
void *ptr; /* ... */ ptr = &&foo; |
To use these values, you need to be able to jump to one. This is done
with the computed goto statement(3), goto *exp;. For example,
goto *ptr; |
Any expression of type void * is allowed.
One way of using these constants is in initializing a static array that will serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
|
Then you can select a label with indexing, like this:
goto *array[i]; |
Note that this does not check whether the subscript is in bounds--array indexing in C never does that.
Such an array of label values serves a purpose much like that of the
switch statement. The switch statement is cleaner, so
use that rather than an array unless the problem does not fit a
switch statement very well.
Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.
You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.
An alternate way to write the above example is
static const int array[] = { &&foo - &&foo, &&bar - &&foo,
&&hack - &&foo };
goto *(&&foo + array[i]);
|
This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.
The &&foo expressions for the same label might have different
values if the containing function is inlined or cloned. If a program
relies on them being always the same,
__attribute__((__noinline__,__noclone__)) should be used to
prevent inlining and cloning. If &&foo is used in a static
variable initializer, inlining and cloning is forbidden.
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A nested function is a function defined inside another function.
(Nested functions are not supported for GNU C++.) The nested function's
name is local to the block where it is defined. For example, here we
define a nested function named square, and call it twice:
foo (double a, double b)
{
double square (double z) { return z * z; }
return square (a) + square (b);
}
|
The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called lexical scoping. For example, here we show a nested
function which uses an inherited variable named offset:
bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
/* ... */
for (i = 0; i < size; i++)
/* ... */ access (array, i) /* ... */
}
|
Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, mixed with the other declarations and statements in the block.
It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:
hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
}
|
Here, the function intermediate receives the address of
store as an argument. If intermediate calls store,
the arguments given to store are used to store into array.
But this technique works only so long as the containing function
(hack, in this example) does not exit.
If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.
GCC implements taking the address of a nested function using a technique called trampolines. This technique was described in Lexical Closures for C++ (Thomas M. Breuel, USENIX C++ Conference Proceedings, October 17-21, 1988).
A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (see section 6.2 Locally Declared Labels). Such a jump returns instantly to the
containing function, exiting the nested function which did the
goto and any intermediate functions as well. Here is an example:
bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
/* ... */
for (i = 0; i < size; i++)
/* ... */ access (array, i) /* ... */
/* ... */
return 0;
/* Control comes here from |
A nested function always has no linkage. Declaring one with
extern or static is erroneous. If you need to declare the nested function
before its definition, use auto (which is otherwise meaningless
for function declarations).
bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
/* ... */
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
/* ... */
}
|
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Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.
You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).
However, these built-in functions may interact badly with some sophisticated features or other extensions of the language. It is, therefore, not recommended to use them outside very simple functions acting as mere forwarders for their arguments.
The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size
of the stack argument data, in bytes.
This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The
value is used by __builtin_apply to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.
__builtin_apply.
__attribute__ ((__always_inline__)) or
__attribute__ ((__gnu_inline__)) extern inline functions.
It must be only passed as last argument to some other function
with variable arguments. This is useful for writing small wrapper
inlines for variable argument functions, when using preprocessor
macros is undesirable. For example:
extern int myprintf (FILE *f, const char *format, ...);
extern inline __attribute__ ((__gnu_inline__)) int
myprintf (FILE *f, const char *format, ...)
{
int r = fprintf (f, "myprintf: ");
if (r < 0)
return r;
int s = fprintf (f, format, __builtin_va_arg_pack ());
if (s < 0)
return s;
return r + s;
}
|
__attribute__ ((__always_inline__)) or
__attribute__ ((__gnu_inline__)) extern inline functions.
For example following will do link or runtime checking of open
arguments for optimized code:
#ifdef __OPTIMIZE__
extern inline __attribute__((__gnu_inline__)) int
myopen (const char *path, int oflag, ...)
{
if (__builtin_va_arg_pack_len () > 1)
warn_open_too_many_arguments ();
if (__builtin_constant_p (oflag))
{
if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1)
{
warn_open_missing_mode ();
return __open_2 (path, oflag);
}
return open (path, oflag, __builtin_va_arg_pack ());
}
if (__builtin_va_arg_pack_len () < 1)
return __open_2 (path, oflag);
return open (path, oflag, __builtin_va_arg_pack ());
}
#endif
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typeof
Another way to refer to the type of an expression is with typeof.
The syntax of using of this keyword looks like sizeof, but the
construct acts semantically like a type name defined with typedef.
There are two ways of writing the argument to typeof: with an
expression or with a type. Here is an example with an expression:
typeof (x[0](1)) |
This assumes that x is an array of pointers to functions;
the type described is that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *) |
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ISO C
programs, write __typeof__ instead of typeof.
See section 6.45 Alternate Keywords.
A typeof-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof or typeof.
The operand of typeof is evaluated for its side effects if and
only if it is an expression of variably modified type or the name of
such a type.
typeof is often useful in conjunction with the
statements-within-expressions feature. Here is how the two together can
be used to define a safe "maximum" macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:
#define max(a,b) \
({ typeof (a) _a = (a); \
typeof (b) _b = (b); \
_a > _b ? _a : _b; })
|
The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for a and b. Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.
Some more examples of the use of typeof:
y with the type of what x points to.
typeof (*x) y; |
y as an array of such values.
typeof (*x) y[4]; |
y as an array of pointers to characters:
typeof (typeof (char *)[4]) y; |
It is equivalent to the following traditional C declaration:
char *y[4]; |
To see the meaning of the declaration using typeof, and why it
might be a useful way to write, rewrite it with these macros:
#define pointer(T) typeof(T *) #define array(T, N) typeof(T [N]) |
Now the declaration can be rewritten this way:
array (pointer (char), 4) y; |
Thus, array (pointer (char), 4) is the type of arrays of 4
pointers to char.
Compatibility Note: In addition to typeof, GCC 2 supported
a more limited extension which permitted one to write
typedef T = expr; |
with the effect of declaring T to have the type of the expression
expr. This extension does not work with GCC 3 (versions between
3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which
relies on it should be rewritten to use typeof:
typedef typeof(expr) T; |
This will work with all versions of GCC.
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The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.
Therefore, the expression
x ? : y |
has the value of x if that is nonzero; otherwise, the value of
y.
This example is perfectly equivalent to
x ? x : y |
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.
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As an extension the integer scalar type __int128 is supported for
targets having an integer mode wide enough to hold 128-bit.
Simply write __int128 for a signed 128-bit integer, or
unsigned __int128 for an unsigned 128-bit integer. There is no
support in GCC to express an integer constant of type __int128
for targets having long long integer with less then 128 bit width.
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ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C90 mode and in C++.
Simply write long long int for a signed integer, or
unsigned long long int for an unsigned integer. To make an
integer constant of type long long int, add the suffix `LL'
to the integer. To make an integer constant of type unsigned long
long int, add the suffix `ULL' to the integer.
You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.
There may be pitfalls when you use long long types for function
arguments, unless you declare function prototypes. If a function
expects type int for its argument, and you pass a value of type
long long int, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects long long int and you pass
int. The best way to avoid such problems is to use prototypes.
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ISO C99 supports complex floating data types, and as an extension GCC
supports them in C90 mode and in C++, and supports complex integer data
types which are not part of ISO C99. You can declare complex types
using the keyword _Complex. As an extension, the older GNU
keyword __complex__ is also supported.
For example, `_Complex double x;' declares x as a
variable whose real part and imaginary part are both of type
double. `_Complex short int y;' declares y to
have real and imaginary parts of type short int; this is not
likely to be useful, but it shows that the set of complex types is
complete.
To write a constant with a complex data type, use the suffix `i' or
`j' (either one; they are equivalent). For example, 2.5fi
has type _Complex float and 3i has type
_Complex int. Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant. This is a GNU extension; if you have an ISO C99
conforming C library (such as GNU libc), and want to construct complex
constants of floating type, you should include <complex.h> and
use the macros I or _Complex_I instead.
To extract the real part of a complex-valued expression exp, write
__real__ exp. Likewise, use __imag__ to
extract the imaginary part. This is a GNU extension; for values of
floating type, you should use the ISO C99 functions crealf,
creal, creall, cimagf, cimag and
cimagl, declared in <complex.h> and also provided as
built-in functions by GCC.
The operator `~' performs complex conjugation when used on a value
with a complex type. This is a GNU extension; for values of
floating type, you should use the ISO C99 functions conjf,
conj and conjl, declared in <complex.h> and also
provided as built-in functions by GCC.
GCC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa). Only the DWARF2
debug info format can represent this, so use of DWARF2 is recommended.
If you are using the stabs debug info format, GCC describes a noncontiguous
complex variable as if it were two separate variables of noncomplex type.
If the variable's actual name is foo, the two fictitious
variables are named foo$real and foo$imag. You can
examine and set these two fictitious variables with your debugger.
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As an extension, the GNU C compiler supports additional floating
types, __float80 and __float128 to support 80bit
(XFmode) and 128 bit (TFmode) floating types.
Support for additional types includes the arithmetic operators:
add, subtract, multiply, divide; unary arithmetic operators;
relational operators; equality operators; and conversions to and from
integer and other floating types. Use a suffix `w' or `W'
in a literal constant of type __float80 and `q' or `Q'
for _float128. You can declare complex types using the
corresponding internal complex type, XCmode for __float80
type and TCmode for __float128 type:
typedef _Complex float __attribute__((mode(TC))) _Complex128; typedef _Complex float __attribute__((mode(XC))) _Complex80; |
Not all targets support additional floating point types. __float80
and __float128 types are supported on i386, x86_64 and ia64 targets.
The __float128 type is supported on hppa HP-UX targets.
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On ARM targets, GCC supports half-precision (16-bit) floating point via
the __fp16 type. You must enable this type explicitly
with the `-mfp16-format' command-line option in order to use it.
ARM supports two incompatible representations for half-precision floating-point values. You must choose one of the representations and use it consistently in your program.
Specifying `-mfp16-format=ieee' selects the IEEE 754-2008 format. This format can represent normalized values in the range of 2^{-14} to 65504. There are 11 bits of significand precision, approximately 3 decimal digits.
Specifying `-mfp16-format=alternative' selects the ARM alternative format. This representation is similar to the IEEE format, but does not support infinities or NaNs. Instead, the range of exponents is extended, so that this format can represent normalized values in the range of 2^{-14} to 131008.
The __fp16 type is a storage format only. For purposes
of arithmetic and other operations, __fp16 values in C or C++
expressions are automatically promoted to float. In addition,
you cannot declare a function with a return value or parameters
of type __fp16.
Note that conversions from double to __fp16
involve an intermediate conversion to float. Because
of rounding, this can sometimes produce a different result than a
direct conversion.
ARM provides hardware support for conversions between
__fp16 and float values
as an extension to VFP and NEON (Advanced SIMD). GCC generates
code using these hardware instructions if you compile with
options to select an FPU that provides them;
for example, `-mfpu=neon-fp16 -mfloat-abi=softfp',
in addition to the `-mfp16-format' option to select
a half-precision format.
Language-level support for the __fp16 data type is
independent of whether GCC generates code using hardware floating-point
instructions. In cases where hardware support is not specified, GCC
implements conversions between __fp16 and float values
as library calls.
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As an extension, the GNU C compiler supports decimal floating types as defined in the N1312 draft of ISO/IEC WDTR24732. Support for decimal floating types in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. Not all targets support decimal floating types.
The decimal floating types are _Decimal32, _Decimal64, and
_Decimal128. They use a radix of ten, unlike the floating types
float, double, and long double whose radix is not
specified by the C standard but is usually two.
Support for decimal floating types includes the arithmetic operators
add, subtract, multiply, divide; unary arithmetic operators;
relational operators; equality operators; and conversions to and from
integer and other floating types. Use a suffix `df' or
`DF' in a literal constant of type _Decimal32, `dd'
or `DD' for _Decimal64, and `dl' or `DL' for
_Decimal128.
GCC support of decimal float as specified by the draft technical report is incomplete:
__STDC_DEC_FP__ to indicate that the implementation conforms to
the technical report.
Types _Decimal32, _Decimal64, and _Decimal128
are supported by the DWARF2 debug information format.
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ISO C99 supports floating-point numbers written not only in the usual
decimal notation, such as 1.55e1, but also numbers such as
0x1.fp3 written in hexadecimal format. As a GNU extension, GCC
supports this in C90 mode (except in some cases when strictly
conforming) and in C++. In that format the
`0x' hex introducer and the `p' or `P' exponent field are
mandatory. The exponent is a decimal number that indicates the power of
2 by which the significant part will be multiplied. Thus `0x1.f' is
1 15/16,
`p3' multiplies it by 8, and the value of 0x1.fp3
is the same as 1.55e1.
Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation. Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., 0x1.f. This
could mean 1.0f or 1.9375 since `f' is also the
extension for floating-point constants of type float.
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As an extension, the GNU C compiler supports fixed-point types as defined in the N1169 draft of ISO/IEC DTR 18037. Support for fixed-point types in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. Not all targets support fixed-point types.
The fixed-point types are
short _Fract,
_Fract,
long _Fract,
long long _Fract,
unsigned short _Fract,
unsigned _Fract,
unsigned long _Fract,
unsigned long long _Fract,
_Sat short _Fract,
_Sat _Fract,
_Sat long _Fract,
_Sat long long _Fract,
_Sat unsigned short _Fract,
_Sat unsigned _Fract,
_Sat unsigned long _Fract,
_Sat unsigned long long _Fract,
short _Accum,
_Accum,
long _Accum,
long long _Accum,
unsigned short _Accum,
unsigned _Accum,
unsigned long _Accum,
unsigned long long _Accum,
_Sat short _Accum,
_Sat _Accum,
_Sat long _Accum,
_Sat long long _Accum,
_Sat unsigned short _Accum,
_Sat unsigned _Accum,
_Sat unsigned long _Accum,
_Sat unsigned long long _Accum.
Fixed-point data values contain fractional and optional integral parts. The format of fixed-point data varies and depends on the target machine.
Support for fixed-point types includes:
++, --)
+, -, !)
+, -, *, /)
<<, >>)
<, <=, >=, >)
==, !=)
+=, -=, *=, /=,
<<=, >>=)
Use a suffix in a fixed-point literal constant:
short _Fract and
_Sat short _Fract
_Fract and _Sat _Fract
long _Fract and
_Sat long _Fract
long long _Fract and
_Sat long long _Fract
unsigned short _Fract and
_Sat unsigned short _Fract
unsigned _Fract and
_Sat unsigned _Fract
unsigned long _Fract and
_Sat unsigned long _Fract
unsigned long long _Fract
and _Sat unsigned long long _Fract
short _Accum and
_Sat short _Accum
_Accum and _Sat _Accum
long _Accum and
_Sat long _Accum
long long _Accum and
_Sat long long _Accum
unsigned short _Accum and
_Sat unsigned short _Accum
unsigned _Accum and
_Sat unsigned _Accum
unsigned long _Accum and
_Sat unsigned long _Accum
unsigned long long _Accum
and _Sat unsigned long long _Accum
GCC support of fixed-point types as specified by the draft technical report is incomplete:
Fixed-point types are supported by the DWARF2 debug information format.
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As an extension, the GNU C compiler supports named address spaces as defined in the N1275 draft of ISO/IEC DTR 18037. Support for named address spaces in GCC will evolve as the draft technical report changes. Calling conventions for any target might also change. At present, only the AVR, SPU, M32C, and RL78 targets support address spaces other than the generic address space.
Address space identifiers may be used exactly like any other C type
qualifier (e.g., const or volatile). See the N1275
document for more details.
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On the M32C target, with the R8C and M16C cpu variants, variables
qualified with __far are accessed using 32-bit addresses in
order to access memory beyond the first 64@tie{}Ki bytes. If
__far is used with the M32CM or M32C cpu variants, it has no
effect.
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On the RL78 target, variables qualified with __far are accessed
with 32-bit pointers (20-bit addresses) rather than the default 16-bit
addresses. Non-far variables are assumed to appear in the topmost
64@tie{}KiB of the address space.
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For any declared symbols matching name, this does three things to that
symbol: it forces the symbol to be located at the given address (a number), it
forces the symbol to be volatile, and it changes the symbol's scope to be
static. Note that this pragma does not work in C++ only C and that the common
1234H numeric syntax is not supported (use 0x1234 instead). Please also not that
this pragma does not work to __far variables and variables which have
initalizers. Example:
#pragma ADDRESS port3 0x8000
char port3;
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Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
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In ISO C90, you would have to give contents a length of 1, which
means either you waste space or complicate the argument to malloc.
In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:
contents[] without
the 0.
sizeof
operator may not be applied. As a quirk of the original implementation
of zero-length arrays, sizeof evaluates to zero.
struct that is otherwise non-empty.
GCC versions before 3.0 allowed zero-length arrays to be statically initialized, as if they were flexible arrays. In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data. Non-empty initialization of zero-length arrays is now treated like any case where there are more initializer elements than the array holds, in that a suitable warning about "excess elements in array" is given, and the excess elements (all of them, in this case) are ignored.
Instead GCC allows static initialization of flexible array members.
This is equivalent to defining a new structure containing the original
structure followed by an array of sufficient size to contain the data.
I.e. in the following, f1 is constructed as if it were declared
like f2.
struct f1 {
int x; int y[];
} f1 = { 1, { 2, 3, 4 } };
struct f2 {
struct f1 f1; int data[3];
} f2 = { { 1 }, { 2, 3, 4 } };
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The convenience of this extension is that f1 has the desired
type, eliminating the need to consistently refer to f2.f1.
This has symmetry with normal static arrays, in that an array of
unknown size is also written with [].
Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example:
struct foo { int x; int y[]; };
struct bar { struct foo z; };
struct foo a = { 1, { 2, 3, 4 } }; // Valid.
struct bar b = { { 1, { 2, 3, 4 } } }; // Invalid.
struct bar c = { { 1, { } } }; // Valid.
struct foo d[1] = { { 1 { 2, 3, 4 } } }; // Invalid.
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GCC permits a C structure to have no members:
struct empty {
};
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The structure will have size zero. In C++, empty structures are part
of the language. G++ treats empty structures as if they had a single
member of type char.
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Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:
FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}
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Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.
You can use the function alloca to get an effect much like
variable-length arrays. The function alloca is available in
many other C implementations (but not in all). On the other hand,
variable-length arrays are more elegant.
There are other differences between these two methods. Space allocated
with alloca exists until the containing function returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends. (If you use both variable-length arrays and
alloca in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with alloca.)
You can also use variable-length arrays as arguments to functions:
struct entry
tester (int len, char data[len][len])
{
/* ... */
}
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The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
sizeof.
If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension.
struct entry
tester (int len; char data[len][len], int len)
{
/* ... */
}
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The `int len' before the semicolon is a parameter forward
declaration, and it serves the purpose of making the name len
known when the declaration of data is parsed.
You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type. ISO C99 does not support parameter forward declarations.
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In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) |
Here `...' is a variable argument. In the invocation of
such a macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas. This set of
tokens replaces the identifier __VA_ARGS__ in the macro body
wherever it appears. See the CPP manual for more information.
GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example:
#define debug(format, args...) fprintf (stderr, format, args) |
This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.
GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.
In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string:
debug ("A message")
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GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.
To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, `##'. If instead you write
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) |
and if the variable arguments are omitted or empty, the `##' operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded.
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Recently, the preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.
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In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary `&' operator may not be applied to them. As an extension, GCC allows such arrays to be subscripted in C90 mode, though otherwise they do not decay to pointers outside C99 mode. For example, this is valid in GNU C though not valid in C90:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}
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void- and Function-Pointers
In GNU C, addition and subtraction operations are supported on pointers to
void and on pointers to functions. This is done by treating the
size of a void or of a function as 1.
A consequence of this is that sizeof is also allowed on void
and on function types, and returns 1.
The option `-Wpointer-arith' requests a warning if these extensions are used.
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As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
/* ... */
}
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ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C90 mode and in C++.
Usually, the specified type is a structure. Assume that
struct foo and structure are declared as shown:
struct foo {int a; char b[2];} structure;
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Here is an example of constructing a struct foo with a compound literal:
structure = ((struct foo) {x + y, 'a', 0});
|
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
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You can also construct an array. If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here:
char **foo = (char *[]) { "x", "y", "z" };
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Compound literals for scalar types and union types are also allowed, but then the compound literal is equivalent to a cast.
As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99, because the initializer is not a constant). It is handled as if the object was initialized only with the bracket enclosed list if the types of the compound literal and the object match. The initializer list of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by compound literal size.
static struct foo x = (struct foo) {1, 'a', 'b'};
static int y[] = (int []) {1, 2, 3};
static int z[] = (int [3]) {1};
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The above lines are equivalent to the following:
static struct foo x = {1, 'a', 'b'};
static int y[] = {1, 2, 3};
static int z[] = {1, 0, 0};
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Standard C90 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.
In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C90 mode as well. This extension is not implemented in GNU C++.
To specify an array index, write `[index] =' before the element value. For example,
int a[6] = { [4] = 29, [2] = 15 };
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is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
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The index values must be constant expressions, even if the array being initialized is automatic.
An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write `[index]' before the element value, with no `='.
To initialize a range of elements to the same value, write `[first ... last] = value'. This is a GNU extension. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
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If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize with `.fieldname =' before the element value. For example, given the following structure,
struct point { int x, y; };
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the following initialization
struct point p = { .y = yvalue, .x = xvalue };
|
is equivalent to
struct point p = { xvalue, yvalue };
|
Another syntax which has the same meaning, obsolete since GCC 2.5, is `fieldname:', as shown here:
struct point p = { y: yvalue, x: xvalue };
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The `[index]' or `.fieldname' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,
union foo { int i; double d; };
union foo f = { .d = 4 };
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will convert 4 to a double to store it in the union using
the second element. By contrast, casting 4 to type union foo
would store it into the union as the integer i, since it is
an integer. (See section 6.28 Cast to a Union Type.)
You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
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is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
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Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum type.
For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
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You can also write a series of `.fieldname' and `[index]' designators before an `=' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the `struct point' declaration above:
struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
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If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, GCC will discard them and issue a warning.
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You can specify a range of consecutive values in a single case label,
like this:
case low ... high: |
This has the same effect as the proper number of individual case
labels, one for each integer value from low to high, inclusive.
This feature is especially useful for ranges of ASCII character codes:
case 'A' ... 'Z': |
Be careful: Write spaces around the ..., for otherwise
it may be parsed wrong when you use it with integer values. For example,
write this:
case 1 ... 5: |
rather than this:
case 1...5: |
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A cast to union type is similar to other casts, except that the type
specified is a union type. You can specify the type either with
union tag or with a typedef name. A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts. (See section 6.25 Compound Literals.)
The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:
union foo { int i; double d; };
int x;
double y;
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both x and y can be cast to type union foo.
Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:
union foo u; /* ... */ u = (union foo) x == u.i = x u = (union foo) y == u.d = y |
You can also use the union cast as a function argument:
void hack (union foo); /* ... */ hack ((union foo) x); |
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ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GCC also allows this in C90 mode. For example, you could do:
int i; /* ... */ i++; int j = i + 2; |
Each identifier is visible from where it is declared until the end of the enclosing block.
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In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword __attribute__ allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. The following
attributes are currently defined for functions on all targets:
aligned, alloc_size, noreturn,
returns_twice, noinline, noclone,
always_inline, flatten, pure, const,
nothrow, sentinel, format, format_arg,
no_instrument_function, no_split_stack,
section, constructor,
destructor, used, unused, deprecated,
weak, malloc, alias, ifunc,
warn_unused_result, nonnull, gnu_inline,
externally_visible, hot, cold, artificial,
error and warning. Several other attributes are defined
for functions on particular target systems. Other attributes,
including section are supported for variables declarations
(see section 6.36 Specifying Attributes of Variables) and for types (see section 6.37 Specifying Attributes of Types).
GCC plugins may provide their own attributes.
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__ instead of noreturn.
See section 6.31 Attribute Syntax, for details of the exact syntax for using attributes.
alias ("target")
alias attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* Do something. */; }
void f () __attribute__ ((weak, alias ("__f")));
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defines `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used. It is an error if `__f' is not defined in the same translation unit.
Not all target machines support this attribute.
aligned (alignment)
You cannot use this attribute to decrease the alignment of a function, only to increase it. However, when you explicitly specify a function alignment this will override the effect of the `-falign-functions' (see section 3.10 Options That Control Optimization) option for this function.
Note that the effectiveness of aligned attributes may be
limited by inherent limitations in your linker. On many systems, the
linker is only able to arrange for functions to be aligned up to a
certain maximum alignment. (For some linkers, the maximum supported
alignment may be very very small.) See your linker documentation for
further information.
The aligned attribute can also be used for variables and fields
(see section 6.36 Specifying Attributes of Variables.)
alloc_size
alloc_size attribute is used to tell the compiler that the
function return value points to memory, where the size is given by
one or two of the functions parameters. GCC uses this
information to improve the correctness of __builtin_object_size.
The function parameter(s) denoting the allocated size are specified by one or two integer arguments supplied to the attribute. The allocated size is either the value of the single function argument specified or the product of the two function arguments specified. Argument numbering starts at one.
For instance,
void* my_calloc(size_t, size_t) __attribute__((alloc_size(1,2))) void my_realloc(void*, size_t) __attribute__((alloc_size(2))) |
declares that my_calloc will return memory of the size given by the product of parameter 1 and 2 and that my_realloc will return memory of the size given by parameter 2.
always_inline
gnu_inline
inline keyword. It directs GCC to treat the function
as if it were defined in gnu90 mode even when compiling in C99 or
gnu99 mode.
If the function is declared extern, then this definition of the
function is used only for inlining. In no case is the function
compiled as a standalone function, not even if you take its address
explicitly. Such an address becomes an external reference, as if you
had only declared the function, and had not defined it. This has
almost the effect of a macro. The way to use this is to put a
function definition in a header file with this attribute, and put
another copy of the function, without extern, in a library
file. The definition in the header file will cause most calls to the
function to be inlined. If any uses of the function remain, they will
refer to the single copy in the library. Note that the two
definitions of the functions need not be precisely the same, although
if they do not have the same effect your program may behave oddly.
In C, if the function is neither extern nor static, then
the function is compiled as a standalone function, as well as being
inlined where possible.
This is how GCC traditionally handled functions declared
inline. Since ISO C99 specifies a different semantics for
inline, this function attribute is provided as a transition
measure and as a useful feature in its own right. This attribute is
available in GCC 4.1.3 and later. It is available if either of the
preprocessor macros __GNUC_GNU_INLINE__ or
__GNUC_STDC_INLINE__ are defined. See section An Inline Function is As Fast As a Macro.
In C++, this attribute does not depend on extern in any way,
but it still requires the inline keyword to enable its special
behavior.
artificial
bank_switch
flatten
error ("message")
__builtin_constant_p
and inline functions where checking the inline function arguments is not
possible through extern char [(condition) ? 1 : -1]; tricks.
While it is possible to leave the function undefined and thus invoke
a link failure, when using this attribute the problem will be diagnosed
earlier and with exact location of the call even in presence of inline
functions or when not emitting debugging information.
warning ("message")
__builtin_constant_p
and inline functions. While it is possible to define the function with
a message in .gnu.warning* section, when using this attribute the problem
will be diagnosed earlier and with exact location of the call even in presence
of inline functions or when not emitting debugging information.
cdecl
cdecl attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments. This is
useful to override the effects of the `-mrtd' switch.
const
pure attribute below, since function is not
allowed to read global memory.
Note that a function that has pointer arguments and examines the data
pointed to must not be declared const. Likewise, a
function that calls a non-const function usually must not be
const. It does not make sense for a const function to
return void.
The attribute const is not implemented in GCC versions earlier
than 2.5. An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:
typedef int intfn (); extern const intfn square; |
This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value.
constructor
destructor
constructor (priority)
destructor (priority)
constructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called
automatically after main () has completed or exit () has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
You may provide an optional integer priority to control the order in which constructor and destructor functions are run. A constructor with a smaller priority number runs before a constructor with a larger priority number; the opposite relationship holds for destructors. So, if you have a constructor that allocates a resource and a destructor that deallocates the same resource, both functions typically have the same priority. The priorities for constructor and destructor functions are the same as those specified for namespace-scope C++ objects (see section 7.7 C++-Specific Variable, Function, and Type Attributes).
These attributes are not currently implemented for Objective-C.
deprecated
deprecated (msg)
deprecated attribute results in a warning if the function
is used anywhere in the source file. This is useful when identifying
functions that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead. Note that the warnings only occurs for uses:
int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn; |
results in a warning on line 3 but not line 2. The optional msg argument, which must be a string, will be printed in the warning if present.
The deprecated attribute can also be used for variables and
types (see section 6.36 Specifying Attributes of Variables, see section 6.37 Specifying Attributes of Types.)
disinterrupt
dllexport
dllexport attribute causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
dllimport attribute. On Microsoft Windows targets, the pointer
name is formed by combining _imp__ and the function or variable
name.
You can use __declspec(dllexport) as a synonym for
__attribute__ ((dllexport)) for compatibility with other
compilers.
On systems that support the visibility attribute, this
attribute also implies "default" visibility. It is an error to
explicitly specify any other visibility.
In previous versions of GCC, the dllexport attribute was ignored
for inlined functions, unless the `-fkeep-inline-functions' flag
had been used. The default behaviour now is to emit all dllexported
inline functions; however, this can cause object file-size bloat, in
which case the old behaviour can be restored by using
`-fno-keep-inline-dllexport'.
The attribute is also ignored for undefined symbols.
When applied to C++ classes, the attribute marks defined non-inlined member functions and static data members as exports. Static consts initialized in-class are not marked unless they are also defined out-of-class.
For Microsoft Windows targets there are alternative methods for
including the symbol in the DLL's export table such as using a
`.def' file with an EXPORTS section or, with GNU ld, using
the `--export-all' linker flag.
dllimport
dllimport
attribute causes the compiler to reference a function or variable via
a global pointer to a pointer that is set up by the DLL exporting the
symbol. The attribute implies extern. On Microsoft Windows
targets, the pointer name is formed by combining _imp__ and the
function or variable name.
You can use __declspec(dllimport) as a synonym for
__attribute__ ((dllimport)) for compatibility with other
compilers.
On systems that support the visibility attribute, this
attribute also implies "default" visibility. It is an error to
explicitly specify any other visibility.
Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol definition, an error is reported.
If a symbol previously declared dllimport is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
dllexport.
When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.
On the SH Symbian OS target the dllimport attribute also has
another affect--it can cause the vtable and run-time type information
for a class to be exported. This happens when the class has a
dllimport'ed constructor or a non-inline, non-pure virtual function
and, for either of those two conditions, the class also has an inline
constructor or destructor and has a key function that is defined in
the current translation unit.
For Microsoft Windows based targets the use of the dllimport
attribute on functions is not necessary, but provides a small
performance benefit by eliminating a thunk in the DLL. The use of the
dllimport attribute on imported variables was required on older
versions of the GNU linker, but can now be avoided by passing the
`--enable-auto-import' switch to the GNU linker. As with
functions, using the attribute for a variable eliminates a thunk in
the DLL.
One drawback to using this attribute is that a pointer to a
variable marked as dllimport cannot be used as a constant
address. However, a pointer to a function with the
dllimport attribute can be used as a constant initializer; in
this case, the address of a stub function in the import lib is
referenced. On Microsoft Windows targets, the attribute can be disabled
for functions by setting the `-mnop-fun-dllimport' flag.
eightbit_data
The "vector_address" is the vector location in the range from 0 to 255.
Compiler uses 2 bytes instead of 4 bytes to call a function declared with this attribute. For this compiler uses memory indirect call using jsr @aa:8 instruction.
For example, following call to "foo" will use 2 bytes instead of 4 bytes.
void foo (void) __attribute__ ((function_vector(vector_address)));
void foo (void)
{
}
void bar (void)
{
foo();
}
|
You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
exception_handler
externally_visible
gold is used as the linker plugin, externally_visible attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file, explicit externally_visible attributes are still necessary.
far
far attribute causes the compiler to
use a calling convention that takes care of switching memory banks when
entering and leaving a function. This calling convention is also the
default when using the `-mlong-calls' option.
On 68HC12 the compiler will use the call and rtc instructions
to call and return from a function.
On 68HC11 the compiler will generate a sequence of instructions
to invoke a board-specific routine to switch the memory bank and call the
real function. The board-specific routine simulates a call.
At the end of a function, it will jump to a board-specific routine
instead of using rts. The board-specific return routine simulates
the rtc.
On MeP targets this causes the compiler to use a calling convention which assumes the called function is too far away for the built-in addressing modes.
fast_interrupt
interrupt attribute, except that freit is used to return
instead of reit.
fastcall
fastcall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX and
the second argument (if of integral type) in the register EDX. Subsequent
and other typed arguments are passed on the stack. The called function will
pop the arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.
thiscall
thiscall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX.
Subsequent and other typed arguments are passed on the stack. The called
function will pop the arguments off the stack.
If the number of arguments is variable all arguments are pushed on the
stack.
The thiscall attribute is intended for C++ non-static member functions.
As gcc extension this calling convention can be used for C-functions
and for static member methods.
format (archetype, string-index, first-to-check)
format attribute specifies that a function takes printf,
scanf, strftime or strfmon style arguments which
should be type-checked against a format string. For example, the
declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
|
causes the compiler to check the arguments in calls to my_printf
for consistency with the printf style format string argument
my_format.
The parameter archetype determines how the format string is
interpreted, and should be printf, scanf, strftime,
gnu_printf, gnu_scanf, gnu_strftime or
strfmon. (You can also use __printf__,
__scanf__, __strftime__ or __strfmon__.) On
MinGW targets, ms_printf, ms_scanf, and
ms_strftime are also present.
archtype values such as printf refer to the formats accepted
by the system's C run-time library, while gnu_ values always refer
to the formats accepted by the GNU C Library. On Microsoft Windows
targets, ms_ values refer to the formats accepted by the
`msvcrt.dll' library.
The parameter string-index
specifies which argument is the format string argument (starting
from 1), while first-to-check is the number of the first
argument to check against the format string. For functions
where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency. For
strftime formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit this argument, the
arguments of such methods should be counted from two, not one, when
giving values for string-index and first-to-check.
In the example above, the format string (my_format) is the second
argument of the function my_print, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
The format attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
`-ffreestanding' or `-fno-builtin' is used) checks formats
for the standard library functions printf, fprintf,
sprintf, scanf, fscanf, sscanf, strftime,
vprintf, vfprintf and vsprintf whenever such
warnings are requested (using `-Wformat'), so there is no need to
modify the header file `stdio.h'. In C99 mode, the functions
snprintf, vsnprintf, vscanf, vfscanf and
vsscanf are also checked. Except in strictly conforming C
standard modes, the X/Open function strfmon is also checked as
are printf_unlocked and fprintf_unlocked.
See section Options Controlling C Dialect.
For Objective-C dialects, NSString (or __NSString__) is
recognized in the same context. Declarations including these format attributes
will be parsed for correct syntax, however the result of checking of such format
strings is not yet defined, and will not be carried out by this version of the
compiler.
format_arg (string-index)
format_arg attribute specifies that a function takes a format
string for a printf, scanf, strftime or
strfmon style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
printf, scanf, strftime or strfmon style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string). For example, the
declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
|
causes the compiler to check the arguments in calls to a printf,
scanf, strftime or strfmon type function, whose
format string argument is a call to the my_dgettext function, for
consistency with the format string argument my_format. If the
format_arg attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
`-Wformat-nonliteral' is used, but the calls could not be checked
without the attribute.
The parameter string-index specifies which argument is the format
string argument (starting from one). Since non-static C++ methods have
an implicit this argument, the arguments of such methods should
be counted from two.
The format-arg attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to printf, scanf, strftime or strfmon
type function whose operands are a call to one of your own function.
The compiler always treats gettext, dgettext, and
dcgettext in this manner except when strict ISO C support is
requested by `-ansi' or an appropriate `-std' option, or
`-ffreestanding' or `-fno-builtin'
is used. See section Options Controlling C Dialect.
For Objective-C dialects, the format-arg attribute may refer to an
NSString reference for compatibility with the format attribute
above.
function_vector (vector_address)
In SH2A target, this attribute declares a function to be called using the TBR relative addressing mode. The argument to this attribute is the entry number of the same function in a vector table containing all the TBR relative addressable functions. For the successful jump, register TBR should contain the start address of this TBR relative vector table. In the startup routine of the user application, user needs to care of this TBR register initialization. The TBR relative vector table can have at max 256 function entries. The jumps to these functions will be generated using a SH2A specific, non delayed branch instruction JSR/N @(disp8,TBR). You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
Please refer the example of M16C target, to see the use of this attribute while declaring a function,
In an application, for a function being called once, this attribute will save at least 8 bytes of code; and if other successive calls are being made to the same function, it will save 2 bytes of code per each of these calls.
On M16C/M32C targets, the function_vector attribute declares a
special page subroutine call function. Use of this attribute reduces
the code size by 2 bytes for each call generated to the
subroutine. The argument to the attribute is the vector number entry
from the special page vector table which contains the 16 low-order
bits of the subroutine's entry address. Each vector table has special
page number (18 to 255) which are used in jsrs instruction.
Jump addresses of the routines are generated by adding 0x0F0000 (in
case of M16C targets) or 0xFF0000 (in case of M32C targets), to the 2
byte addresses set in the vector table. Therefore you need to ensure
that all the special page vector routines should get mapped within the
address range 0x0F0000 to 0x0FFFFF (for M16C) and 0xFF0000 to 0xFFFFFF
(for M32C).
In the following example 2 bytes will be saved for each call to
function foo.
void foo (void) __attribute__((function_vector(0x18)));
void foo (void)
{
}
void bar (void)
{
foo();
}
|
If functions are defined in one file and are called in another file, then be sure to write this declaration in both files.
This attribute is ignored for R8C target.
interrupt
Note, interrupt handlers for the Blackfin, H8/300, H8/300H, H8S, MicroBlaze,
and SH processors can be specified via the interrupt_handler attribute.
Note, on the AVR, interrupts will be enabled inside the function.
Note, for the ARM, you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:
void f () __attribute__ ((interrupt ("IRQ")));
|
Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.
On ARMv7-M the interrupt type is ignored, and the attribute means the function may be called with a word aligned stack pointer.
On Epiphany targets one or more optional parameters can be added like this:
void __attribute__ ((interrupt ("dma0, dma1"))) universal_dma_handler ();
|
Permissible values for these parameters are: reset,
software_exception, page_miss,
timer0, timer1, message,
dma0, dma1, wand and swi.
Multiple parameters indicate that multiple entries in the interrupt
vector table should be initialized for this function, i.e. for each
parameter name, a jump to the function will be emitted in
the section ivt_entry_name. The parameter(s) may be omitted
entirely, in which case no interrupt vector table entry will be provided.
Note, on Epiphany targets, interrupts are enabled inside the function
unless the disinterrupt attribute is also specified.
On Epiphany targets, you can also use the following attribute to modify the behavior of an interrupt handler:
forwarder_section
The following examples are all valid uses of these attributes on Epiphany targets:
void __attribute__ ((interrupt)) universal_handler ();
void __attribute__ ((interrupt ("dma1"))) dma1_handler ();
void __attribute__ ((interrupt ("dma0, dma1"))) universal_dma_handler ();
void __attribute__ ((interrupt ("timer0"), disinterrupt))
fast_timer_handler ();
void __attribute__ ((interrupt ("dma0, dma1"), forwarder_section ("tramp")))
external_dma_handler ();
|
On MIPS targets, you can use the following attributes to modify the behavior of an interrupt handler:
use_shadow_register_set
keep_interrupts_masked
use_debug_exception_return
deret instruction. Interrupt handlers that don't
have this attribute return using eret instead.
You can use any combination of these attributes, as shown below:
void __attribute__ ((interrupt)) v0 ();
void __attribute__ ((interrupt, use_shadow_register_set)) v1 ();
void __attribute__ ((interrupt, keep_interrupts_masked)) v2 ();
void __attribute__ ((interrupt, use_debug_exception_return)) v3 ();
void __attribute__ ((interrupt, use_shadow_register_set,
keep_interrupts_masked)) v4 ();
void __attribute__ ((interrupt, use_shadow_register_set,
use_debug_exception_return)) v5 ();
void __attribute__ ((interrupt, keep_interrupts_masked,
use_debug_exception_return)) v6 ();
void __attribute__ ((interrupt, use_shadow_register_set,
keep_interrupts_masked,
use_debug_exception_return)) v7 ();
|
On RL78, use brk_interrupt instead of interrupt for
handlers intended to be used with the BRK opcode (i.e. those
that must end with RETB instead of RETI).
On RX targets, you may specify one or more vector numbers as arguments
to the attribute, as well as naming an alternate table name.
Parameters are handled sequentially, so one handler can be assigned to
multiple entries in multiple tables. One may also pass the magic
string "$default" which causes the function to be used for any
unfilled slots in the current table.
This example shows a simple assignment of a function to one vector in the default table (note that preprocessor macros may be used for chip-specific symbolic vector names):
void __attribute__ ((interrupt (5))) txd1_handler (); |
This example assigns a function to two slots in the default table
(using preprocessor macros defined elsewhere) and makes it the default
for the dct table:
void __attribute__ ((interrupt (RXD1_VECT,RXD2_VECT,"dct","$default"))) txd1_handler (); |
ifunc ("resolver")
ifunc attribute is used to mark a function as an indirect
function using the STT_GNU_IFUNC symbol type extension to the ELF
standard. This allows the resolution of the symbol value to be
determined dynamically at load time, and an optimized version of the
routine can be selected for the particular processor or other system
characteristics determined then. To use this attribute, first define
the implementation functions available, and a resolver function that
returns a pointer to the selected implementation function. The
implementation functions' declarations must match the API of the
function being implemented, the resolver's declaration is be a
function returning pointer to void function returning void:
void *my_memcpy (void *dst, const void *src, size_t len)
{
...
}
static void (*resolve_memcpy (void)) (void)
{
return my_memcpy; // we'll just always select this routine
}
|
The exported header file declaring the function the user calls would contain:
extern void *memcpy (void *, const void *, size_t); |
allowing the user to call this as a regular function, unaware of the implementation. Finally, the indirect function needs to be defined in the same translation unit as the resolver function:
void *memcpy (void *, const void *, size_t)
__attribute__ ((ifunc ("resolve_memcpy")));
|
Indirect functions cannot be weak, and require a recent binutils (at least version 2.20.1), and GNU C library (at least version 2.11.1).
interrupt_handler
interrupt_thread
sleep
instruction. This attribute is available only on fido.
isr
interrupt attribute above.
kspisusp
interrupt_handler, exception_handler
or nmi_handler, code will be generated to load the stack pointer
from the USP register in the function prologue.
l1_text
.l1.text.
With `-mfdpic', function calls with a such function as the callee
or caller will use inlined PLT.
l2
.l1.text. With `-mfdpic', callers of such functions will use
an inlined PLT.
leaf
The attribute is intended for library functions to improve dataflow analysis.
The compiler takes the hint that any data not escaping the current compilation unit can
not be used or modified by the leaf function. For example, the sin function
is a leaf function, but qsort is not.
Note that leaf functions might invoke signals and signal handlers might be
defined in the current compilation unit and use static variables. The only
compliant way to write such a signal handler is to declare such variables
volatile.
The attribute has no effect on functions defined within the current compilation unit. This is to allow easy merging of multiple compilation units into one, for example, by using the link time optimization. For this reason the attribute is not allowed on types to annotate indirect calls.
malloc
malloc attribute is used to tell the compiler that a function
may be treated as if any non-NULL pointer it returns cannot
alias any other pointer valid when the function returns and that the memory
has undefined content.
This will often improve optimization.
Standard functions with this property include malloc and
calloc. realloc-like functions do not have this
property as the memory pointed to does not have undefined content.
naked
asm statements that do not have operands. All other statements,
including declarations of local variables, if statements, and so
forth, should be avoided. Naked functions should be used to implement the
body of an assembly function, while allowing the compiler to construct
the requisite function declaration for the assembler.
near
near attribute causes the compiler to
use the normal calling convention based on jsr and rts.
This attribute can be used to cancel the effect of the `-mlong-calls'
option.
On MeP targets this attribute causes the compiler to assume the called
function is close enough to use the normal calling convention,
overriding the -mtf command line option.
nesting
interrupt_handler,
exception_handler or nmi_handler to indicate that the function
entry code should enable nested interrupts or exceptions.
nmi_handler
no_instrument_function
no_split_stack
no_split_stack attribute will not have that prologue, and thus
may run with only a small amount of stack space available.
noinline
asm ("");
|
noclone
nonnull (arg-index, ...)
nonnull attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
|
causes the compiler to check that, in calls to my_memcpy,
arguments dest and src are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the `-Wnonnull' option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.
If no argument index list is given to the nonnull attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
|
noreturn
abort and exit,
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (/* ... */)
{
/* ... */ /* Print error message. */ /* ... */
exit (1);
}
|
The noreturn keyword tells the compiler to assume that
fatal cannot return. It can then optimize without regard to what
would happen if fatal ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
The noreturn keyword does not affect the exceptional path when that
applies: a noreturn-marked function may still return to the caller
by throwing an exception or calling longjmp.
Do not assume that registers saved by the calling function are
restored before calling the noreturn function.
It does not make sense for a noreturn function to have a return
type other than void.
The attribute noreturn is not implemented in GCC versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
typedef void voidfn (); volatile voidfn fatal; |
This approach does not work in GNU C++.
nothrow
nothrow attribute is used to inform the compiler that a
function cannot throw an exception. For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of qsort and bsearch that
take function pointer arguments. The nothrow attribute is not
implemented in GCC versions earlier than 3.3.
optimize
optimize attribute is used to specify that a function is to
be compiled with different optimization options than specified on the
command line. Arguments can either be numbers or strings. Numbers
are assumed to be an optimization level. Strings that begin with
O are assumed to be an optimization option, while other options
are assumed to be used with a -f prefix. You can also use the
`#pragma GCC optimize' pragma to set the optimization options
that affect more than one function.
See section 6.56.8 Function Specific Option Pragmas, for details about the
`#pragma GCC optimize' pragma.
This can be used for instance to have frequently executed functions compiled with more aggressive optimization options that produce faster and larger code, while other functions can be called with less aggressive options.
OS_main/OS_task
OS_main or OS_task attribute
do not save/restore any call-saved register in their prologue/epilogue.
The OS_main attribute can be used when there is
guarantee that interrupts are disabled at the time when the function
is entered. This will save resources when the stack pointer has to be
changed to set up a frame for local variables.
The OS_task attribute can be used when there is no
guarantee that interrupts are disabled at that time when the function
is entered like for, e.g. task functions in a multi-threading operating
system. In that case, changing the stack pointer register will be
guarded by save/clear/restore of the global interrupt enable flag.
The differences to the naked function attribute are:
naked functions do not have a return instruction whereas
OS_main and OS_task functions will have a RET or
RETI return instruction.
naked functions do not set up a frame for local variables
or a frame pointer whereas OS_main and OS_task do this
as needed.
pcs
The pcs attribute can be used to control the calling convention
used for a function on ARM. The attribute takes an argument that specifies
the calling convention to use.
When compiling using the AAPCS ABI (or a variant of that) then valid
values for the argument are "aapcs" and "aapcs-vfp". In
order to use a variant other than "aapcs" then the compiler must
be permitted to use the appropriate co-processor registers (i.e., the
VFP registers must be available in order to use "aapcs-vfp").
For example,
/* Argument passed in r0, and result returned in r0+r1. */
double f2d (float) __attribute__((pcs("aapcs")));
|
Variadic functions always use the "aapcs" calling convention and
the compiler will reject attempts to specify an alternative.
pure
pure. For example,
int square (int) __attribute__ ((pure)); |
says that the hypothetical function square is safe to call
fewer times than the program says.
Some of common examples of pure functions are strlen or memcmp.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as feof in a multithreading environment).
The attribute pure is not implemented in GCC versions earlier
than 2.96.
hot
hot attribute is used to inform the compiler that a function is a
hot spot of the compiled program. The function is optimized more aggressively
and on many target it is placed into special subsection of the text section so
all hot functions appears close together improving locality.
When profile feedback is available, via `-fprofile-use', hot functions are automatically detected and this attribute is ignored.
The hot attribute is not implemented in GCC versions earlier
than 4.3.
cold
cold attribute is used to inform the compiler that a function is
unlikely executed. The function is optimized for size rather than speed and on
many targets it is placed into special subsection of the text section so all
cold functions appears close together improving code locality of non-cold parts
of program. The paths leading to call of cold functions within code are marked
as unlikely by the branch prediction mechanism. It is thus useful to mark
functions used to handle unlikely conditions, such as perror, as cold to
improve optimization of hot functions that do call marked functions in rare
occasions.
When profile feedback is available, via `-fprofile-use', hot functions are automatically detected and this attribute is ignored.
The cold attribute is not implemented in GCC versions earlier than 4.3.
regparm (number)
regparm attribute causes the compiler to
pass arguments number one to number if they are of integral type
in registers EAX, EDX, and ECX instead of on the stack. Functions that
take a variable number of arguments will continue to be passed all of their
arguments on the stack.
Beware that on some ELF systems this attribute is unsuitable for global functions in shared libraries with lazy binding (which is the default). Lazy binding will send the first call via resolving code in the loader, which might assume EAX, EDX and ECX can be clobbered, as per the standard calling conventions. Solaris 8 is affected by this. GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be safe since the loaders there save EAX, EDX and ECX. (Lazy binding can be disabled with the linker or the loader if desired, to avoid the problem.)
sseregparm
sseregparm attribute
causes the compiler to pass up to 3 floating point arguments in
SSE registers instead of on the stack. Functions that take a
variable number of arguments will continue to pass all of their
floating point arguments on the stack.
force_align_arg_pointer
force_align_arg_pointer attribute may be
applied to individual function definitions, generating an alternate
prologue and epilogue that realigns the runtime stack if necessary.
This supports mixing legacy codes that run with a 4-byte aligned stack
with modern codes that keep a 16-byte stack for SSE compatibility.
resbank
interrupt_handler
routines. Saving to the bank is performed automatically after the CPU
accepts an interrupt that uses a register bank.
The nineteen 32-bit registers comprising general register R0 to R14, control register GBR, and system registers MACH, MACL, and PR and the vector table address offset are saved into a register bank. Register banks are stacked in first-in last-out (FILO) sequence. Restoration from the bank is executed by issuing a RESBANK instruction.
returns_twice
returns_twice attribute tells the compiler that a function may
return more than one time. The compiler will ensure that all registers
are dead before calling such a function and will emit a warning about
the variables that may be clobbered after the second return from the
function. Examples of such functions are setjmp and vfork.
The longjmp-like counterpart of such function, if any, might need
to be marked with the noreturn attribute.
saveall
save_volatiles
section ("section-name")
text section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The section
attribute specifies that a function lives in a particular section.
For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
|
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
sentinel
NULL. The attribute is only valid on variadic
functions. By default, the sentinel is located at position zero, the
last parameter of the function call. If an optional integer position
argument P is supplied to the attribute, the sentinel must be located at
position P counting backwards from the end of the argument list.
__attribute__ ((sentinel)) is equivalent to __attribute__ ((sentinel(0))) |
The attribute is automatically set with a position of 0 for the built-in
functions execl and execlp. The built-in function
execle has the attribute set with a position of 1.
A valid NULL in this context is defined as zero with any pointer
type. If your system defines the NULL macro with an integer type
then you need to add an explicit cast. GCC replaces stddef.h
with a copy that redefines NULL appropriately.
The warnings for missing or incorrect sentinels are enabled with `-Wformat'.
short_call
shortcall
signal
sp_switch
interrupt_handler
function should switch to an alternate stack. It expects a string
argument that names a global variable holding the address of the
alternate stack.
void *alt_stack;
void f () __attribute__ ((interrupt_handler,
sp_switch ("alt_stack")));
|
stdcall
stdcall attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
syscall_linkage
target
target attribute is used to specify that a function is to
be compiled with different target options than specified on the
command line. This can be used for instance to have functions
compiled with a different ISA (instruction set architecture) than the
default. You can also use the `#pragma GCC target' pragma to set
more than one function to be compiled with specific target options.
See section 6.56.8 Function Specific Option Pragmas, for details about the
`#pragma GCC target' pragma.
For instance on a 386, you could compile one function with
target("sse4.1,arch=core2") and another with
target("sse4a,arch=amdfam10") that would be equivalent to
compiling the first function with `-msse4.1' and
`-march=core2' options, and the second function with
`-msse4a' and `-march=amdfam10' options. It is up to the
user to make sure that a function is only invoked on a machine that
supports the particular ISA it was compiled for (for example by using
cpuid on 386 to determine what feature bits and architecture
family are used).
int core2_func (void) __attribute__ ((__target__ ("arch=core2")));
int sse3_func (void) __attribute__ ((__target__ ("sse3")));
|
On the 386, the following options are allowed:
sin, cos, and
sqrt instructions on the 387 floating point unit.
target("fpmath=sse,387") option must be specified as
target("fpmath=sse+387") because the comma would separate
different options.
tinydata
trap_exit
interrupt_handler to return using
trapa instead of rte. This attribute expects an integer
argument specifying the trap number to be used.
unused
used
When applied to a member function of a C++ class template, the attribute also means that the function will be instantiated if the class itself is instantiated.
version_id
extern int foo () __attribute__((version_id ("20040821")));
|
Calls to foo will be mapped to calls to foo{20040821}.
visibility ("visibility_type")
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
int i __attribute__ ((visibility ("hidden")));
|
The possible values of visibility_type correspond to the visibility settings in the ELF gABI.
On ELF, default visibility means that the declaration is visible to other modules and, in shared libraries, means that the declared entity may be overridden.
On Darwin, default visibility means that the declaration is visible to other modules.
Default visibility corresponds to "external linkage" in the language.
All visibilities are supported on many, but not all, ELF targets (supported when the assembler supports the `.visibility' pseudo-op). Default visibility is supported everywhere. Hidden visibility is supported on Darwin targets.
The visibility attribute should be applied only to declarations which would otherwise have external linkage. The attribute should be applied consistently, so that the same entity should not be declared with different settings of the attribute.
In C++, the visibility attribute applies to types as well as functions and objects, because in C++ types have linkage. A class must not have greater visibility than its non-static data member types and bases, and class members default to the visibility of their class. Also, a declaration without explicit visibility is limited to the visibility of its type.
In C++, you can mark member functions and static member variables of a class with the visibility attribute. This is useful if you know a particular method or static member variable should only be used from one shared object; then you can mark it hidden while the rest of the class has default visibility. Care must be taken to avoid breaking the One Definition Rule; for example, it is usually not useful to mark an inline method as hidden without marking the whole class as hidden.
A C++ namespace declaration can also have the visibility attribute. This attribute applies only to the particular namespace body, not to other definitions of the same namespace; it is equivalent to using `#pragma GCC visibility' before and after the namespace definition (see section 6.56.6 Visibility Pragmas).
In C++, if a template argument has limited visibility, this restriction is implicitly propagated to the template instantiation. Otherwise, template instantiations and specializations default to the visibility of their template.
If both the template and enclosing class have explicit visibility, the visibility from the template is used.
vliw
vliw attribute tells the compiler to emit
instructions in VLIW mode instead of core mode. Note that this
attribute is not allowed unless a VLIW coprocessor has been configured
and enabled through command line options.
warn_unused_result
warn_unused_result attribute causes a warning to be emitted
if a caller of the function with this attribute does not use its
return value. This is useful for functions where not checking
the result is either a security problem or always a bug, such as
realloc.
int fn () __attribute__ ((warn_unused_result));
int foo ()
{
if (fn () < 0) return -1;
fn ();
return 0;
}
|
results in warning on line 5.
weak
weak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
weakref
weakref ("target")
weakref attribute marks a declaration as a weak reference.
Without arguments, it should be accompanied by an alias attribute
naming the target symbol. Optionally, the target may be given as
an argument to weakref itself. In either case, weakref
implicitly marks the declaration as weak. Without a
target, given as an argument to weakref or to alias,
weakref is equivalent to weak.
static int x() __attribute__ ((weakref ("y")));
/* is equivalent to... */
static int x() __attribute__ ((weak, weakref, alias ("y")));
/* and to... */
static int x() __attribute__ ((weakref));
static int x() __attribute__ ((alias ("y")));
|
A weak reference is an alias that does not by itself require a
definition to be given for the target symbol. If the target symbol is
only referenced through weak references, then it becomes a weak
undefined symbol. If it is directly referenced, however, then such
strong references prevail, and a definition will be required for the
symbol, not necessarily in the same translation unit.
The effect is equivalent to moving all references to the alias to a separate translation unit, renaming the alias to the aliased symbol, declaring it as weak, compiling the two separate translation units and performing a reloadable link on them.
At present, a declaration to which weakref is attached can
only be static.
You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.
Some people object to the __attribute__ feature, suggesting that
ISO C's #pragma should be used instead. At the time
__attribute__ was designed, there were two reasons for not doing
this.
#pragma commands from a macro.
#pragma might mean in another
compiler.
These two reasons applied to almost any application that might have been
proposed for #pragma. It was basically a mistake to use
#pragma for anything.
The ISO C99 standard includes _Pragma, which now allows pragmas
to be generated from macros. In addition, a #pragma GCC
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use __attribute__ to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma GCC is of use for constructs that do not naturally form
part of the grammar. See section `Miscellaneous Preprocessing Directives' in The GNU C Preprocessor.
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This section describes the syntax with which __attribute__ may be
used, and the constructs to which attribute specifiers bind, for the C
language. Some details may vary for C++ and Objective-C. Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.
There are some problems with the semantics of attributes in C++. For
example, there are no manglings for attributes, although they may affect
code generation, so problems may arise when attributed types are used in
conjunction with templates or overloading. Similarly, typeid
does not distinguish between types with different attributes. Support
for attributes in C++ may be restricted in future to attributes on
declarations only, but not on nested declarators.
See section 6.30 Declaring Attributes of Functions, for details of the semantics of attributes applying to functions. See section 6.36 Specifying Attributes of Variables, for details of the semantics of attributes applying to variables. See section 6.37 Specifying Attributes of Types, for details of the semantics of attributes applying to structure, union and enumerated types.
An attribute specifier is of the form
__attribute__ ((attribute-list)). An attribute list
is a possibly empty comma-separated sequence of attributes, where
each attribute is one of the following:
unused, or a reserved
word such as const).
mode attributes use this form.
format attributes use this form.
format_arg attributes use this form with the list being a single
integer constant expression, and alias attributes use this form
with the list being a single string constant.
An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.
In GNU C, an attribute specifier list may appear after the colon following a
label, other than a case or default label. The only
attribute it makes sense to use after a label is unused. This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with `-Wall'. It would
not normally be appropriate to use in it human-written code, though it
could be useful in cases where the code that jumps to the label is
contained within an #ifdef conditional. GNU C++ only permits
attributes on labels if the attribute specifier is immediately
followed by a semicolon (i.e., the label applies to an empty
statement). If the semicolon is missing, C++ label attributes are
ambiguous, as it is permissible for a declaration, which could begin
with an attribute list, to be labelled in C++. Declarations cannot be
labelled in C90 or C99, so the ambiguity does not arise there.
An attribute specifier list may appear as part of a struct,
union or enum specifier. It may go either immediately
after the struct, union or enum keyword, or after
the closing brace. The former syntax is preferred.
Where attribute specifiers follow the closing brace, they are considered
to relate to the structure, union or enumerated type defined, not to any
enclosing declaration the type specifier appears in, and the type
defined is not complete until after the attribute specifiers.
Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.
Any list of specifiers and qualifiers at the start of a declaration may
contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers. (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
section.) There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case). In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler. All attribute specifiers in this place relate to the
declaration as a whole. In the obsolescent usage where a type of
int is implied by the absence of type specifiers, such a list of
specifiers and qualifiers may be an attribute specifier list with no
other specifiers or qualifiers.
At present, the first parameter in a function prototype must have some
type specifier which is not an attribute specifier; this resolves an
ambiguity in the interpretation of void f(int
(__attribute__((foo)) x)), but is subject to change. At present, if
the parentheses of a function declarator contain only attributes then
those attributes are ignored, rather than yielding an error or warning
or implying a single parameter of type int, but this is subject to
change.
An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in
__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...),
d2 (void)
|
the noreturn attribute applies to all the functions
declared; the format attribute only applies to d1.
An attribute specifier list may appear immediately before the comma,
= or semicolon terminating the declaration of an identifier other
than a function definition. Such attribute specifiers apply
to the declared object or function. Where an
assembler name for an object or function is specified (see section 6.43 Controlling Names Used in Assembler Code), the attribute must follow the asm
specification.
An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).
Attribute specifiers may be mixed with type qualifiers appearing inside
the [] of a parameter array declarator, in the C99 construct by
which such qualifiers are applied to the pointer to which the array is
implicitly converted. Such attribute specifiers apply to the pointer,
not to the array, but at present this is not implemented and they are
ignored.
An attribute specifier list may appear at the start of a nested
declarator. At present, there are some limitations in this usage: the
attributes correctly apply to the declarator, but for most individual
attributes the semantics this implies are not implemented.
When attribute specifiers follow the * of a pointer
declarator, they may be mixed with any type qualifiers present.
The following describes the formal semantics of this syntax. It will make the
most sense if you are familiar with the formal specification of
declarators in the ISO C standard.
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T
D1, where T contains declaration specifiers that specify a type
Type (such as int) and D1 is a declarator that
contains an identifier ident. The type specified for ident
for derived declarators whose type does not include an attribute
specifier is as in the ISO C standard.
If D1 has the form ( attribute-specifier-list D ),
and the declaration T D specifies the type
"derived-declarator-type-list Type" for ident, then
T D1 specifies the type "derived-declarator-type-list
attribute-specifier-list Type" for ident.
If D1 has the form *
type-qualifier-and-attribute-specifier-list D, and the
declaration T D specifies the type
"derived-declarator-type-list Type" for ident, then
T D1 specifies the type "derived-declarator-type-list
type-qualifier-and-attribute-specifier-list pointer to Type" for
ident.
For example,
void (__attribute__((noreturn)) ****f) (void); |
specifies the type "pointer to pointer to pointer to pointer to
non-returning function returning void". As another example,
char *__attribute__((aligned(8))) *f; |
specifies the type "pointer to 8-byte-aligned pointer to char".
Note again that this does not work with most attributes; for example,
the usage of `aligned' and `noreturn' attributes given above
is not yet supported.
For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.
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GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:
/* Use prototypes unless the compiler is old-fashioned. */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif
/* Prototype function declaration. */
int isroot P((uid_t));
/* Old-style function definition. */
int
isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
|
Suppose the type uid_t happens to be short. ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted. Therefore in this example the
function definition's argument is really an int, which does not
match the prototype argument type of short.
This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the uid_t type is short, int, or
long. Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition. More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion. Thus in GNU C the above example is
equivalent to the following:
int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
|
GNU C++ does not support old-style function definitions, so this extension is irrelevant.
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In GNU C, you may use C++ style comments, which start with `//' and continue until the end of the line. Many other C implementations allow such comments, and they are included in the 1999 C standard. However, C++ style comments are not recognized if you specify an `-std' option specifying a version of ISO C before C99, or `-ansi' (equivalent to `-std=c90').
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In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.
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You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC.
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The keyword __attribute__ allows you to specify special
attributes of variables or structure fields. This keyword is followed
by an attribute specification inside double parentheses. Some
attributes are currently defined generically for variables.
Other attributes are defined for variables on particular target
systems. Other attributes are available for functions
(see section 6.30 Declaring Attributes of Functions) and for types (see section 6.37 Specifying Attributes of Types).
Other front ends might define more attributes
(see section Extensions to the C++ Language).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __aligned__ instead of aligned.
See section 6.31 Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment)
int x __attribute__ ((aligned (16))) = 0; |
causes the compiler to allocate the global variable x on a
16-byte boundary. On a 68040, this could be used in conjunction with
an asm expression to access the move16 instruction which
requires 16-byte aligned operands.
You can also specify the alignment of structure fields. For example, to
create a double-word aligned int pair, you could write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
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This is an alternative to creating a union with a double member
that forces the union to be double-word aligned.
As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the default alignment for the target architecture you are compiling for. The default alignment is sufficient for all scalar types, but may not be enough for all vector types on a target which supports vector operations. The default alignment is fixed for a particular target ABI.
Gcc also provides a target specific macro __BIGGEST_ALIGNMENT__,
which is the largest alignment ever used for any data type on the
target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned (__BIGGEST_ALIGNMENT__))); |
The compiler automatically sets the alignment for the declared
variable or field to __BIGGEST_ALIGNMENT__. Doing this can
often make copy operations more efficient, because the compiler can
use whatever instructions copy the biggest chunks of memory when
performing copies to or from the variables or fields that you have
aligned this way. Note that the value of __BIGGEST_ALIGNMENT__
may change depending on command line options.
When used on a struct, or struct member, the aligned attribute can
only increase the alignment; in order to decrease it, the packed
attribute must be specified as well. When used as part of a typedef, the
aligned attribute can both increase and decrease alignment, and
specifying the packed attribute will generate a warning.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
The aligned attribute can also be used for functions
(see section 6.30 Declaring Attributes of Functions.)
cleanup (cleanup_function)
cleanup attribute runs a function when the variable goes
out of scope. This attribute can only be applied to auto function
scope variables; it may not be applied to parameters or variables
with static storage duration. The function must take one parameter,
a pointer to a type compatible with the variable. The return value
of the function (if any) is ignored.
If `-fexceptions' is enabled, then cleanup_function
will be run during the stack unwinding that happens during the
processing of the exception. Note that the cleanup attribute
does not allow the exception to be caught, only to perform an action.
It is undefined what happens if cleanup_function does not
return normally.
common
nocommon
common attribute requests GCC to place a variable in
"common" storage. The nocommon attribute requests the
opposite--to allocate space for it directly.
These attributes override the default chosen by the `-fno-common' and `-fcommon' flags respectively.
deprecated
deprecated (msg)
deprecated attribute results in a warning if the variable
is used anywhere in the source file. This is useful when identifying
variables that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated variable, to enable users to easily find further
information about why the variable is deprecated, or what they should
do instead. Note that the warning only occurs for uses:
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () { return old_var; }
|
results in a warning on line 3 but not line 2. The optional msg argument, which must be a string, will be printed in the warning if present.
The deprecated attribute can also be used for functions and
types (see section 6.30 Declaring Attributes of Functions, see section 6.37 Specifying Attributes of Types.)
mode (mode)
You may also specify a mode of `byte' or `__byte__' to indicate the mode corresponding to a one-byte integer, `word' or `__word__' for the mode of a one-word integer, and `pointer' or `__pointer__' for the mode used to represent pointers.
packed
packed attribute specifies that a variable or structure field
should have the smallest possible alignment--one byte for a variable,
and one bit for a field, unless you specify a larger value with the
aligned attribute.
Here is a structure in which the field x is packed, so that it
immediately follows a:
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
|
Note: The 4.1, 4.2 and 4.3 series of GCC ignore the
packed attribute on bit-fields of type char. This has
been fixed in GCC 4.4 but the change can lead to differences in the
structure layout. See the documentation of
`-Wpacked-bitfield-compat' for more information.
section ("section-name")
data and bss. Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware. The section
attribute specifies that a variable (or function) lives in a particular
section. For example, this small program uses several specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA")));
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
|
Use the section attribute with
global variables and not local variables,
as shown in the example.
You may use the section attribute with initialized or
uninitialized global variables but the linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the common (or bss) section
and can be multiply "defined". Using the section attribute
will change what section the variable goes into and may cause the
linker to issue an error if an uninitialized variable has multiple
definitions. You can force a variable to be initialized with the
`-fno-common' flag or the nocommon attribute.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
shared
shared and marking the section
shareable:
int foo __attribute__((section ("shared"), shared)) = 0;
int
main()
{
/* Read and write foo. All running
copies see the same value. */
return 0;
}
|
You may only use the shared attribute along with section
attribute with a fully initialized global definition because of the way
linkers work. See section attribute for more information.
The shared attribute is only available on Microsoft Windows.
tls_model ("tls_model")
tls_model attribute sets thread-local storage model
(see section 6.58 Thread-Local Storage) of a particular __thread variable,
overriding `-ftls-model=' command-line switch on a per-variable
basis.
The tls_model argument should be one of global-dynamic,
local-dynamic, initial-exec or local-exec.
Not all targets support this attribute.
unused
used
When applied to a static data member of a C++ class template, the attribute also means that the member will be instantiated if the class itself is instantiated.
vector_size (bytes)
int foo __attribute__ ((vector_size (16))); |
causes the compiler to set the mode for foo, to be 16 bytes,
divided into int sized units. Assuming a 32-bit int (a vector of
4 units of 4 bytes), the corresponding mode of foo will be V4SI.
This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.
Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:
struct S { int a; };
struct S __attribute__ ((vector_size (16))) foo;
|
is invalid even if the size of the structure is the same as the size of
the int.
selectany
selectany attribute causes an initialized global variable to
have link-once semantics. When multiple definitions of the variable are
encountered by the linker, the first is selected and the remainder are
discarded. Following usage by the Microsoft compiler, the linker is told
not to warn about size or content differences of the multiple
definitions.
Although the primary usage of this attribute is for POD types, the attribute can also be applied to global C++ objects that are initialized by a constructor. In this case, the static initialization and destruction code for the object is emitted in each translation defining the object, but the calls to the constructor and destructor are protected by a link-once guard variable.
The selectany attribute is only available on Microsoft Windows
targets. You can use __declspec (selectany) as a synonym for
__attribute__ ((selectany)) for compatibility with other
compilers.
weak
weak attribute is described in 6.30 Declaring Attributes of Functions.
dllimport
dllimport attribute is described in 6.30 Declaring Attributes of Functions.
dllexport
dllexport attribute is described in 6.30 Declaring Attributes of Functions.
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The keyword __attribute__ allows you to specify special
attributes of struct and union types when you define
such types. This keyword is followed by an attribute specification
inside double parentheses. Seven attributes are currently defined for
types: aligned, packed, transparent_union,
unused, deprecated, visibility, and
may_alias. Other attributes are defined for functions
(see section 6.30 Declaring Attributes of Functions) and for variables (see section 6.36 Specifying Attributes of Variables).
You may also specify any one of these attributes with `__'
preceding and following its keyword. This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name. For example, you may use __aligned__
instead of aligned.
You may specify type attributes in an enum, struct or union type
declaration or definition, or for other types in a typedef
declaration.
For an enum, struct or union type, you may specify attributes either between the enum, struct or union tag and the name of the type, or just past the closing curly brace of the definition. The former syntax is preferred.
See section 6.31 Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment)
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
|
force the compiler to insure (as far as it can) that each variable whose
type is struct S or more_aligned_int will be allocated and
aligned at least on a 8-byte boundary. On a SPARC, having all
variables of type struct S aligned to 8-byte boundaries allows
the compiler to use the ldd and std (doubleword load and
store) instructions when copying one variable of type struct S to
another, thus improving run-time efficiency.
Note that the alignment of any given struct or union type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the struct or union in question. This means that you can
effectively adjust the alignment of a struct or union
type by attaching an aligned attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire struct or union type.
As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given struct
or union type. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
|
Whenever you leave out the alignment factor in an aligned
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.
In the example above, if the size of each short is 2 bytes, then
the size of the entire struct S type is 6 bytes. The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire struct S type to 8
bytes.
Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
packed
struct or union type
definition, specifies that each member (other than zero-width bitfields)
of the structure or union is placed to minimize the memory required. When
attached to an enum definition, it indicates that the smallest
integral type should be used.
Specifying this attribute for struct and union types is
equivalent to specifying the packed attribute on each of the
structure or union members. Specifying the `-fshort-enums'
flag on the line is equivalent to specifying the packed
attribute on all enum definitions.
In the following example struct my_packed_struct's members are
packed closely together, but the internal layout of its s member
is not packed--to do that, struct my_unpacked_struct would need to
be packed too.
struct my_unpacked_struct
{
char c;
int i;
};
struct __attribute__ ((__packed__)) my_packed_struct
{
char c;
int i;
struct my_unpacked_struct s;
};
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You may only specify this attribute on the definition of an enum,
struct or union, not on a typedef which does not
also define the enumerated type, structure or union.
transparent_union
union type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required. Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like const on
the referenced type must be respected, just as with normal pointer
conversions.
Second, the argument is passed to the function using the calling conventions of the first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.
Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons. For example, suppose the
wait function must accept either a value of type int * to
comply with Posix, or a value of type union wait * to comply with
the 4.1BSD interface. If wait's parameter were void *,
wait would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful. Instead, <sys/wait.h> might define the interface
as follows:
typedef union __attribute__ ((__transparent_union__))
{
int *__ip;
union wait *__up;
} wait_status_ptr_t;
pid_t wait (wait_status_ptr_t);
|
This interface allows either int * or union wait *
arguments to be passed, using the int * calling convention.
The program can call wait with arguments of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
|
With this interface, wait's implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
|
unused
union or a struct),
this attribute means that variables of that type are meant to appear
possibly unused. GCC will not produce a warning for any variables of
that type, even if the variable appears to do nothing. This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.
deprecated
deprecated (msg)
deprecated attribute results in a warning if the type
is used anywhere in the source file. This is useful when identifying
types that are expected to be removed in a future version of a program.
If possible, the warning also includes the location of the declaration
of the deprecated type, to enable users to easily find further
information about why the type is deprecated, or what they should do
instead. Note that the warnings only occur for uses and then only
if the type is being applied to an identifier that itself is not being
declared as deprecated.
typedef int T1 __attribute__ ((deprecated)); T1 x; typedef T1 T2; T2 y; typedef T1 T3 __attribute__ ((deprecated)); T3 z __attribute__ ((deprecated)); |
results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6. The optional msg argument, which must be a string, will be printed in the warning if present.
The deprecated attribute can also be used for functions and
variables (see section 6.30 Declaring Attributes of Functions, see section 6.36 Specifying Attributes of Variables.)
may_alias
Note that an object of a type with this attribute does not have any special semantics.
Example of use:
typedef short __attribute__((__may_alias__)) short_a;
int
main (void)
{
int a = 0x12345678;
short_a *b = (short_a *) &a;
b[1] = 0;
if (a == 0x12345678)
abort();
exit(0);
}
|
If you replaced short_a with short in the variable
declaration, the above program would abort when compiled with
`-fstrict-aliasing', which is on by default at `-O2' or
above in recent GCC versions.
visibility
Note that the type visibility is applied to vague linkage entities associated with the class (vtable, typeinfo node, etc.). In particular, if a class is thrown as an exception in one shared object and caught in another, the class must have default visibility. Otherwise the two shared objects will be unable to use the same typeinfo node and exception handling will break.
bit_order
bit_order
attribute overrides this default, allowing you to force allocation to
be MSB-first, LSB-first, or the opposite of whatever gcc defaults to. The
bit_order attribute takes an optional argument:
native
swapped
native.
lsb
msb
A short example demonstrates bitfield allocation:
struct __attribute__((bit_order(msb))) {
char a:3;
char b:3;
} foo = { 3, 5 };
|
With LSB-first allocation, foo.a will be in the 3 least
significant bits (mask 0x07) and foo.b will be in the next 3
bits (mask 0x38). With MSB-first allocation, foo.a will be in
the 3 most significant bits (mask 0xE0) and foo.b will be in
the next 3 bits (mask 0x1C).
Note that it is entirely up to the programmer to define bitfields that make sense when swapped. Consider:
struct __attribute__((bit_order(msb))) {
short a:7;
char b:6;
} foo = { 3, 5 };
|
On some targets, or if the struct is packed, GCC may only use
one byte of storage for A despite it being a short type.
Swapping the bit order of A would cause it to overlap B. Worse, the
bitfield for B may span bytes, so "swapping" would no longer be
defined as there is no "char" to swap within. To avoid such
problems, the programmer should either fully-define each underlying
type, or ensure that their target's ABI allocates enough space for
each underlying type regardless of how much of it is used.
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The keyword __alignof__ allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type. Its
syntax is just like sizeof.
For example, if the target machine requires a double value to be
aligned on an 8-byte boundary, then __alignof__ (double) is 8.
This is true on many RISC machines. On more traditional machine
designs, __alignof__ (double) is 4 or even 2.
Some machines never actually require alignment; they allow reference to any
data type even at an odd address. For these machines, __alignof__
reports the smallest alignment that GCC will give the data type, usually as
mandated by the target ABI.
If the operand of __alignof__ is an lvalue rather than a type,
its value is the required alignment for its type, taking into account
any minimum alignment specified with GCC's __attribute__
extension (see section 6.36 Specifying Attributes of Variables). For example, after this
declaration:
struct foo { int x; char y; } foo1;
|
the value of __alignof__ (foo1.y) is 1, even though its actual
alignment is probably 2 or 4, the same as __alignof__ (int).
It is an error to ask for the alignment of an incomplete type.
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By declaring a function inline, you can direct GCC to make calls to that function faster. One way GCC can achieve this is to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. You can also direct GCC to try to integrate all "simple enough" functions into their callers with the option `-finline-functions'.
GCC implements three different semantics of declaring a function
inline. One is available with `-std=gnu89' or
`-fgnu89-inline' or when gnu_inline attribute is present
on all inline declarations, another when
`-std=c99', `-std=c11',
`-std=gnu99' or `-std=gnu11'
(without `-fgnu89-inline'), and the third
is used when compiling C++.
To declare a function inline, use the inline keyword in its
declaration, like this:
static inline int
inc (int *a)
{
return (*a)++;
}
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If you are writing a header file to be included in ISO C90 programs, write
__inline__ instead of inline. See section 6.45 Alternate Keywords.
The three types of inlining behave similarly in two important cases:
when the inline keyword is used on a static function,
like the example above, and when a function is first declared without
using the inline keyword and then is defined with
inline, like this:
extern int inc (int *a);
inline int
inc (int *a)
{
return (*a)++;
}
|
In both of these common cases, the program behaves the same as if you
had not used the inline keyword, except for its speed.
When a function is both inline and static, if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GCC does not actually output assembler code for the
function, unless you specify the option `-fkeep-inline-functions'.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition). If there is a
nonintegrated call, then the function is compiled to assembler code as
usual. The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.
Note that certain usages in a function definition can make it unsuitable
for inline substitution. Among these usages are: use of varargs, use of
alloca, use of variable sized data types (see section 6.19 Arrays of Variable Length),
use of computed goto (see section 6.3 Labels as Values), use of nonlocal goto,
and nested functions (see section 6.4 Nested Functions). Using `-Winline'
will warn when a function marked inline could not be substituted,
and will give the reason for the failure.
As required by ISO C++, GCC considers member functions defined within
the body of a class to be marked inline even if they are
not explicitly declared with the inline keyword. You can
override this with `-fno-default-inline'; see section Options Controlling C++ Dialect.
GCC does not inline any functions when not optimizing unless you specify the `always_inline' attribute for the function, like this:
/* Prototype. */ inline void foo (const char) __attribute__((always_inline)); |
The remainder of this section is specific to GNU C90 inlining.
When an inline function is not static, then the compiler must assume
that there may be calls from other source files; since a global symbol can
be defined only once in any program, the function must not be defined in
the other source files, so the calls therein cannot be integrated.
Therefore, a non-static inline function is always compiled on its
own in the usual fashion.
If you specify both inline and extern in the function
definition, then the definition is used only for inlining. In no case
is the function compiled on its own, not even if you refer to its
address explicitly. Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.
This combination of inline and extern has almost the
effect of a macro. The way to use it is to put a function definition in
a header file with these keywords, and put another copy of the
definition (lacking inline and extern) in a library file.
The definition in the header file will cause most calls to the function
to be inlined. If any uses of the function remain, they will refer to
the single copy in the library.
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C has the concept of volatile objects. These are normally accessed by pointers and used for accessing hardware or inter-thread communication. The standard encourages compilers to refrain from optimizations concerning accesses to volatile objects, but leaves it implementation defined as to what constitutes a volatile access. The minimum requirement is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred. Thus an implementation is free to reorder and combine volatile accesses which occur between sequence points, but cannot do so for accesses across a sequence point. The use of volatile does not allow you to violate the restriction on updating objects multiple times between two sequence points.
Accesses to non-volatile objects are not ordered with respect to volatile accesses. You cannot use a volatile object as a memory barrier to order a sequence of writes to non-volatile memory. For instance:
int *ptr = something; volatile int vobj; *ptr = something; vobj = 1; |
Unless *ptr and vobj can be aliased, it is not guaranteed that the write to *ptr will have occurred by the time the update of vobj has happened. If you need this guarantee, you must use a stronger memory barrier such as:
int *ptr = something;
volatile int vobj;
*ptr = something;
asm volatile ("" : : : "memory");
vobj = 1;
|
A scalar volatile object is read when it is accessed in a void context:
volatile int *src = somevalue; *src; |
Such expressions are rvalues, and GCC implements this as a read of the volatile object being pointed to.
Assignments are also expressions and have an rvalue. However when assigning to a scalar volatile, the volatile object is not reread, regardless of whether the assignment expression's rvalue is used or not. If the assignment's rvalue is used, the value is that assigned to the volatile object. For instance, there is no read of vobj in all the following cases:
int obj; volatile int vobj; vobj = something; obj = vobj = something; obj ? vobj = onething : vobj = anotherthing; obj = (something, vobj = anotherthing); |
If you need to read the volatile object after an assignment has occurred, you must use a separate expression with an intervening sequence point.
As bitfields are not individually addressable, volatile bitfields may be implicitly read when written to, or when adjacent bitfields are accessed. Bitfield operations may be optimized such that adjacent bitfields are only partially accessed, if they straddle a storage unit boundary. For these reasons it is unwise to use volatile bitfields to access hardware.
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In an assembler instruction using asm, you can specify the
operands of the instruction using C expressions. This means you need not
guess which registers or memory locations will contain the data you want
to use.
You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.
For example, here is how to use the 68881's fsinx instruction:
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
|
Here angle is the C expression for the input operand while
result is that of the output operand. Each has `"f"' as its
operand constraint, saying that a floating point register is required.
The `=' in `=f' indicates that the operand is an output; all
output operands' constraints must use `='. The constraints use the
same language used in the machine description (see section 6.42 Constraints for asm Operands).
Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.
If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.
As of GCC version 3.1, it is also possible to specify input and output
operands using symbolic names which can be referenced within the
assembler code. These names are specified inside square brackets
preceding the constraint string, and can be referenced inside the
assembler code using %[name] instead of a percentage sign
followed by the operand number. Using named operands the above example
could look like:
asm ("fsinx %[angle],%[output]"
: [output] "=f" (result)
: [angle] "f" (angle));
|
Note that the symbolic operand names have no relation whatsoever to other C identifiers. You may use any name you like, even those of existing C symbols, but you must ensure that no two operands within the same assembler construct use the same symbolic name.
Output operand expressions must be lvalues; the compiler can check this.
The input operands need not be lvalues. The compiler cannot check
whether the operands have data types that are reasonable for the
instruction being executed. It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input. The extended asm feature is most often used for
machine instructions the compiler itself does not know exist. If
the output expression cannot be directly addressed (for example, it is a
bit-field), your constraint must allow a register. In that case, GCC
will use the register as the output of the asm, and then store
that register into the output.
The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character `+' to indicate such an operand and list it with the output operands. You should only use read-write operands when the constraints for the operand (or the operand in which only some of the bits are to be changed) allow a register.
You may, as an alternative, logically split its function into two
separate operands, one input operand and one write-only output
operand. The connection between them is expressed by constraints
which say they need to be in the same location when the instruction
executes. You can use the same C expression for both operands, or
different expressions. For example, here we write the (fictitious)
`combine' instruction with bar as its read-only source
operand and foo as its read-write destination:
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
|
The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A number in constraint is allowed only in an input operand and it must refer to an output operand.
Only a number in the constraint can guarantee that one operand will be in
the same place as another. The mere fact that foo is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code. The following would not
work reliably:
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
|
Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GCC knows no reason not to do so. For example, the
compiler might find a copy of the value of foo in one register and
use it for operand 1, but generate the output operand 0 in a different
register (copying it afterward to foo's own address). Of course,
since the register for operand 1 is not even mentioned in the assembler
code, the result will not work, but GCC can't tell that.
As of GCC version 3.1, one may write [name] instead of
the operand number for a matching constraint. For example:
asm ("cmoveq %1,%2,%[result]"
: [result] "=r"(result)
: "r" (test), "r"(new), "[result]"(old));
|
Sometimes you need to make an asm operand be a specific register,
but there's no matching constraint letter for that register by
itself. To force the operand into that register, use a local variable
for the operand and specify the register in the variable declaration.
See section 6.44 Variables in Specified Registers. Then for the asm operand, use any
register constraint letter that matches the register:
register int *p1 asm ("r0") = ...;
register int *p2 asm ("r1") = ...;
register int *result asm ("r0");
asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));
|
In the above example, beware that a register that is call-clobbered by
the target ABI will be overwritten by any function call in the
assignment, including library calls for arithmetic operators.
Also a register may be clobbered when generating some operations,
like variable shift, memory copy or memory move on x86.
Assuming it is a call-clobbered register, this may happen to r0
above by the assignment to p2. If you have to use such a
register, use temporary variables for expressions between the register
assignment and use:
int t1 = ...;
register int *p1 asm ("r0") = ...;
register int *p2 asm ("r1") = t1;
register int *result asm ("r0");
asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));
|
Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:
asm volatile ("movc3 %0,%1,%2"
: /* no outputs */
: "g" (from), "g" (to), "g" (count)
: "r0", "r1", "r2", "r3", "r4", "r5");
|
You may not write a clobber description in a way that overlaps with an
input or output operand. For example, you may not have an operand
describing a register class with one member if you mention that register
in the clobber list. Variables declared to live in specific registers
(see section 6.44 Variables in Specified Registers), and used as asm input or output operands must
have no part mentioned in the clobber description.
There is no way for you to specify that an input
operand is modified without also specifying it as an output
operand. Note that if all the output operands you specify are for this
purpose (and hence unused), you will then also need to specify
volatile for the asm construct, as described below, to
prevent GCC from deleting the asm statement as unused.
If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input.
If your assembler instruction can alter the condition code register, add `cc' to the list of clobbered registers. GCC on some machines represents the condition codes as a specific hardware register; `cc' serves to name this register. On other machines, the condition code is handled differently, and specifying `cc' has no effect. But it is valid no matter what the machine.
If your assembler instructions access memory in an unpredictable
fashion, add `memory' to the list of clobbered registers. This
will cause GCC to not keep memory values cached in registers across the
assembler instruction and not optimize stores or loads to that memory.
You will also want to add the volatile keyword if the memory
affected is not listed in the inputs or outputs of the asm, as
the `memory' clobber does not count as a side-effect of the
asm. If you know how large the accessed memory is, you can add
it as input or output but if this is not known, you should add
`memory'. As an example, if you access ten bytes of a string, you
can use a memory input like:
{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.
|
Note that in the following example the memory input is necessary,
otherwise GCC might optimize the store to x away:
int foo ()
{
int x = 42;
int *y = &x;
int result;
asm ("magic stuff accessing an 'int' pointed to by '%1'"
"=&d" (r) : "a" (y), "m" (*y));
return result;
}
|
You can put multiple assembler instructions together in a single
asm template, separated by the characters normally used in assembly
code for the system. A combination that works in most places is a newline
to break the line, plus a tab character to move to the instruction field
(written as `\n\t'). Sometimes semicolons can be used, if the
assembler allows semicolons as a line-breaking character. Note that some
assembler dialects use semicolons to start a comment.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like. Here
is an example of multiple instructions in a template; it assumes the
subroutine _foo accepts arguments in registers 9 and 10:
asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
: /* no outputs */
: "g" (from), "g" (to)
: "r9", "r10");
|
Unless an output operand has the `&' constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. See section 6.42.3 Constraint Modifier Characters.
If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the asm
construct, as follows:
asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
: "g" (result)
: "g" (input));
|
This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.
Speaking of labels, jumps from one asm to another are not
supported. The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize. See Extended asm with goto.
Usually the most convenient way to use these asm instructions is to
encapsulate them in macros that look like functions. For example,
#define sin(x) \
({ double __value, __arg = (x); \
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
__value; })
|
Here the variable __arg is used to make sure that the instruction
operates on a proper double value, and to accept only those
arguments x which can convert automatically to a double.
Another way to make sure the instruction operates on the correct data
type is to use a cast in the asm. This is different from using a
variable __arg in that it converts more different types. For
example, if the desired type were int, casting the argument to
int would accept a pointer with no complaint, while assigning the
argument to an int variable named __arg would warn about
using a pointer unless the caller explicitly casts it.
If an asm has output operands, GCC assumes for optimization
purposes the instruction has no side effects except to change the output
operands. This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression. Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.
You can prevent an asm instruction from being deleted
by writing the keyword volatile after
the asm. For example:
#define get_and_set_priority(new) \
({ int __old; \
asm volatile ("get_and_set_priority %0, %1" \
: "=g" (__old) : "g" (new)); \
__old; })
|
The volatile keyword indicates that the instruction has
important side-effects. GCC will not delete a volatile asm if
it is reachable. (The instruction can still be deleted if GCC can
prove that control-flow will never reach the location of the
instruction.) Note that even a volatile asm instruction
can be moved relative to other code, including across jump
instructions. For example, on many targets there is a system
register which can be set to control the rounding mode of
floating point operations. You might try
setting it with a volatile asm, like this PowerPC example:
asm volatile("mtfsf 255,%0" : : "f" (fpenv));
sum = x + y;
|
This will not work reliably, as the compiler may move the addition back
before the volatile asm. To make it work you need to add an
artificial dependency to the asm referencing a variable in the code
you don't want moved, for example:
asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv));
sum = x + y;
|
Similarly, you can't expect a
sequence of volatile asm instructions to remain perfectly
consecutive. If you want consecutive output, use a single asm.
Also, GCC will perform some optimizations across a volatile asm
instruction; GCC does not "forget everything" when it encounters
a volatile asm instruction the way some other compilers do.
An asm instruction without any output operands will be treated
identically to a volatile asm instruction.
It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following "store" instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary "test" and "compare" instructions because they don't have any output operands.
For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.
As of GCC version 4.5, asm goto may be used to have the assembly
jump to one or more C labels. In this form, a fifth section after the
clobber list contains a list of all C labels to which the assembly may jump.
Each label operand is implicitly self-named. The asm is also assumed
to fall through to the next statement.
This form of asm is restricted to not have outputs. This is due
to a internal restriction in the compiler that control transfer instructions
cannot have outputs. This restriction on asm goto may be lifted
in some future version of the compiler. In the mean time, asm goto
may include a memory clobber, and so leave outputs in memory.
int frob(int x)
{
int y;
asm goto ("frob %%r5, %1; jc %l[error]; mov (%2), %%r5"
: : "r"(x), "r"(&y) : "r5", "memory" : error);
return y;
error:
return -1;
}
|
In this (inefficient) example, the frob instruction sets the
carry bit to indicate an error. The jc instruction detects
this and branches to the error label. Finally, the output
of the frob instruction (%r5) is stored into the memory
for variable y, which is later read by the return statement.
void doit(void)
{
int i = 0;
asm goto ("mfsr %%r1, 123; jmp %%r1;"
".pushsection doit_table;"
".long %l0, %l1, %l2, %l3;"
".popsection"
: : : "r1" : label1, label2, label3, label4);
__builtin_unreachable ();
label1:
f1();
return;
label2:
f2();
return;
label3:
i = 1;
label4:
f3(i);
}
|
In this (also inefficient) example, the mfsr instruction reads
an address from some out-of-band machine register, and the following
jmp instruction branches to that address. The address read by
the mfsr instruction is assumed to have been previously set via
some application-specific mechanism to be one of the four values stored
in the doit_table section. Finally, the asm is followed
by a call to __builtin_unreachable to indicate that the asm
does not in fact fall through.
#define TRACE1(NUM) \
do { \
asm goto ("0: nop;" \
".pushsection trace_table;" \
".long 0b, %l0;" \
".popsection" \
: : : : trace#NUM); \
if (0) { trace#NUM: trace(); } \
} while (0)
#define TRACE TRACE1(__COUNTER__)
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In this example (which in fact inspired the asm goto feature)
we want on rare occasions to call the trace function; on other
occasions we'd like to keep the overhead to the absolute minimum.
The normal code path consists of a single nop instruction.
However, we record the address of this nop together with the
address of a label that calls the trace function. This allows
the nop instruction to be patched at runtime to be an
unconditional branch to the stored label. It is assumed that an
optimizing compiler will move the labeled block out of line, to
optimize the fall through path from the asm.
If you are writing a header file that should be includable in ISO C
programs, write __asm__ instead of asm. See section 6.45 Alternate Keywords.
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asm
Some targets require that GCC track the size of each instruction used in
order to generate correct code. Because the final length of an
asm is only known by the assembler, GCC must make an estimate as
to how big it will be. The estimate is formed by counting the number of
statements in the pattern of the asm and multiplying that by the
length of the longest instruction on that processor. Statements in the
asm are identified by newline characters and whatever statement
separator characters are supported by the assembler; on most processors
this is the `;' character.
Normally, GCC's estimate is perfectly adequate to ensure that correct code is generated, but it is possible to confuse the compiler if you use pseudo instructions or assembler macros that expand into multiple real instructions or if you use assembler directives that expand to more space in the object file than would be needed for a single instruction. If this happens then the assembler will produce a diagnostic saying that a label is unreachable.
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asm Operands
Here are specific details on what constraint letters you can use with
asm operands.
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
Side-effects aren't allowed in operands of inline asm, unless
`<' or `>' constraints are used, because there is no guarantee
that the side-effects will happen exactly once in an instruction that can update
the addressing register.
6.42.1 Simple Constraints Basic use of constraints. 6.42.2 Multiple Alternative Constraints When an insn has two alternative constraint-patterns. 6.42.3 Constraint Modifier Characters More precise control over effects of constraints. 6.42.4 Constraints for Particular Machines Special constraints for some particular machines.
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The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
TARGET_MEM_CONSTRAINT macro.
For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.
Note that in an output operand which can be matched by another operand, the constraint letter `o' is valid only when accompanied by both `<' (if the target machine has predecrement addressing) and `>' (if the target machine has preincrement addressing).
asm this constraint is only
allowed if the operand is used exactly once in an instruction that can
handle the side-effects. Not using an operand with `<' in constraint
string in the inline asm pattern at all or using it in multiple
instructions isn't valid, because the side-effects wouldn't be performed
or would be performed more than once. Furthermore, on some targets
the operand with `<' in constraint string must be accompanied by
special instruction suffixes like %U0 instruction suffix on PowerPC
or %P0 on IA-64.
asm the same restrictions
as for `<' apply.
const_double) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).
const_double or
const_vector) is allowed.
This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use `s' instead of `i'? Sometimes it allows better code to be generated.
For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a `moveq' instruction. We arrange for this to happen by defining the letter `K' to mean "any integer outside the range -128 to 127", and then specifying `Ks' in the operand constraints.
This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that `10' be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.
This is called a matching constraint and what it really means is
that the assembler has only a single operand that fills two roles
which asm distinguishes. For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
input-output operand:
addl #35,r12 |
Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.
`p' in the constraint must be accompanied by address_operand
as the predicate in the match_operand. This predicate interprets
the mode specified in the match_operand as the mode of the memory
reference for which the address would be valid.
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Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the `?' and `!' characters:
?
!
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Here are constraint modifier characters.
When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. `=' identifies an output; `+' identifies an operand that is both input and output; all other operands are assumed to be input only.
If you specify `=' or `+' in a constraint, you put it in the first character of the constraint string.
`&' applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires `&' while others do not. See, for example, the `movdf' insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber. See, for example, the `mulsi3' insn of the ARM.
`&' does not obviate the need to write `='.
define_peephole2
and define_splits performed after reload cannot rely on
`%' to make the intended insn match.
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Whenever possible, you should use the general-purpose constraint letters
in asm arguments, since they will convey meaning more readily to
people reading your code. Failing that, use the constraint letters
that usually have very similar meanings across architectures. The most
commonly used constraints are `m' and `r' (for memory and
general-purpose registers respectively; see section 6.42.1 Simple Constraints), and
`I', usually the letter indicating the most common
immediate-constant format.
Each architecture defines additional constraints. These constraints
are used by the compiler itself for instruction generation, as well as
for asm statements; therefore, some of the constraints are not
particularly useful for asm. Here is a summary of some of the
machine-dependent constraints available on some particular machines;
it includes both constraints that are useful for asm and
constraints that aren't. The compiler source file mentioned in the
table heading for each architecture is the definitive reference for
the meanings of that architecture's constraints.
Rsp
Rfb
Rsb
Rcr
Rcl
R0w
R1w
R2w
R3w
R02
R13
Rdi
Rhl
R23
Raa
Raw
Ral
Rqi
Rad
Rsi
Rhi
Rhc
Rra
Rfl
Rmm
Rpi
Rpa
Is3
IS1
IS2
IU2
In4
In5
In6
IM2
Ilb
Ilw
Sd
Sa
Si
Ss
Sf
Ss
S1
Int3
Int8
J
K
L
M
N
O
P
Qbi
Qsc
Wab
Wbc
BC as a base register, with an optional offset.
Wca
AX, BC, DE, or HL for the address, for calls.
Wcv
Wd2
DE as a base register, with an optional offset.
Wde
DE as a base register, without any offset.
Wfr
Wh1
HL as a base register, with an optional one-byte offset.
Whb
HL as a base register, with B or C as the index register.
Whl
HL as a base register, without any offset.
Ws1
SP as a base register, with an optional one-byte offset.
Y
A
AX register.
B
BC register.
D
DE register.
R
A through L registers.
S
SP register.
T
HL register.
Z08W
R8 register.
Z10W
R10 register.
Zint
R24 to R31).
a
A register.
b
B register.
c
C register.
d
D register.
e
E register.
h
H register.
l
L register.
v
w
PSW register.
x
X register.
Q
Symbol
Int08
Sint08
Sint16
Sint24
Uint04
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You can specify the name to be used in the assembler code for a C
function or variable by writing the asm (or __asm__)
keyword after the declarator as follows:
int foo asm ("myfoo") = 2;
|
This specifies that the name to be used for the variable foo in
the assembler code should be `myfoo' rather than the usual
`_foo'.
On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.
It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see 6.44 Variables in Specified Registers. GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.
You cannot use asm in this way in a function definition; but
you can get the same effect by writing a declaration for the function
before its definition and putting asm there, like this:
extern func () asm ("FUNC");
func (x, y)
int x, y;
/* ... */
|
It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.
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GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.
asm statement and the asm statement itself is
not deleted. The compiler's data flow analysis is capable of determining
where the specified registers contain live values, and where they are
available for other uses. Stores into local register variables may be deleted
when they appear to be dead according to dataflow analysis. References
to local register variables may be deleted or moved or simplified.
These local variables are sometimes convenient for use with the extended
asm feature (see section 6.41 Assembler Instructions with C Expression Operands), if you want to write one
output of the assembler instruction directly into a particular register.
(This will work provided the register you specify fits the constraints
specified for that operand in the asm.)
6.44.1 Defining Global Register Variables 6.44.2 Specifying Registers for Local Variables
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You can define a global register variable in GNU C like this:
register int *foo asm ("a5");
|
Here a5 is the name of the register which should be used. Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.
Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type. The register
a5 would be a good choice on a 68000 for a variable of pointer
type. On machines with register windows, be sure to choose a "global"
register that is not affected magically by the function call mechanism.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.
Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.
It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).
It is not safe for one function that uses a global register variable to
call another such function foo by way of a third function
lose that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared). This is
because lose might save the register and put some other value there.
For example, you can't expect a global register variable to be available in
the comparison-function that you pass to qsort, since qsort
might have put something else in that register. (If you are prepared to
recompile qsort with the same global register variable, you can
solve this problem.)
If you want to recompile qsort or other source files which do not
actually use your global register variable, so that they will not use that
register for any other purpose, then it suffices to specify the compiler
option `-ffixed-reg'. You need not actually add a global
register declaration to their source code.
A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.
On most machines, longjmp will restore to each global register
variable the value it had at the time of the setjmp. On some
machines, however, longjmp will not change the value of global
register variables. To be portable, the function that called setjmp
should make other arrangements to save the values of the global register
variables, and to restore them in a longjmp. This way, the same
thing will happen regardless of what longjmp does.
All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.
Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.
On the SPARC, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as getwd, as well
as the subroutines for division and remainder, modify g3 and g4. g1 and
g2 are local temporaries.
On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those.
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You can define a local register variable with a specified register like this:
register int *foo asm ("a5");
|
Here a5 is the name of the register which should be used. Note
that this is the same syntax used for defining global register
variables, but for a local variable it would appear within a function.
Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section 6.41 Assembler Instructions with C Expression Operands). Both of these things generally require that you conditionalize your program according to cpu type.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live.
This option does not guarantee that GCC will generate code that has
this variable in the register you specify at all times. You may not
code an explicit reference to this register in the assembler
instruction template part of an asm statement and assume it will
always refer to this variable. However, using the variable as an
asm operand guarantees that the specified register is used
for the operand.
Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.
As for global register variables, it's recommended that you choose a
register which is normally saved and restored by function calls on
your machine, so that library routines will not clobber it. A common
pitfall is to initialize multiple call-clobbered registers with
arbitrary expressions, where a function call or library call for an
arithmetic operator will overwrite a register value from a previous
assignment, for example r0 below:
register int *p1 asm ("r0") = ...;
register int *p2 asm ("r1") = ...;
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`-ansi' and the various `-std' options disable certain
keywords. This causes trouble when you want to use GNU C extensions, or
a general-purpose header file that should be usable by all programs,
including ISO C programs. The keywords asm, typeof and
inline are not available in programs compiled with
`-ansi' or `-std' (although inline can be used in a
program compiled with `-std=c99' or `-std=c11'). The
ISO C99 keyword
restrict is only available when `-std=gnu99' (which will
eventually be the default) or `-std=c99' (or the equivalent
`-std=iso9899:1999'), or an option for a later standard
version, is used.
The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword. For example, use __asm__
instead of asm, and __inline__ instead of inline.
Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:
#ifndef __GNUC__ #define __asm__ asm #endif |
`-pedantic' and other options cause warnings for many GNU C extensions.
You can
prevent such warnings within one expression by writing
__extension__ before the expression. __extension__ has no
effect aside from this.
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enum Types
You can define an enum tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
struct foo without describing the elements. A later declaration
which does specify the possible values completes the type.
You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.
This extension may not be very useful, but it makes the handling of
enum more consistent with the way struct and union
are handled.
This extension is not supported by GNU C++.
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GCC provides three magic variables which hold the name of the current
function, as a string. The first of these is __func__, which
is part of the C99 standard:
The identifier __func__ is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration
static const char __func__[] = "function-name"; |
appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.
__FUNCTION__ is another name for __func__. Older
versions of GCC recognize only this name. However, it is not
standardized. For maximum portability, we recommend you use
__func__, but provide a fallback definition with the
preprocessor:
#if __STDC_VERSION__ < 199901L # if __GNUC__ >= 2 # define __func__ __FUNCTION__ # else # define __func__ "<unknown>" # endif #endif |
In C, __PRETTY_FUNCTION__ is yet another name for
__func__. However, in C++, __PRETTY_FUNCTION__ contains
the type signature of the function as well as its bare name. For
example, this program:
extern "C" {
extern int printf (char *, ...);
}
class a {
public:
void sub (int i)
{
printf ("__FUNCTION__ = %s\n", __FUNCTION__);
printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
}
};
int
main (void)
{
a ax;
ax.sub (0);
return 0;
}
|
gives this output:
__FUNCTION__ = sub __PRETTY_FUNCTION__ = void a::sub(int) |
These identifiers are not preprocessor macros. In GCC 3.3 and
earlier, in C only, __FUNCTION__ and __PRETTY_FUNCTION__
were treated as string literals; they could be used to initialize
char arrays, and they could be concatenated with other string
literals. GCC 3.4 and later treat them as variables, like
__func__. In C++, __FUNCTION__ and
__PRETTY_FUNCTION__ have always been variables.
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These functions may be used to get information about the callers of a function.
0 yields the return address
of the current function, a value of 1 yields the return address
of the caller of the current function, and so forth. When inlining
the expected behavior is that the function will return the address of
the function that will be returned to. To work around this behavior use
the noinline function attribute.
The level argument must be a constant integer.
On some machines it may be impossible to determine the return address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return 0 or a
random value. In addition, __builtin_frame_address may be used
to determine if the top of the stack has been reached.
Additional post-processing of the returned value may be needed, see
__builtin_extract_return_address.
This function should only be used with a nonzero argument for debugging purposes.
__builtin_return_address may have to be fed
through this function to get the actual encoded address. For example, on the
31-bit S/390 platform the highest bit has to be masked out, or on SPARC
platforms an offset has to be added for the true next instruction to be
executed.
If no fixup is needed, this function simply passes through addr.
__builtin_extract_return_address.
__builtin_return_address, but it
returns the address of the function frame rather than the return address
of the function. Calling __builtin_frame_address with a value of
0 yields the frame address of the current function, a value of
1 yields the frame address of the caller of the current function,
and so forth.
The frame is the area on the stack which holds local variables and saved
registers. The frame address is normally the address of the first word
pushed on to the stack by the function. However, the exact definition
depends upon the processor and the calling convention. If the processor
has a dedicated frame pointer register, and the function has a frame,
then __builtin_frame_address will return the value of the frame
pointer register.
On some machines it may be impossible to determine the frame address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return 0 if
the first frame pointer is properly initialized by the startup code.
This function should only be used with a nonzero argument for debugging purposes.
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On some targets, the instruction set contains SIMD vector instructions that operate on multiple values contained in one large register at the same time. For example, on the i386 the MMX, 3DNow! and SSE extensions can be used this way.
The first step in using these extensions is to provide the necessary data
types. This should be done using an appropriate typedef:
typedef int v4si __attribute__ ((vector_size (16))); |
The int type specifies the base type, while the attribute specifies
the vector size for the variable, measured in bytes. For example, the
declaration above causes the compiler to set the mode for the v4si
type to be 16 bytes wide and divided into int sized units. For
a 32-bit int this means a vector of 4 units of 4 bytes, and the
corresponding mode of foo will be V4SI.
The vector_size attribute is only applicable to integral and
float scalars, although arrays, pointers, and function return values
are allowed in conjunction with this construct.
All the basic integer types can be used as base types, both as signed
and as unsigned: char, short, int, long,
long long. In addition, float and double can be
used to build floating-point vector types.
Specifying a combination that is not valid for the current architecture
will cause GCC to synthesize the instructions using a narrower mode.
For example, if you specify a variable of type V4SI and your
architecture does not allow for this specific SIMD type, GCC will
produce code that uses 4 SIs.
The types defined in this manner can be used with a subset of normal C
operations. Currently, GCC will allow using the following operators
on these types: +, -, *, /, unary minus, ^, |, &, ~, %.
The operations behave like C++ valarrays. Addition is defined as
the addition of the corresponding elements of the operands. For
example, in the code below, each of the 4 elements in a will be
added to the corresponding 4 elements in b and the resulting
vector will be stored in c.
typedef int v4si __attribute__ ((vector_size (16))); v4si a, b, c; c = a + b; |
Subtraction, multiplication, division, and the logical operations operate in a similar manner. Likewise, the result of using the unary minus or complement operators on a vector type is a vector whose elements are the negative or complemented values of the corresponding elements in the operand.
In C it is possible to use shifting operators <<, >> on
integer-type vectors. The operation is defined as following: {a0,
a1, ..., an} >> {b0, b1, ..., bn} == {a0 >> b0, a1 >> b1,
..., an >> bn}. Vector operands must have the same number of
elements.
For the convenience in C it is allowed to use a binary vector operation where one operand is a scalar. In that case the compiler will transform the scalar operand into a vector where each element is the scalar from the operation. The transformation will happen only if the scalar could be safely converted to the vector-element type. Consider the following code.
typedef int v4si __attribute__ ((vector_size (16)));
v4si a, b, c;
long l;
a = b + 1; /* a = b + {1,1,1,1}; */
a = 2 * b; /* a = {2,2,2,2} * b; */
a = l + a; /* Error, cannot convert long to int. */
|
In C vectors can be subscripted as if the vector were an array with the same number of elements and base type. Out of bound accesses invoke undefined behavior at runtime. Warnings for out of bound accesses for vector subscription can be enabled with `-Warray-bounds'.
In GNU C vector comparison is supported within standard comparison
operators: ==, !=, <, <=, >, >=. Comparison operands can be
vector expressions of integer-type or real-type. Comparison between
integer-type vectors and real-type vectors are not supported. The
result of the comparison is a vector of the same width and number of
elements as the comparison operands with a signed integral element
type.
Vectors are compared element-wise producing 0 when comparison is false and -1 (constant of the appropriate type where all bits are set) otherwise. Consider the following example.
typedef int v4si __attribute__ ((vector_size (16)));
v4si a = {1,2,3,4};
v4si b = {3,2,1,4};
v4si c;
c = a > b; /* The result would be {0, 0,-1, 0} */
c = a == b; /* The result would be {0,-1, 0,-1} */
|
Vector shuffling is available using functions
__builtin_shuffle (vec, mask) and
__builtin_shuffle (vec0, vec1, mask).
Both functions construct a permutation of elements from one or two
vectors and return a vector of the same type as the input vector(s).
The mask is an integral vector with the same width (W)
and element count (N) as the output vector.
The elements of the input vectors are numbered in memory ordering of vec0 beginning at 0 and vec1 beginning at N. The elements of mask are considered modulo N in the single-operand case and modulo 2*N in the two-operand case.
Consider the following example,
typedef int v4si __attribute__ ((vector_size (16)));
v4si a = {1,2,3,4};
v4si b = {5,6,7,8};
v4si mask1 = {0,1,1,3};
v4si mask2 = {0,4,2,5};
v4si res;
res = __builtin_shuffle (a, mask1); /* res is {1,2,2,4} */
res = __builtin_shuffle (a, b, mask2); /* res is {1,5,3,6} */
|
Note that __builtin_shuffle is intentionally semantically
compatible with the OpenCL shuffle and shuffle2 functions.
You can declare variables and use them in function calls and returns, as well as in assignments and some casts. You can specify a vector type as a return type for a function. Vector types can also be used as function arguments. It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size).
You cannot operate between vectors of different lengths or different signedness without a cast.
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GCC implements for both C and C++ a syntactic extension to implement
the offsetof macro.
primary:
"__builtin_offsetof" "(" |
This extension is sufficient such that
#define offsetof(type, member) __builtin_offsetof (type, member) |
is a suitable definition of the offsetof macro. In C++, type
may be dependent. In either case, member may consist of a single
identifier, or a sequence of member accesses and array references.
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The following builtins are intended to be compatible with those described in the Intel Itanium Processor-specific Application Binary Interface, section 7.4. As such, they depart from the normal GCC practice of using the "__builtin_" prefix, and further that they are overloaded such that they work on multiple types.
The definition given in the Intel documentation allows only for the use of
the types int, long, long long as well as their unsigned
counterparts. GCC will allow any integral scalar or pointer type that is
1, 2, 4 or 8 bytes in length.
Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning will be generated and a call an external function will be generated. The external function will carry the same name as the builtin, with an additional suffix `_n' where n is the size of the data type.
In most cases, these builtins are considered a full barrier. That is, no memory operand will be moved across the operation, either forward or backward. Further, instructions will be issued as necessary to prevent the processor from speculating loads across the operation and from queuing stores after the operation.
All of the routines are described in the Intel documentation to take "an optional list of variables protected by the memory barrier". It's not clear what is meant by that; it could mean that only the following variables are protected, or it could mean that these variables should in addition be protected. At present GCC ignores this list and protects all variables which are globally accessible. If in the future we make some use of this list, an empty list will continue to mean all globally accessible variables.
type __sync_fetch_and_add (type *ptr, type value, ...)
type __sync_fetch_and_sub (type *ptr, type value, ...)
type __sync_fetch_and_or (type *ptr, type value, ...)
type __sync_fetch_and_and (type *ptr, type value, ...)
type __sync_fetch_and_xor (type *ptr, type value, ...)
type __sync_fetch_and_nand (type *ptr, type value, ...)
{ tmp = *ptr; *ptr op= value; return tmp; }
{ tmp = *ptr; *ptr = ~(tmp & value); return tmp; } // nand
|
Note: GCC 4.4 and later implement __sync_fetch_and_nand
builtin as *ptr = ~(tmp & value) instead of *ptr = ~tmp & value.
type __sync_add_and_fetch (type *ptr, type value, ...)
type __sync_sub_and_fetch (type *ptr, type value, ...)
type __sync_or_and_fetch (type *ptr, type value, ...)
type __sync_and_and_fetch (type *ptr, type value, ...)
type __sync_xor_and_fetch (type *ptr, type value, ...)
type __sync_nand_and_fetch (type *ptr, type value, ...)
{ *ptr op= value; return *ptr; }
{ *ptr = ~(*ptr & value); return *ptr; } // nand
|
Note: GCC 4.4 and later implement __sync_nand_and_fetch
builtin as *ptr = ~(*ptr & value) instead of
*ptr = ~*ptr & value.
bool __sync_bool_compare_and_swap (type *ptr, type oldval, type newval, ...)
type __sync_val_compare_and_swap (type *ptr, type oldval, type newval, ...)
*ptr is oldval, then write newval into
*ptr.
The "bool" version returns true if the comparison is successful and
newval was written. The "val" version returns the contents
of *ptr before the operation.
__sync_synchronize (...)
type __sync_lock_test_and_set (type *ptr, type value, ...)
*ptr, and returns the previous contents of
*ptr.
Many targets have only minimal support for such locks, and do not support
a full exchange operation. In this case, a target may support reduced
functionality here by which the only valid value to store is the
immediate constant 1. The exact value actually stored in *ptr
is implementation defined.
This builtin is not a full barrier, but rather an acquire barrier. This means that references after the builtin cannot move to (or be speculated to) before the builtin, but previous memory stores may not be globally visible yet, and previous memory loads may not yet be satisfied.
void __sync_lock_release (type *ptr, ...)
__sync_lock_test_and_set.
Normally this means writing the constant 0 to *ptr.
This builtin is not a full barrier, but rather a release barrier. This means that all previous memory stores are globally visible, and all previous memory loads have been satisfied, but following memory reads are not prevented from being speculated to before the barrier.
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The following built-in functions approximately match the requirements for C++11 memory model. Many are similar to the `__sync' prefixed built-in functions, but all also have a memory model parameter. These are all identified by being prefixed with `__atomic', and most are overloaded such that they work with multiple types.
GCC will allow any integral scalar or pointer type that is 1, 2, 4, or 8 bytes in length. 16-byte integral types are also allowed if `__int128' (see section 6.8 128-bits integers) is supported by the architecture.
Target architectures are encouraged to provide their own patterns for each of these built-in functions. If no target is provided, the original non-memory model set of `__sync' atomic built-in functions will be utilized, along with any required synchronization fences surrounding it in order to achieve the proper behaviour. Execution in this case is subject to the same restrictions as those built-in functions.
If there is no pattern or mechanism to provide a lock free instruction sequence, a call is made to an external routine with the same parameters to be resolved at runtime.
The four non-arithmetic functions (load, store, exchange, and compare_exchange) all have a generic version as well. This generic version will work on any data type. If the data type size maps to one of the integral sizes which may have lock free support, the generic version will utilize the lock free built-in function. Otherwise an external call is left to be resolved at runtime. This external call will be the same format with the addition of a `size_t' parameter inserted as the first parameter indicating the size of the object being pointed to. All objects must be the same size.
There are 6 different memory models which can be specified. These map to the same names in the C++11 standard. Refer there or to the GCC wiki on atomic synchronization for more detailed definitions. These memory models integrate both barriers to code motion as well as synchronization requirements with other threads. These are listed in approximately ascending order of strength.
__ATOMIC_RELAXED
__ATOMIC_CONSUME
__ATOMIC_ACQUIRE
__ATOMIC_RELEASE
__ATOMIC_ACQ_REL
__ATOMIC_SEQ_CST
When implementing patterns for these built-in functions , the memory model
parameter can be ignored as long as the pattern implements the most
restrictive __ATOMIC_SEQ_CST model. Any of the other memory models
will execute correctly with this memory model but they may not execute as
efficiently as they could with a more appropriate implemention of the
relaxed requirements.
Note that the C++11 standard allows for the memory model parameter to be
determined at runtime rather than at compile time. These built-in
functions will map any runtime value to __ATOMIC_SEQ_CST rather
than invoke a runtime library call or inline a switch statement. This is
standard compliant, safe, and the simplest approach for now.
The memory model parameter is a signed int, but only the lower 8 bits are reserved for the memory model. The remainder of the signed int is reserved for future use and should be 0. Use of the predefined atomic values will ensure proper usage.
*ptr.
The valid memory model variants are
__ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE,
and __ATOMIC_CONSUME.
*ptr in *ret.
val into *ptr.
The valid memory model variants are
__ATOMIC_RELAXED, __ATOMIC_SEQ_CST, and __ATOMIC_RELEASE.
*val into *ptr.
*ptr, and returns the previous contents of
*ptr.
The valid memory model variants are
__ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE,
__ATOMIC_RELEASE, and __ATOMIC_ACQ_REL.
*val into *ptr. The original value
of *ptr will be copied into *ret.
*ptr with the contents of
*expected and if equal, writes desired into
*ptr. If they are not equal, the current contents of
*ptr is written into *expected. weak is true
for weak compare_exchange, and false for the strong variation. Many targets
only offer the strong variation and ignore the parameter. When in doubt, use
the strong variation.
True is returned if desired is written into
*ptr and the execution is considered to conform to the
memory model specified by success_memmodel. There are no
restrictions on what memory model can be used here.
False is returned otherwise, and the execution is considered to conform
to failure_memmodel. This memory model cannot be
__ATOMIC_RELEASE nor __ATOMIC_ACQ_REL. It also cannot be a
stronger model than that specified by success_memmodel.
__atomic_compare_exchange. The function is virtually identical to
__atomic_compare_exchange_n, except the desired value is also a
pointer.
{ *ptr op= val; return *ptr; }
|
All memory models are valid.
*ptr. That is,
{ tmp = *ptr; *ptr op= val; return tmp; }
|
All memory models are valid.
This built-in function performs an atomic test-and-set operation on
the byte at *ptr. The byte is set to some implementation
defined non-zero "set" value and the return value is true if and only
if the previous contents were "set".
All memory models are valid.
This built-in function performs an atomic clear operation on
*ptr. After the operation, *ptr will contain 0.
The valid memory model variants are
__ATOMIC_RELAXED, __ATOMIC_SEQ_CST, and
__ATOMIC_RELEASE.
This built-in function acts as a synchronization fence between threads based on the specified memory model.
All memory orders are valid.
This built-in function acts as a synchronization fence between a thread and signal handlers based in the same thread.
All memory orders are valid.
This built-in function returns true if objects of size bytes will always generate lock free atomic instructions for the target architecture. size must resolve to a compile time constant and the result also resolves to compile time constant.
ptr is an optional pointer to the object which may be used to determine alignment. A value of 0 indicates typical alignment should be used. The compiler may also ignore this parameter.
if (_atomic_always_lock_free (sizeof (long long), 0)) |
This built-in function returns true if objects of size bytes will always
generate lock free atomic instructions for the target architecture. If
it is not known to be lock free a call is made to a runtime routine named
__atomic_is_lock_free.
ptr is an optional pointer to the object which may be used to determine alignment. A value of 0 indicates typical alignment should be used. The compiler may also ignore this parameter.
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GCC implements a limited buffer overflow protection mechanism that can prevent some buffer overflow attacks.
__builtin_object_size never evaluates
its arguments for side-effects. If there are any side-effects in them, it
returns (size_t) -1 for type 0 or 1 and (size_t) 0
for type 2 or 3. If there are multiple objects ptr can
point to and all of them are known at compile time, the returned number
is the maximum of remaining byte counts in those objects if type & 2 is
0 and minimum if nonzero. If it is not possible to determine which objects
ptr points to at compile time, __builtin_object_size should
return (size_t) -1 for type 0 or 1 and (size_t) 0
for type 2 or 3.
type is an integer constant from 0 to 3. If the least significant bit is clear, objects are whole variables, if it is set, a closest surrounding subobject is considered the object a pointer points to. The second bit determines if maximum or minimum of remaining bytes is computed.
struct V { char buf1[10]; int b; char buf2[10]; } var;
char *p = &var.buf1[1], *q = &var.b;
/* Here the object p points to is var. */
assert (__builtin_object_size (p, 0) == sizeof (var) - 1);
/* The subobject p points to is var.buf1. */
assert (__builtin_object_size (p, 1) == sizeof (var.buf1) - 1);
/* The object q points to is var. */
assert (__builtin_object_size (q, 0)
== (char *) (&var + 1) - (char *) &var.b);
/* The subobject q points to is var.b. */
assert (__builtin_object_size (q, 1) == sizeof (var.b));
|
There are built-in functions added for many common string operation
functions, e.g., for memcpy __builtin___memcpy_chk
built-in is provided. This built-in has an additional last argument,
which is the number of bytes remaining in object the dest
argument points to or (size_t) -1 if the size is not known.
The built-in functions are optimized into the normal string functions
like memcpy if the last argument is (size_t) -1 or if
it is known at compile time that the destination object will not
be overflown. If the compiler can determine at compile time the
object will be always overflown, it issues a warning.
The intended use can be e.g.
#undef memcpy #define bos0(dest) __builtin_object_size (dest, 0) #define memcpy(dest, src, n) \ __builtin___memcpy_chk (dest, src, n, bos0 (dest)) char *volatile p; char buf[10]; /* It is unknown what object p points to, so this is optimized into plain memcpy - no checking is possible. */ memcpy (p, "abcde", n); /* Destination is known and length too. It is known at compile time there will be no overflow. */ memcpy (&buf[5], "abcde", 5); /* Destination is known, but the length is not known at compile time. This will result in __memcpy_chk call that can check for overflow at runtime. */ memcpy (&buf[5], "abcde", n); /* Destination is known and it is known at compile time there will be overflow. There will be a warning and __memcpy_chk call that will abort the program at runtime. */ memcpy (&buf[6], "abcde", 5); |
Such built-in functions are provided for memcpy, mempcpy,
memmove, memset, strcpy, stpcpy, strncpy,
strcat and strncat.
There are also checking built-in functions for formatted output functions.
int __builtin___sprintf_chk (char *s, int flag, size_t os, const char *fmt, ...);
int __builtin___snprintf_chk (char *s, size_t maxlen, int flag, size_t os,
const char *fmt, ...);
int __builtin___vsprintf_chk (char *s, int flag, size_t os, const char *fmt,
va_list ap);
int __builtin___vsnprintf_chk (char *s, size_t maxlen, int flag, size_t os,
const char *fmt, va_list ap);
|
The added flag argument is passed unchanged to __sprintf_chk
etc. functions and can contain implementation specific flags on what
additional security measures the checking function might take, such as
handling %n differently.
The os argument is the object size s points to, like in the
other built-in functions. There is a small difference in the behavior
though, if os is (size_t) -1, the built-in functions are
optimized into the non-checking functions only if flag is 0, otherwise
the checking function is called with os argument set to
(size_t) -1.
In addition to this, there are checking built-in functions
__builtin___printf_chk, __builtin___vprintf_chk,
__builtin___fprintf_chk and __builtin___vfprintf_chk.
These have just one additional argument, flag, right before
format string fmt. If the compiler is able to optimize them to
fputc etc. functions, it will, otherwise the checking function
should be called and the flag argument passed to it.
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GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.
The remaining functions are provided for optimization purposes.
GCC includes built-in versions of many of the functions in the standard
C library. The versions prefixed with __builtin_ will always be
treated as having the same meaning as the C library function even if you
specify the `-fno-builtin' option. (see section 3.4 Options Controlling C Dialect)
Many of these functions are only optimized in certain cases; if they are
not optimized in a particular case, a call to the library function will
be emitted.
Outside strict ISO C mode (`-ansi', `-std=c90',
`-std=c99' or `-std=c11'), the functions
_exit, alloca, bcmp, bzero,
dcgettext, dgettext, dremf, dreml,
drem, exp10f, exp10l, exp10, ffsll,
ffsl, ffs, fprintf_unlocked,
fputs_unlocked, gammaf, gammal, gamma,
gammaf_r, gammal_r, gamma_r, gettext,
index, isascii, j0f, j0l, j0,
j1f, j1l, j1, jnf, jnl, jn,
lgammaf_r, lgammal_r, lgamma_r, mempcpy,
pow10f, pow10l, pow10, printf_unlocked,
rindex, scalbf, scalbl, scalb,
signbit, signbitf, signbitl, signbitd32,
signbitd64, signbitd128, significandf,
significandl, significand, sincosf,
sincosl, sincos, stpcpy, stpncpy,
strcasecmp, strdup, strfmon, strncasecmp,
strndup, toascii, y0f, y0l, y0,
y1f, y1l, y1, ynf, ynl and
yn
may be handled as built-in functions.
All these functions have corresponding versions
prefixed with __builtin_, which may be used even in strict C90
mode.
The ISO C99 functions
_Exit, acoshf, acoshl, acosh, asinhf,
asinhl, asinh, atanhf, atanhl, atanh,
cabsf, cabsl, cabs, cacosf, cacoshf,
cacoshl, cacosh, cacosl, cacos,
cargf, cargl, carg, casinf, casinhf,
casinhl, casinh, casinl, casin,
catanf, catanhf, catanhl, catanh,
catanl, catan, cbrtf, cbrtl, cbrt,
ccosf, ccoshf, ccoshl, ccosh, ccosl,
ccos, cexpf, cexpl, cexp, cimagf,
cimagl, cimag, clogf, clogl, clog,
conjf, conjl, conj, copysignf, copysignl,
copysign, cpowf, cpowl, cpow, cprojf,
cprojl, cproj, crealf, creall, creal,
csinf, csinhf, csinhl, csinh, csinl,
csin, csqrtf, csqrtl, csqrt, ctanf,
ctanhf, ctanhl, ctanh, ctanl, ctan,
erfcf, erfcl, erfc, erff, erfl,
erf, exp2f, exp2l, exp2, expm1f,
expm1l, expm1, fdimf, fdiml, fdim,
fmaf, fmal, fmaxf, fmaxl, fmax,
fma, fminf, fminl, fmin, hypotf,
hypotl, hypot, ilogbf, ilogbl, ilogb,
imaxabs, isblank, iswblank, lgammaf,
lgammal, lgamma, llabs, llrintf, llrintl,
llrint, llroundf, llroundl, llround,
log1pf, log1pl, log1p, log2f, log2l,
log2, logbf, logbl, logb, lrintf,
lrintl, lrint, lroundf, lroundl,
lround, nearbyintf, nearbyintl, nearbyint,
nextafterf, nextafterl, nextafter,
nexttowardf, nexttowardl, nexttoward,
remainderf, remainderl, remainder, remquof,
remquol, remquo, rintf, rintl, rint,
roundf, roundl, round, scalblnf,
scalblnl, scalbln, scalbnf, scalbnl,
scalbn, snprintf, tgammaf, tgammal,
tgamma, truncf, truncl, trunc,
vfscanf, vscanf, vsnprintf and vsscanf
are handled as built-in functions
except in strict ISO C90 mode (`-ansi' or `-std=c90').
There are also built-in versions of the ISO C99 functions
acosf, acosl, asinf, asinl, atan2f,
atan2l, atanf, atanl, ceilf, ceill,
cosf, coshf, coshl, cosl, expf,
expl, fabsf, fabsl, floorf, floorl,
fmodf, fmodl, frexpf, frexpl, ldexpf,
ldexpl, log10f, log10l, logf, logl,
modfl, modf, powf, powl, sinf,
sinhf, sinhl, sinl, sqrtf, sqrtl,
tanf, tanhf, tanhl and tanl
that are recognized in any mode since ISO C90 reserves these names for
the purpose to which ISO C99 puts them. All these functions have
corresponding versions prefixed with __builtin_.
The ISO C94 functions
iswalnum, iswalpha, iswcntrl, iswdigit,
iswgraph, iswlower, iswprint, iswpunct,
iswspace, iswupper, iswxdigit, towlower and
towupper
are handled as built-in functions
except in strict ISO C90 mode (`-ansi' or `-std=c90').
The ISO C90 functions
abort, abs, acos, asin, atan2,
atan, calloc, ceil, cosh, cos,
exit, exp, fabs, floor, fmod,
fprintf, fputs, frexp, fscanf,
isalnum, isalpha, iscntrl, isdigit,
isgraph, islower, isprint, ispunct,
isspace, isupper, isxdigit, tolower,
toupper, labs, ldexp, log10, log,
malloc, memchr, memcmp, memcpy,
memset, modf, pow, printf, putchar,
puts, scanf, sinh, sin, snprintf,
sprintf, sqrt, sscanf, strcat,
strchr, strcmp, strcpy, strcspn,
strlen, strncat, strncmp, strncpy,
strpbrk, strrchr, strspn, strstr,
tanh, tan, vfprintf, vprintf and vsprintf
are all recognized as built-in functions unless
`-fno-builtin' is specified (or `-fno-builtin-function'
is specified for an individual function). All of these functions have
corresponding versions prefixed with __builtin_.
GCC provides built-in versions of the ISO C99 floating point comparison
macros that avoid raising exceptions for unordered operands. They have
the same names as the standard macros ( isgreater,
isgreaterequal, isless, islessequal,
islessgreater, and isunordered) , with __builtin_
prefixed. We intend for a library implementor to be able to simply
#define each standard macro to its built-in equivalent.
In the same fashion, GCC provides fpclassify, isfinite,
isinf_sign and isnormal built-ins used with
__builtin_ prefixed. The isinf and isnan
builtins appear both with and without the __builtin_ prefix.
You can use the built-in function __builtin_types_compatible_p to
determine whether two types are the same.
This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions.
This built-in function ignores top level qualifiers (e.g., const,
volatile). For example, int is equivalent to const
int.
The type int[] and int[5] are compatible. On the other
hand, int and char * are not compatible, even if the size
of their types, on the particular architecture are the same. Also, the
amount of pointer indirection is taken into account when determining
similarity. Consequently, short * is not similar to
short **. Furthermore, two types that are typedefed are
considered compatible if their underlying types are compatible.
An enum type is not considered to be compatible with another
enum type even if both are compatible with the same integer
type; this is what the C standard specifies.
For example, enum {foo, bar} is not similar to
enum {hot, dog}.
You would typically use this function in code whose execution varies depending on the arguments' types. For example:
#define foo(x) \
({ \
typeof (x) tmp = (x); \
if (__builtin_types_compatible_p (typeof (x), long double)) \
tmp = foo_long_double (tmp); \
else if (__builtin_types_compatible_p (typeof (x), double)) \
tmp = foo_double (tmp); \
else if (__builtin_types_compatible_p (typeof (x), float)) \
tmp = foo_float (tmp); \
else \
abort (); \
tmp; \
})
|
Note: This construct is only available for C.
You can use the built-in function __builtin_choose_expr to
evaluate code depending on the value of a constant expression. This
built-in function returns exp1 if const_exp, which is an
integer constant expression, is nonzero. Otherwise it returns exp2.
This built-in function is analogous to the `? :' operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that was not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.
This built-in function can return an lvalue if the chosen argument is an lvalue.
If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2.
Example:
#define foo(x) \
__builtin_choose_expr ( \
__builtin_types_compatible_p (typeof (x), double), \
foo_double (x), \
__builtin_choose_expr ( \
__builtin_types_compatible_p (typeof (x), float), \
foo_float (x), \
/* The void expression results in a compile-time error \
when assigning the result to something. */ \
(void)0))
|
Note: This construct is only available for C. Furthermore, the unused expression (exp1 or exp2 depending on the value of const_exp) may still generate syntax errors. This may change in future revisions.
The built-in function __builtin_complex is provided for use in
implementing the ISO C11 macros CMPLXF, CMPLX and
CMPLXL. real and imag must have the same type, a
real binary floating-point type, and the result has the corresponding
complex type with real and imaginary parts real and imag.
Unlike `real + I * imag', this works even when
infinities, NaNs and negative zeros are involved.
__builtin_constant_p to
determine if a value is known to be constant at compile-time and hence
that GCC can perform constant-folding on expressions involving that
value. The argument of the function is the value to test. The function
returns the integer 1 if the argument is known to be a compile-time
constant and 0 if it is not known to be a compile-time constant. A
return of 0 does not indicate that the value is not a constant,
but merely that GCC cannot prove it is a constant with the specified
value of the `-O' option.
You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:
#define Scale_Value(X) \ (__builtin_constant_p (X) \ ? ((X) * SCALE + OFFSET) : Scale (X)) |
You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see section 6.25 Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the `-O' option.
You may also use __builtin_constant_p in initializers for static
data. For instance, you can write
static const int table[] = {
__builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
/* ... */
};
|
This is an acceptable initializer even if EXPRESSION is not a
constant expression, including the case where
__builtin_constant_p returns 1 because EXPRESSION can be
folded to a constant but EXPRESSION contains operands that would
not otherwise be permitted in a static initializer (for example,
0 && foo ()). GCC must be more conservative about evaluating the
built-in in this case, because it has no opportunity to perform
optimization.
Previous versions of GCC did not accept this built-in in data initializers. The earliest version where it is completely safe is 3.0.1.
__builtin_expect to provide the compiler with
branch prediction information. In general, you should prefer to
use actual profile feedback for this (`-fprofile-arcs'), as
programmers are notoriously bad at predicting how their programs
actually perform. However, there are applications in which this
data is hard to collect.
The return value is the value of exp, which should be an integral expression. The semantics of the built-in are that it is expected that exp == c. For example:
if (__builtin_expect (x, 0)) foo (); |
would indicate that we do not expect to call foo, since
we expect x to be zero. Since you are limited to integral
expressions for exp, you should use constructions such as
if (__builtin_expect (ptr != NULL, 1)) foo (*ptr); |
when testing pointer or floating-point values.
abort. The mechanism used may vary from release to release so
you should not rely on any particular implementation.
__builtin_unreachable,
the program is undefined. It is useful in situations where the
compiler cannot deduce the unreachability of the code.
One such case is immediately following an asm statement that
will either never terminate, or one that transfers control elsewhere
and never returns. In this example, without the
__builtin_unreachable, GCC would issue a warning that control
reaches the end of a non-void function. It would also generate code
to return after the asm.
int f (int c, int v)
{
if (c)
{
return v;
}
else
{
asm("jmp error_handler");
__builtin_unreachable ();
}
}
|
Because the asm statement unconditionally transfers control out
of the function, control will never reach the end of the function
body. The __builtin_unreachable is in fact unreachable and
communicates this fact to the compiler.
Another use for __builtin_unreachable is following a call a
function that never returns but that is not declared
__attribute__((noreturn)), as in this example:
void function_that_never_returns (void);
int g (int c)
{
if (c)
{
return 1;
}
else
{
function_that_never_returns ();
__builtin_unreachable ();
}
}
|
void *x = __builtin_assume_aligned (arg, 16); |
means that the compiler can assume x, set to arg, is at least 16 byte aligned, while:
void *x = __builtin_assume_aligned (arg, 32, 8); |
means that the compiler can assume for x, set to arg, that (char *) x - 8 is 32 byte aligned.
If the target does not require instruction cache flushes,
__builtin___clear_cache has no effect. Otherwise either
instructions are emitted in-line to clear the instruction cache or a
call to the __clear_cache function in libgcc is made.
__builtin_prefetch into code for which
you know addresses of data in memory that is likely to be accessed soon.
If the target supports them, data prefetch instructions will be generated.
If the prefetch is done early enough before the access then the data will
be in the cache by the time it is accessed.
The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three.
for (i = 0; i < n; i++)
{
a[i] = a[i] + b[i];
__builtin_prefetch (&a[i+j], 1, 1);
__builtin_prefetch (&b[i+j], 0, 1);
/* ... */
}
|
Data prefetch does not generate faults if addr is invalid, but
the address expression itself must be valid. For example, a prefetch
of p->next will not fault if p->next is not a valid
address, but evaluation will fault if p is not a valid address.
If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.
DBL_MAX. This function is suitable for implementing the
ISO C macro HUGE_VAL.
__builtin_huge_val, except the return type is float.
__builtin_huge_val, except the return
type is long double.
FP_NAN,
FP_INFINITE, FP_NORMAL, FP_SUBNORMAL and
FP_ZERO. The ellipsis is for exactly one floating point value
to classify. GCC treats the last argument as type-generic, which
means it does not do default promotion from float to double.
__builtin_huge_val, except a warning is generated
if the target floating-point format does not support infinities.
__builtin_inf, except the return type is _Decimal32.
__builtin_inf, except the return type is _Decimal64.
__builtin_inf, except the return type is _Decimal128.
__builtin_inf, except the return type is float.
This function is suitable for implementing the ISO C99 macro INFINITY.
__builtin_inf, except the return
type is long double.
isinf, except the return value will be negative for
an argument of -Inf. Note while the parameter list is an
ellipsis, this function only accepts exactly one floating point
argument. GCC treats this parameter as type-generic, which means it
does not do default promotion from float to double.
nan.
Since ISO C99 defines this function in terms of strtod, which we
do not implement, a description of the parsing is in order. The string
is parsed as by strtol; that is, the base is recognized by
leading `0' or `0x' prefixes. The number parsed is placed
in the significand such that the least significant bit of the number
is at the least significant bit of the significand. The number is
truncated to fit the significand field provided. The significand is
forced to be a quiet NaN.
This function, if given a string literal all of which would have been consumed by strtol, is evaluated early enough that it is considered a compile-time constant.
__builtin_nan, except the return type is _Decimal32.
__builtin_nan, except the return type is _Decimal64.
__builtin_nan, except the return type is _Decimal128.
__builtin_nan, except the return type is float.
__builtin_nan, except the return type is long double.
__builtin_nan, except the significand is forced
to be a signaling NaN. The nans function is proposed by
WG14 N965.
__builtin_nans, except the return type is float.
__builtin_nans, except the return type is long double.
__builtin_ffs, except the argument type is
unsigned long.
__builtin_clz, except the argument type is
unsigned long.
__builtin_ctz, except the argument type is
unsigned long.
__builtin_clrsb, except the argument type is
long.
__builtin_popcount, except the argument type is
unsigned long.
__builtin_parity, except the argument type is
unsigned long.
__builtin_ffs, except the argument type is
unsigned long long.
__builtin_clz, except the argument type is
unsigned long long.
__builtin_ctz, except the argument type is
unsigned long long.
__builtin_clrsb, except the argument type is
long long.
__builtin_popcount, except the argument type is
unsigned long long.
__builtin_parity, except the argument type is
unsigned long long.
pow function no guarantees about precision and rounding are made.
__builtin_powi, except the argument and return types
are float.
__builtin_powi, except the argument and return types
are long double.
0xaabbccdd becomes 0xddccbbaa. Byte here always means
exactly 8 bits.
__builtin_bswap32, except the argument and return types
are 64-bit.
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On some target machines, GCC supports many built-in functions specific to those machines. Generally these generate calls to specific machine instructions, but allow the compiler to schedule those calls.
6.55.1 RX Built-in Functions
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brk machine instruction.
clrpsw machine instruction to clear the specified
bit in the processor status word.
int machine instruction to generate an interrupt
with the specified value.
machi machine instruction to add the result of
multiplying the top 16-bits of the two arguments into the
accumulator.
maclo machine instruction to add the result of
multiplying the bottom 16-bits of the two arguments into the
accumulator.
mulhi machine instruction to place the result of
multiplying the top 16-bits of the two arguments into the
accumulator.
mullo machine instruction to place the result of
multiplying the bottom 16-bits of the two arguments into the
accumulator.
mvfachi machine instruction to read the top
32-bits of the accumulator.
mvfacmi machine instruction to read the middle
32-bits of the accumulator.
mvfc machine instruction which reads the control
register specified in its argument and returns its value.
mvtachi machine instruction to set the top
32-bits of the accumulator.
mvtaclo machine instruction to set the bottom
32-bits of the accumulator.
mvtc machine instruction which sets control
register number reg to val.
mvtipl machine instruction set the interrupt
priority level.
racw machine instruction to round the accumulator
according to the specified mode.
revw machine instruction which swaps the bytes in
the argument so that bits 0--7 now occupy bits 8--15 and vice versa,
and also bits 16--23 occupy bits 24--31 and vice versa.
rmpa machine instruction which initiates a
repeated multiply and accumulate sequence.
round machine instruction which returns the
floating point argument rounded according to the current rounding mode
set in the floating point status word register.
sat machine instruction which returns the
saturated value of the argument.
setpsw machine instruction to set the specified
bit in the processor status word.
wait machine instruction.
bset machine instruction.
bclr machine instruction.
bnot machine instruction.
xchg machine instruction.| [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers. Note that in general we do not recommend the use of pragmas; See section 6.30 Declaring Attributes of Functions, for further explanation.
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GCC memregs number
-memregs= for the current
file. Use with care! This pragma must be before any function in the
file, and mixing different memregs values in different objects may
make them incompatible. This pragma is useful when a
performance-critical function uses a memreg for temporary values,
as it may allow you to reduce the number of memregs used.
ADDRESS name address
1234H numeric syntax is not supported (use 0x1234
instead). Example:
#pragma ADDRESS port3 0x103 char port3; |
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For compatibility with the Solaris and Tru64 UNIX system headers, GCC
supports two #pragma directives which change the name used in
assembly for a given declaration. #pragma extern_prefix is only
available on platforms whose system headers need it. To get this effect
on all platforms supported by GCC, use the asm labels extension (see section 6.43 Controlling Names Used in Assembler Code).
redefine_extname oldname newname
This pragma gives the C function oldname the assembly symbol
newname. The preprocessor macro __PRAGMA_REDEFINE_EXTNAME
will be defined if this pragma is available (currently on all platforms).
extern_prefix string
This pragma causes all subsequent external function and variable
declarations to have string prepended to their assembly symbols.
This effect may be terminated with another extern_prefix pragma
whose argument is an empty string. The preprocessor macro
__PRAGMA_EXTERN_PREFIX will be defined if this pragma is
available (currently only on Tru64 UNIX).
These pragmas and the asm labels extension interact in a complicated manner. Here are some corner cases you may want to be aware of.
#pragma redefine_extname is
always the C-language name.
#pragma extern_prefix is in effect, and a declaration
occurs with an asm label attached, the prefix is silently ignored for
that declaration.
#pragma extern_prefix and #pragma redefine_extname
apply to the same declaration, whichever triggered first wins, and a
warning issues if they contradict each other. (We would like to have
#pragma redefine_extname always win, for consistency with asm
labels, but if #pragma extern_prefix triggers first we have no
way of knowing that that happened.)
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For compatibility with Microsoft Windows compilers, GCC supports a
set of #pragma directives which change the maximum alignment of
members of structures (other than zero-width bitfields), unions, and
classes subsequently defined. The n value below always is required
to be a small power of two and specifies the new alignment in bytes.
#pragma pack(n) simply sets the new alignment.
#pragma pack() sets the alignment to the one that was in
effect when compilation started (see also command-line option
`-fpack-struct[=n]' see section 3.18 Options for Code Generation Conventions).
#pragma pack(push[,n]) pushes the current alignment
setting on an internal stack and then optionally sets the new alignment.
#pragma pack(pop) restores the alignment setting to the one
saved at the top of the internal stack (and removes that stack entry).
Note that #pragma pack([n]) does not influence this internal
stack; thus it is possible to have #pragma pack(push) followed by
multiple #pragma pack(n) instances and finalized by a single
#pragma pack(pop).
Some targets, e.g. i386 and powerpc, support the ms_struct
#pragma which lays out a structure as the documented
__attribute__ ((ms_struct)).
#pragma ms_struct on turns on the layout for structures
declared.
#pragma ms_struct off turns off the layout for structures
declared.
#pragma ms_struct reset goes back to the default layout.
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For compatibility with SVR4, GCC supports a set of #pragma
directives for declaring symbols to be weak, and defining weak
aliases.
#pragma weak symbol
#pragma weak symbol1 = symbol2
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GCC allows the user to selectively enable or disable certain types of diagnostics, and change the kind of the diagnostic. For example, a project's policy might require that all sources compile with `-Werror' but certain files might have exceptions allowing specific types of warnings. Or, a project might selectively enable diagnostics and treat them as errors depending on which preprocessor macros are defined.
#pragma GCC diagnostic kind option
Modifies the disposition of a diagnostic. Note that not all diagnostics are modifiable; at the moment only warnings (normally controlled by `-W...') can be controlled, and not all of them. Use `-fdiagnostics-show-option' to determine which diagnostics are controllable and which option controls them.
kind is `error' to treat this diagnostic as an error, `warning' to treat it like a warning (even if `-Werror' is in effect), or `ignored' if the diagnostic is to be ignored. option is a double quoted string which matches the command-line option.
#pragma GCC diagnostic warning "-Wformat" #pragma GCC diagnostic error "-Wformat" #pragma GCC diagnostic ignored "-Wformat" |
Note that these pragmas override any command-line options. GCC keeps track of the location of each pragma, and issues diagnostics according to the state as of that point in the source file. Thus, pragmas occurring after a line do not affect diagnostics caused by that line.
#pragma GCC diagnostic push
#pragma GCC diagnostic pop
Causes GCC to remember the state of the diagnostics as of each
push, and restore to that point at each pop. If a
pop has no matching push, the command line options are
restored.
#pragma GCC diagnostic error "-Wuninitialized" foo(a); /* error is given for this one */ #pragma GCC diagnostic push #pragma GCC diagnostic ignored "-Wuninitialized" foo(b); /* no diagnostic for this one */ #pragma GCC diagnostic pop foo(c); /* error is given for this one */ #pragma GCC diagnostic pop foo(d); /* depends on command line options */ |
GCC also offers a simple mechanism for printing messages during compilation.
#pragma message string
Prints string as a compiler message on compilation. The message is informational only, and is neither a compilation warning nor an error.
#pragma message "Compiling " __FILE__ "..." |
string may be parenthesized, and is printed with location information. For example,
#define DO_PRAGMA(x) _Pragma (#x)
#define TODO(x) DO_PRAGMA(message ("TODO - " #x))
TODO(Remember to fix this)
|
prints `/tmp/file.c:4: note: #pragma message: TODO - Remember to fix this'.
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#pragma GCC visibility push(visibility)
#pragma GCC visibility pop
This pragma allows the user to set the visibility for multiple declarations without having to give each a visibility attribute See section 6.30 Declaring Attributes of Functions, for more information about visibility and the attribute syntax.
In C++, `#pragma GCC visibility' affects only namespace-scope declarations. Class members and template specializations are not affected; if you want to override the visibility for a particular member or instantiation, you must use an attribute.
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For compatibility with Microsoft Windows compilers, GCC supports `#pragma push_macro("macro_name")' and `#pragma pop_macro("macro_name")'.
#pragma push_macro("macro_name")
#pragma pop_macro("macro_name")
For example:
#define X 1
#pragma push_macro("X")
#undef X
#define X -1
#pragma pop_macro("X")
int x [X];
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In this example, the definition of X as 1 is saved by #pragma
push_macro and restored by #pragma pop_macro.
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#pragma GCC optimize ("string"...)
This pragma allows you to set global optimization options for functions
defined later in the source file. One or more strings can be
specified. Each function that is defined after this point will be as
if attribute((optimize("STRING"))) was specified for that
function. The parenthesis around the options is optional.
See section 6.30 Declaring Attributes of Functions, for more information about the
optimize attribute and the attribute syntax.
The `#pragma GCC optimize' pragma is not implemented in GCC versions earlier than 4.4.
#pragma GCC push_options
#pragma GCC pop_options
These pragmas maintain a stack of the current target and optimization options. It is intended for include files where you temporarily want to switch to using a different `#pragma GCC target' or `#pragma GCC optimize' and then to pop back to the previous options.
The `#pragma GCC push_options' and `#pragma GCC pop_options' pragmas are not implemented in GCC versions earlier than 4.4.
#pragma GCC reset_options
This pragma clears the current #pragma GCC target and
#pragma GCC optimize to use the default switches as specified
on the command line.
The `#pragma GCC reset_options' pragma is not implemented in GCC versions earlier than 4.4.
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As permitted by ISO C11 and for compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example:
struct {
int a;
union {
int b;
float c;
};
int d;
} foo;
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In this example, the user would be able to access members of the unnamed
union with code like `foo.b'. Note that only unnamed structs and
unions are allowed, you may not have, for example, an unnamed
int.
You must never create such structures that cause ambiguous field definitions. For example, this structure:
struct {
int a;
struct {
int a;
};
} foo;
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It is ambiguous which a is being referred to with `foo.a'.
The compiler gives errors for such constructs.
Unless `-fms-extensions' is used, the unnamed field must be a
structure or union definition without a tag (for example, `struct
{ int a; };'). If `-fms-extensions' is used, the field may
also be a definition with a tag such as `struct foo { int a;
};', a reference to a previously defined structure or union such as
`struct foo;', or a reference to a typedef name for a
previously defined structure or union type.
The option `-fplan9-extensions' enables `-fms-extensions' as well as two other extensions. First, a pointer to a structure is automatically converted to a pointer to an anonymous field for assignments and function calls. For example:
struct s1 { int a; };
struct s2 { struct s1; };
extern void f1 (struct s1 *);
void f2 (struct s2 *p) { f1 (p); }
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In the call to f1 inside f2, the pointer p is
converted into a pointer to the anonymous field.
Second, when the type of an anonymous field is a typedef for a
struct or union, code may refer to the field using the
name of the typedef.
typedef struct { int a; } s1;
struct s2 { s1; };
s1 f1 (struct s2 *p) { return p->s1; }
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These usages are only permitted when they are not ambiguous.
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Thread-local storage (TLS) is a mechanism by which variables
are allocated such that there is one instance of the variable per extant
thread. The run-time model GCC uses to implement this originates
in the IA-64 processor-specific ABI, but has since been migrated
to other processors as well. It requires significant support from
the linker (ld), dynamic linker (ld.so), and
system libraries (`libc.so' and `libpthread.so'), so it
is not available everywhere.
At the user level, the extension is visible with a new storage
class keyword: __thread. For example:
__thread int i; extern __thread struct state s; static __thread char *p; |
The __thread specifier may be used alone, with the extern
or static specifiers, but with no other storage class specifier.
When used with extern or static, __thread must appear
immediately after the other storage class specifier.
The __thread specifier may be applied to any global, file-scoped
static, function-scoped static, or static data member of a class. It may
not be applied to block-scoped automatic or non-static data member.
When the address-of operator is applied to a thread-local variable, it is evaluated at run-time and returns the address of the current thread's instance of that variable. An address so obtained may be used by any thread. When a thread terminates, any pointers to thread-local variables in that thread become invalid.
No static initialization may refer to the address of a thread-local variable.
In C++, if an initializer is present for a thread-local variable, it must be a constant-expression, as defined in 5.19.2 of the ANSI/ISO C++ standard.
See ELF Handling For Thread-Local Storage for a detailed explanation of the four thread-local storage addressing models, and how the run-time is expected to function.
6.58.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage 6.58.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage
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The following are a set of changes to ISO/IEC 9899:1999 (aka C99) that document the exact semantics of the language extension.
Add new text after paragraph 1
Within either execution environment, a thread is a flow of control within a program. It is implementation defined whether or not there may be more than one thread associated with a program. It is implementation defined how threads beyond the first are created, the name and type of the function called at thread startup, and how threads may be terminated. However, objects with thread storage duration shall be initialized before thread startup.
Add new text before paragraph 3
An object whose identifier is declared with the storage-class
specifier __thread has thread storage duration.
Its lifetime is the entire execution of the thread, and its
stored value is initialized only once, prior to thread startup.
Add __thread.
Add __thread to the list of storage class specifiers in
paragraph 1.
Change paragraph 2 to
With the exception of__thread, at most one storage-class specifier may be given [...]. The__threadspecifier may be used alone, or immediately followingexternorstatic.
Add new text after paragraph 6
The declaration of an identifier for a variable that has block scope that specifies__threadshall also specify eitherexternorstatic.The
__threadspecifier shall be used only with variables.
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The following are a set of changes to ISO/IEC 14882:1998 (aka C++98) that document the exact semantics of the language extension.
New text after paragraph 4
A thread is a flow of control within the abstract machine. It is implementation defined whether or not there may be more than one thread.
New text after paragraph 7
It is unspecified whether additional action must be taken to ensure when and whether side effects are visible to other threads.
Add __thread.
Add after paragraph 5
The thread that begins execution at themainfunction is called the main thread. It is implementation defined how functions beginning threads other than the main thread are designated or typed. A function so designated, as well as themainfunction, is called a thread startup function. It is implementation defined what happens if a thread startup function returns. It is implementation defined what happens to other threads when any thread callsexit.
Add after paragraph 4
The storage for an object of thread storage duration shall be statically initialized before the first statement of the thread startup function. An object of thread storage duration shall not require dynamic initialization.
Add after paragraph 3
The type of an object with thread storage duration shall not have a non-trivial destructor, nor shall it be an array type whose elements (directly or indirectly) have non-trivial destructors.
Add "thread storage duration" to the list in paragraph 1.
Change paragraph 2
Thread, static, and automatic storage durations are associated with objects introduced by declarations [...].
Add __thread to the list of specifiers in paragraph 3.
New section before [basic.stc.static]
The keyword__threadapplied to a non-local object gives the object thread storage duration.A local variable or class data member declared both
staticand__threadgives the variable or member thread storage duration.
Change paragraph 1
All objects which have neither thread storage duration, dynamic storage duration nor are local [...].
Add __thread to the list in paragraph 1.
Change paragraph 1
With the exception of__thread, at most one storage-class-specifier shall appear in a given decl-specifier-seq. The__threadspecifier may be used alone, or immediately following theexternorstaticspecifiers. [...]
Add after paragraph 5
The __thread specifier can be applied only to the names of objects
and to anonymous unions.
Add after paragraph 6
Non-staticmembers shall not be__thread.
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Integer constants can be written as binary constants, consisting of a sequence of `0' and `1' digits, prefixed by `0b' or `0B'. This is particularly useful in environments that operate a lot on the bit-level (like microcontrollers).
The following statements are identical:
i = 42; i = 0x2a; i = 052; i = 0b101010; |
The type of these constants follows the same rules as for octal or hexadecimal integer constants, so suffixes like `L' or `UL' can be applied.
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The GNU compiler provides these extensions to the C++ language (and you
can also use most of the C language extensions in your C++ programs). If you
want to write code that checks whether these features are available, you can
test for the GNU compiler the same way as for C programs: check for a
predefined macro __GNUC__. You can also use __GNUG__ to
test specifically for GNU C++ (see section `Predefined Macros' in The GNU C Preprocessor).
7.1 When is a Volatile C++ Object Accessed? What constitutes an access to a volatile object. 7.2 Restricting Pointer Aliasing C99 restricted pointers and references. 7.3 Vague Linkage Where G++ puts inlines, vtables and such. 7.4 #pragma interface and implementation You can use a single C++ header file for both declarations and definitions. 7.5 Where's the Template? Methods for ensuring that exactly one copy of each needed template instantiation is emitted. 7.6 Extracting the function pointer from a bound pointer to member function You can extract a function pointer to the method denoted by a `->*' or `.*' expression. 7.7 C++-Specific Variable, Function, and Type Attributes Variable, function, and type attributes for C++ only. 7.8 Namespace Association Strong using-directives for namespace association. 7.9 Type Traits Compiler support for type traits 7.10 Java Exceptions Tweaking exception handling to work with Java. 7.11 Deprecated Features Things will disappear from g++. 7.12 Backwards Compatibility Compatibilities with earlier definitions of C++.
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The C++ standard differs from the C standard in its treatment of volatile objects. It fails to specify what constitutes a volatile access, except to say that C++ should behave in a similar manner to C with respect to volatiles, where possible. However, the different lvalueness of expressions between C and C++ complicate the behavior. G++ behaves the same as GCC for volatile access, See section Volatiles, for a description of GCC's behavior.
The C and C++ language specifications differ when an object is accessed in a void context:
volatile int *src = somevalue; *src; |
The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete. The C++ standard does not specify explicitly that it is lvalue to rvalue conversion which is responsible for causing an access. There is reason to believe that it is, because otherwise certain simple expressions become undefined. However, because it would surprise most programmers, G++ treats dereferencing a pointer to volatile object of complete type as GCC would do for an equivalent type in C. When the object has incomplete type, G++ issues a warning; if you wish to force an error, you must force a conversion to rvalue with, for instance, a static cast.
When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue.
G++ implements the same behavior as GCC does when assigning to a volatile object -- there is no reread of the assigned-to object, the assigned rvalue is reused. Note that in C++ assignment expressions are lvalues, and if used as an lvalue, the volatile object will be referred to. For instance, vref will refer to vobj, as expected, in the following example:
volatile int vobj; volatile int &vref = vobj = something; |
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As with the C front end, G++ understands the C99 feature of restricted pointers,
specified with the __restrict__, or __restrict type
qualifier. Because you cannot compile C++ by specifying the `-std=c99'
language flag, restrict is not a keyword in C++.
In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context.
void fn (int *__restrict__ rptr, int &__restrict__ rref)
{
/* ... */
}
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In the body of fn, rptr points to an unaliased integer and
rref refers to a (different) unaliased integer.
You may also specify whether a member function's this pointer is
unaliased by using __restrict__ as a member function qualifier.
void T::fn () __restrict__
{
/* ... */
}
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Within the body of T::fn, this will have the effective
definition T *__restrict__ const this. Notice that the
interpretation of a __restrict__ member function qualifier is
different to that of const or volatile qualifier, in that it
is applied to the pointer rather than the object. This is consistent with
other compilers which implement restricted pointers.
As with all outermost parameter qualifiers, __restrict__ is
ignored in function definition matching. This means you only need to
specify __restrict__ in a function definition, rather than
in a function prototype as well.
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There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have "vague linkage". Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever.
Local static variables and string constants used in an inline function are also considered to have vague linkage, since they must be shared between all inlined and out-of-line instances of the function.
Note: If the chosen key method is later defined as inline, the vtable will still be emitted in every translation unit which defines it. Make sure that any inline virtuals are declared inline in the class body, even if they are not defined there.
type_info objects
When used with GNU ld version 2.8 or later on an ELF system such as GNU/Linux or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support.
On targets that don't support COMDAT, but do support weak symbols, GCC will use them. This way one copy will override all the others, but the unused copies will still take up space in the executable.
For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker. This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things.
See section Declarations and Definitions in One Header, for another way to control placement of these constructs.
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#pragma interface and #pragma implementation provide the
user with a way of explicitly directing the compiler to emit entities
with vague linkage (and debugging information) in a particular
translation unit.
Note: As of GCC 2.7.2, these #pragmas are not useful in
most cases, because of COMDAT support and the "key method" heuristic
mentioned in 7.3 Vague Linkage. Using them can actually cause your
program to grow due to unnecessary out-of-line copies of inline
functions. Currently (3.4) the only benefit of these
#pragmas is reduced duplication of debugging information, and
that should be addressed soon on DWARF 2 targets with the use of
COMDAT groups.
#pragma interface
#pragma interface "subdir/objects.h"
The second form of this directive is useful for the case where you have multiple headers with the same name in different directories. If you use this form, you must specify the same string to `#pragma implementation'.
#pragma implementation
#pragma implementation "objects.h"
If you use `#pragma implementation' with no argument, it applies to an include file with the same basename(4) as your source file. For example, in `allclass.cc', giving just `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'.
In versions of GNU C++ prior to 2.6.0 `allclass.h' was treated as an implementation file whenever you would include it from `allclass.cc' even if you never specified `#pragma implementation'. This was deemed to be more trouble than it was worth, however, and disabled.
Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use `#include' to include the header file; `#pragma implementation' only specifies how to use the file--it doesn't actually include it.)
There is no way to split up the contents of a single header file into multiple implementation files.
`#pragma implementation' and `#pragma interface' also have an effect on function inlining.
If you define a class in a header file marked with `#pragma
interface', the effect on an inline function defined in that class is
similar to an explicit extern declaration--the compiler emits
no code at all to define an independent version of the function. Its
definition is used only for inlining with its callers.
Conversely, when you include the same header file in a main source file that declares it as `#pragma implementation', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with `-fno-implement-inlines'. If any calls were not inlined, you will get linker errors.
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C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which are referred to as the Borland model and the Cfront model.
When used with GNU ld version 2.8 or later on an ELF system such as GNU/Linux or Solaris 2, or on Microsoft Windows, G++ supports the Borland model. On other systems, G++ implements neither automatic model.
A future version of G++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations.
In the mean time, you have the following options for dealing with template instantiations:
This is your best option for application code written for the Borland
model, as it will just work. Code written for the Cfront model will
need to be modified so that the template definitions are available at
one or more points of instantiation; usually this is as simple as adding
#include <tmethods.cc> to the end of each template header.
For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option.
#include "Foo.h"
#include "Foo.cc"
template class Foo<int>;
template ostream& operator <<
(ostream&, const Foo<int>&);
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for each of the instances you need, and create a template instantiation library from those.
If you are using Cfront-model code, you can probably get away with not using `-fno-implicit-templates' when compiling files that don't `#include' the member template definitions.
If you use one big file to do the instantiations, you may want to compile it without `-fno-implicit-templates' so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well.
G++ has extended the template instantiation syntax given in the ISO
standard to allow forward declaration of explicit instantiations
(with extern), instantiation of the compiler support data for a
template class (i.e. the vtable) without instantiating any of its
members (with inline), and instantiation of only the static data
members of a template class, without the support data or member
functions (with (static):
extern template int max (int, int); inline template class Foo<int>; static template class Foo<int>; |
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In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the `this' pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time.
Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls.
The syntax for this extension is
extern A a; extern int (A::*fp)(); typedef int (*fptr)(A *); fptr p = (fptr)(a.*fp); |
For PMF constants (i.e. expressions of the form `&Klasse::Member'), no object is needed to obtain the address of the function. They can be converted to function pointers directly:
fptr p1 = (fptr)(&A::foo); |
You must specify `-Wno-pmf-conversions' to use this extension.
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Some attributes only make sense for C++ programs.
init_priority (priority)
In Standard C++, objects defined at namespace scope are guaranteed to be
initialized in an order in strict accordance with that of their definitions
in a given translation unit. No guarantee is made for initializations
across translation units. However, GNU C++ allows users to control the
order of initialization of objects defined at namespace scope with the
init_priority attribute by specifying a relative priority,
a constant integral expression currently bounded between 101 and 65535
inclusive. Lower numbers indicate a higher priority.
In the following example, A would normally be created before
B, but the init_priority attribute has reversed that order:
Some_Class A __attribute__ ((init_priority (2000))); Some_Class B __attribute__ ((init_priority (543))); |
Note that the particular values of priority do not matter; only their relative ordering.
java_interface
This type attribute informs C++ that the class is a Java interface. It may
only be applied to classes declared within an extern "Java" block.
Calls to methods declared in this interface will be dispatched using GCJ's
interface table mechanism, instead of regular virtual table dispatch.
See also 7.8 Namespace Association.
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Caution: The semantics of this extension are not fully defined. Users should refrain from using this extension as its semantics may change subtly over time. It is possible that this extension will be removed in future versions of G++.
A using-directive with __attribute ((strong)) is stronger
than a normal using-directive in two ways:
The used namespace must be nested within the using namespace so that normal unqualified lookup works properly.
This is useful for composing a namespace transparently from implementation namespaces. For example:
namespace std {
namespace debug {
template <class T> struct A { };
}
using namespace debug __attribute ((__strong__));
template <> struct A<int> { }; // ok to specialize
template <class T> void f (A<T>);
}
int main()
{
f (std::A<float>()); // lookup finds std::f
f (std::A<int>());
}
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The C++ front-end implements syntactic extensions that allow to determine at compile time various characteristics of a type (or of a pair of types).
__has_nothrow_assign (type)
type is const qualified or is a reference type then the trait is
false. Otherwise if __has_trivial_assign (type) is true then the trait
is true, else if type is a cv class or union type with copy assignment
operators that are known not to throw an exception then the trait is true,
else it is false. Requires: type shall be a complete type,
(possibly cv-qualified) void, or an array of unknown bound.
__has_nothrow_copy (type)
__has_trivial_copy (type) is true then the trait is true, else if
type is a cv class or union type with copy constructors that
are known not to throw an exception then the trait is true, else it is false.
Requires: type shall be a complete type, (possibly cv-qualified)
void, or an array of unknown bound.
__has_nothrow_constructor (type)
__has_trivial_constructor (type) is true then the trait is
true, else if type is a cv class or union type (or array
thereof) with a default constructor that is known not to throw an
exception then the trait is true, else it is false. Requires:
type shall be a complete type, (possibly cv-qualified)
void, or an array of unknown bound.
__has_trivial_assign (type)
type is const qualified or is a reference type then the trait is
false. Otherwise if __is_pod (type) is true then the trait is
true, else if type is a cv class or union type with a trivial
copy assignment ([class.copy]) then the trait is true, else it is
false. Requires: type shall be a complete type, (possibly
cv-qualified) void, or an array of unknown bound.
__has_trivial_copy (type)
__is_pod (type) is true or type is a reference type
then the trait is true, else if type is a cv class or union type
with a trivial copy constructor ([class.copy]) then the trait
is true, else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__has_trivial_constructor (type)
__is_pod (type) is true then the trait is true, else if
type is a cv class or union type (or array thereof) with a
trivial default constructor ([class.ctor]) then the trait is true,
else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__has_trivial_destructor (type)
__is_pod (type) is true or type is a reference type then
the trait is true, else if type is a cv class or union type (or
array thereof) with a trivial destructor ([class.dtor]) then the trait
is true, else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__has_virtual_destructor (type)
type is a class type with a virtual destructor
([class.dtor]) then the trait is true, else it is false. Requires:
type shall be a complete type, (possibly cv-qualified)
void, or an array of unknown bound.
__is_abstract (type)
type is an abstract class ([class.abstract]) then the trait
is true, else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__is_base_of (base_type, derived_type)
base_type is a base class of derived_type
([class.derived]) then the trait is true, otherwise it is false.
Top-level cv qualifications of base_type and
derived_type are ignored. For the purposes of this trait, a
class type is considered is own base. Requires: if __is_class
(base_type) and __is_class (derived_type) are true and
base_type and derived_type are not the same type
(disregarding cv-qualifiers), derived_type shall be a complete
type. Diagnostic is produced if this requirement is not met.
__is_class (type)
type is a cv class type, and not a union type
([basic.compound]) the trait is true, else it is false.
__is_empty (type)
__is_class (type) is false then the trait is false.
Otherwise type is considered empty if and only if: type
has no non-static data members, or all non-static data members, if
any, are bit-fields of length 0, and type has no virtual
members, and type has no virtual base classes, and type
has no base classes base_type for which
__is_empty (base_type) is false. Requires: type shall
be a complete type, (possibly cv-qualified) void, or an array
of unknown bound.
__is_enum (type)
type is a cv enumeration type ([basic.compound]) the trait is
true, else it is false.
__is_literal_type (type)
type is a literal type ([basic.types]) the trait is
true, else it is false. Requires: type shall be a complete type,
(possibly cv-qualified) void, or an array of unknown bound.
__is_pod (type)
type is a cv POD type ([basic.types]) then the trait is true,
else it is false. Requires: type shall be a complete type,
(possibly cv-qualified) void, or an array of unknown bound.
__is_polymorphic (type)
type is a polymorphic class ([class.virtual]) then the trait
is true, else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__is_standard_layout (type)
type is a standard-layout type ([basic.types]) the trait is
true, else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__is_trivial (type)
type is a trivial type ([basic.types]) the trait is
true, else it is false. Requires: type shall be a complete
type, (possibly cv-qualified) void, or an array of unknown bound.
__is_union (type)
type is a cv union type ([basic.compound]) the trait is
true, else it is false.
__underlying_type (type)
type. Requires: type shall be
an enumeration type ([dcl.enum]).
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The Java language uses a slightly different exception handling model from C++. Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is:
struct S { ~S(); };
extern void bar(); // is written in Java, and may throw exceptions
void foo()
{
S s;
bar();
}
|
The usual effect of an incorrect guess is a link failure, complaining of a missing routine called `__gxx_personality_v0'.
You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing `#pragma GCC java_exceptions' at the head of the file. This `#pragma' must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.
You cannot mix Java and C++ exceptions in the same translation unit. It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.
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In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.
While the list below is not exhaustive, it documents some of the options that are now deprecated:
-fexternal-templates
-falt-external-templates
-fstrict-prototype
-fno-strict-prototype
G++ allows a virtual function returning `void *' to be overridden by one returning a different pointer type. This extension to the covariant return type rules is now deprecated and will be removed from a future version.
The G++ minimum and maximum operators (`<?' and `>?') and
their compound forms (`<?=') and `>?=') have been deprecated
and are now removed from G++. Code using these operators should be
modified to use std::min and std::max instead.
The named return value extension has been deprecated, and is now removed from G++.
The use of initializer lists with new expressions has been deprecated, and is now removed from G++.
Floating and complex non-type template parameters have been deprecated, and are now removed from G++.
The implicit typename extension has been deprecated and is now removed from G++.
The use of default arguments in function pointers, function typedefs and other places where they are not permitted by the standard is deprecated and will be removed from a future version of G++.
G++ allows floating-point literals to appear in integral constant expressions, e.g. ` enum E { e = int(2.2 * 3.7) } ' This extension is deprecated and will be removed from a future version.
G++ allows static data members of const floating-point type to be declared with an initializer in a class definition. The standard only allows initializers for static members of const integral types and const enumeration types so this extension has been deprecated and will be removed from a future version.
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Now that there is a definitive ISO standard C++, G++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted. In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities. All such backwards compatibility features are liable to disappear in future versions of G++. They should be considered deprecated. See section 7.11 Deprecated Features.
For scope
Implicit C language
extern "C" {...}
scope to set the language. On such systems, all header files are
implicitly scoped inside a C language scope. Also, an empty prototype
() will be treated as an unspecified number of arguments, rather
than no arguments, as C++ demands.
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This document is meant to describe some of the GNU Objective-C features. It is not intended to teach you Objective-C. There are several resources on the Internet that present the language.
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This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
The GNU Objective-C runtime provides an API that allows you to interact with the Objective-C runtime system, querying the live runtime structures and even manipulating them. This allows you for example to inspect and navigate classes, methods and protocols; to define new classes or new methods, and even to modify existing classes or protocols.
If you are using a "Foundation" library such as GNUstep-Base, this library will provide you with a rich set of functionality to do most of the inspection tasks, and you probably will only need direct access to the GNU Objective-C runtime API to define new classes or methods.
8.1.1 Modern GNU Objective-C runtime API 8.1.2 Traditional GNU Objective-C runtime API
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The GNU Objective-C runtime provides an API which is similar to the one provided by the "Objective-C 2.0" Apple/NeXT Objective-C runtime. The API is documented in the public header files of the GNU Objective-C runtime:
id, Class
and BOOL. You have to include this header to do almost
anything with Objective-C.
class_getName(), declared in
`objc/runtime.h'.
@synchronized() syntax, allowing
you to emulate an Objective-C @synchronized() block in plain
C/C++ code.
objc_mutex_lock(), which provide a
platform-independent set of threading functions.
The header files contain detailed documentation for each function in the GNU Objective-C runtime API.
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The GNU Objective-C runtime used to provide a different API, which we
call the "traditional" GNU Objective-C runtime API. Functions
belonging to this API are easy to recognize because they use a
different naming convention, such as class_get_super_class()
(traditional API) instead of class_getSuperclass() (modern
API). Software using this API includes the file
`objc/objc-api.h' where it is declared.
Starting with GCC 4.7.0, the traditional GNU runtime API is no longer available.
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+load: Executing code before main This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
The GNU Objective-C runtime provides a way that allows you to execute
code before the execution of the program enters the main
function. The code is executed on a per-class and a per-category basis,
through a special class method +load.
This facility is very useful if you want to initialize global variables
which can be accessed by the program directly, without sending a message
to the class first. The usual way to initialize global variables, in the
+initialize method, might not be useful because
+initialize is only called when the first message is sent to a
class object, which in some cases could be too late.
Suppose for example you have a FileStream class that declares
Stdin, Stdout and Stderr as global variables, like
below:
FileStream *Stdin = nil;
FileStream *Stdout = nil;
FileStream *Stderr = nil;
@implementation FileStream
+ (void)initialize
{
Stdin = [[FileStream new] initWithFd:0];
Stdout = [[FileStream new] initWithFd:1];
Stderr = [[FileStream new] initWithFd:2];
}
/* Other methods here */
@end
|
In this example, the initialization of Stdin, Stdout and
Stderr in +initialize occurs too late. The programmer can
send a message to one of these objects before the variables are actually
initialized, thus sending messages to the nil object. The
+initialize method which actually initializes the global
variables is not invoked until the first message is sent to the class
object. The solution would require these variables to be initialized
just before entering main.
The correct solution of the above problem is to use the +load
method instead of +initialize:
@implementation FileStream
+ (void)load
{
Stdin = [[FileStream new] initWithFd:0];
Stdout = [[FileStream new] initWithFd:1];
Stderr = [[FileStream new] initWithFd:2];
}
/* Other methods here */
@end
|
The +load is a method that is not overridden by categories. If a
class and a category of it both implement +load, both methods are
invoked. This allows some additional initializations to be performed in
a category.
This mechanism is not intended to be a replacement for +initialize.
You should be aware of its limitations when you decide to use it
instead of +initialize.
8.2.1 What you can and what you cannot do in +load
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+load
+load is to be used only as a last resort. Because it is
executed very early, most of the Objective-C runtime machinery will
not be ready when +load is executed; hence +load works
best for executing C code that is independent on the Objective-C
runtime.
The +load implementation in the GNU runtime guarantees you the
following things:
+load implementation of all super classes of a class are
executed before the +load of that class is executed;
+load implementation of a class is executed before the
+load implementation of any category.
In particular, the following things, even if they can work in a particular case, are not guaranteed:
@"this is a
constant string");
You should make no assumptions about receiving +load in sibling
classes when you write +load of a class. The order in which
sibling classes receive +load is not guaranteed.
The order in which +load and +initialize are called could
be problematic if this matters. If you don't allocate objects inside
+load, it is guaranteed that +load is called before
+initialize. If you create an object inside +load the
+initialize method of object's class is invoked even if
+load was not invoked. Note if you explicitly call +load
on a class, +initialize will be called first. To avoid possible
problems try to implement only one of these methods.
The +load method is also invoked when a bundle is dynamically
loaded into your running program. This happens automatically without any
intervening operation from you. When you write bundles and you need to
write +load you can safely create and send messages to objects whose
classes already exist in the running program. The same restrictions as
above apply to classes defined in bundle.
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This is an advanced section. Type encodings are used extensively by the compiler and by the runtime, but you generally do not need to know about them to use Objective-C.
The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes.
The types are encoded in the following way:
_Bool |
B
|
char |
c
|
unsigned char |
C
|
short |
s
|
unsigned short |
S
|
int |
i
|
unsigned int |
I
|
long |
l
|
unsigned long |
L
|
long long |
q
|
unsigned long long |
Q
|
float |
f
|
double |
d
|
long double |
D
|
void |
v
|
id |
@
|
Class |
#
|
SEL |
:
|
char* |
*
|
enum |
an enum is encoded exactly as the integer type that the compiler uses for it, which depends on the enumeration
values. Often the compiler users unsigned int, which is then encoded as I.
|
| unknown type | ?
|
| Complex types | j followed by the inner type. For example _Complex double is encoded as "jd".
|
| bit-fields | b followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)
|
The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object.
The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure.
The non-atomic types are encoded as follows:
| pointers | `^' followed by the pointed type. |
| arrays | `[' followed by the number of elements in the array followed by the type of the elements followed by `]' |
| structures | `{' followed by the name of the structure (or `?' if the structure is unnamed), the `=' sign, the type of the members and by `}' |
| unions | `(' followed by the name of the structure (or `?' if the union is unnamed), the `=' sign, the type of the members followed by `)' |
| vectors | `![' followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by `]' |
Here are some types and their encodings, as they are generated by the compiler on an i386 machine:
| Objective-C type | Compiler encoding | ||
int a[10]; |
[10i]
struct {
int i;
float f[3];
int a:3;
int b:2;
char c;
}
|
{?=i[3f]b128i3b131i2c}
int a __attribute__ ((vector_size (16))); |
![16,16i] (alignment would depend on the machine)
In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers:
| Specifier | Encoding |
const |
r
|
in |
n
|
inout |
N
|
out |
o
|
bycopy |
O
|
byref |
R
|
oneway |
V
|
The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types.
Note how const interacts with pointers:
| Objective-C type | Compiler encoding | ||
const int |
ri
const int* |
^ri
int *const |
r^i
const int* is a pointer to a const int, and so is
encoded as ^ri. int* const, instead, is a const
pointer to an int, and so is encoded as r^i.
Finally, there is a complication when encoding const char *
versus char * const. Because char * is encoded as
* and not as ^c, there is no way to express the fact
that r applies to the pointer or to the pointee.
Hence, it is assumed as a convention that r* means const
char * (since it is what is most often meant), and there is no way to
encode char *const. char *const would simply be encoded
as *, and the const is lost.
8.3.1 Legacy type encoding 8.3.2 @encode 8.3.3 Method signatures
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Unfortunately, historically GCC used to have a number of bugs in its encoding code. The NeXT runtime expects GCC to emit type encodings in this historical format (compatible with GCC-3.3), so when using the NeXT runtime, GCC will introduce on purpose a number of incorrect encodings:
enums are always encoded as 'i' (int) even if they are actually
unsigned or long.
In addition to that, the NeXT runtime uses a different encoding for
bitfields. It encodes them as b followed by the size, without
a bit offset or the underlying field type.
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GNU Objective-C supports the @encode syntax that allows you to
create a type encoding from a C/Objective-C type. For example,
@encode(int) is compiled by the compiler into "i".
@encode does not support type qualifiers other than
const. For example, @encode(const char*) is valid and
is compiled into "r*", while @encode(bycopy char *) is
invalid and will cause a compilation error.
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This section documents the encoding of method types, which is rarely needed to use Objective-C. You should skip it at a first reading; the runtime provides functions that will work on methods and can walk through the list of parameters and interpret them for you. These functions are part of the public "API" and are the preferred way to interact with method signatures from user code.
But if you need to debug a problem with method signatures and need to know how they are implemented (i.e., the "ABI"), read on.
Methods have their "signature" encoded and made available to the runtime. The "signature" encodes all the information required to dynamically build invocations of the method at runtime: return type and arguments.
The "signature" is a null-terminated string, composed of the following:
int would have i here.
self and the
method selector _cmd).
For example, a method with no arguments and returning int would
have the signature i8@0:4 if the size of a pointer is 4. The
signature is interpreted as follows: the i is the return type
(an int), the 8 is the total size of the parameters in
bytes (two pointers each of size 4), the @0 is the first
parameter (an object at byte offset 0) and :4 is the
second parameter (a SEL at byte offset 4).
You can easily find more examples by running the "strings" program
on an Objective-C object file compiled by GCC. You'll see a lot of
strings that look very much like i8@0:4. They are signatures
of Objective-C methods.
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This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
Support for garbage collection with the GNU runtime has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector.
To enable the support for it you have to configure the compiler using an additional argument, `--enable-objc-gc'. This will build the boehm-gc library, and build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, `libobjc_gc.a' to not conflict with the non-garbage-collected library.
When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized.
There is a new runtime function class_ivar_set_gcinvisible()
which can be used to declare a so-called weak pointer
reference. Such a pointer is basically hidden for the garbage collector;
this can be useful in certain situations, especially when you want to
keep track of the allocated objects, yet allow them to be
collected. This kind of pointers can only be members of objects, you
cannot declare a global pointer as a weak reference. Every type which is
a pointer type can be declared a weak pointer, including id,
Class and SEL.
Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this:
@interface WeakPointer : Object
{
const void* weakPointer;
}
- initWithPointer:(const void*)p;
- (const void*)weakPointer;
@end
@implementation WeakPointer
+ (void)initialize
{
if (self == objc_lookUpClass ("WeakPointer"))
class_ivar_set_gcinvisible (self, "weakPointer", YES);
}
- initWithPointer:(const void*)p
{
weakPointer = p;
return self;
}
- (const void*)weakPointer
{
return weakPointer;
}
@end
|
Weak pointers are supported through a new type character specifier
represented by the `!' character. The
class_ivar_set_gcinvisible() function adds or removes this
specifier to the string type description of the instance variable named
as argument.
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GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character `@':
id myString = @"this is a constant string object"; |
The constant string objects are by default instances of the
NXConstantString class which is provided by the GNU Objective-C
runtime. To get the definition of this class you must include the
`objc/NXConstStr.h' header file.
User defined libraries may want to implement their own constant string
class. To be able to support them, the GNU Objective-C compiler provides
a new command line options `-fconstant-string-class=class-name'.
The provided class should adhere to a strict structure, the same
as NXConstantString's structure:
@interface MyConstantStringClass
{
Class isa;
char *c_string;
unsigned int len;
}
@end
|
NXConstantString inherits from Object; user class
libraries may choose to inherit the customized constant string class
from a different class than Object. There is no requirement in
the methods the constant string class has to implement, but the final
ivar layout of the class must be the compatible with the given
structure.
When the compiler creates the statically allocated constant string
object, the c_string field will be filled by the compiler with
the string; the length field will be filled by the compiler with
the string length; the isa pointer will be filled with
NULL by the compiler, and it will later be fixed up automatically
at runtime by the GNU Objective-C runtime library to point to the class
which was set by the `-fconstant-string-class' option when the
object file is loaded (if you wonder how it works behind the scenes, the
name of the class to use, and the list of static objects to fixup, are
stored by the compiler in the object file in a place where the GNU
runtime library will find them at runtime).
As a result, when a file is compiled with the `-fconstant-string-class' option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this.
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The keyword @compatibility_alias allows you to define a class name
as equivalent to another class name. For example:
@compatibility_alias WOApplication GSWApplication; |
tells the compiler that each time it encounters WOApplication as
a class name, it should replace it with GSWApplication (that is,
WOApplication is just an alias for GSWApplication).
There are some constraints on how this can be used---
WOApplication (the alias) must not be an existing class;
GSWApplication (the real class) must be an existing class.
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GNU Objective-C provides exception support built into the language, as in the following example:
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
|
The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in
which case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be caught
by the nearest @catch clause capable of handling objects of
that type, analogously to how catch blocks work in C++ and
Java. A @catch(id ...) clause (as shown above) may also
be provided to catch any and all Objective-C exceptions not caught by
previous @catch clauses (if any).
The @finally clause, if present, will be executed upon exit
from the immediately preceding @try ... @catch section.
This will happen regardless of whether any exceptions are thrown,
caught or rethrown inside the @try ... @catch section,
analogously to the behavior of the finally clause in Java.
There are several caveats to using the new exception mechanism:
NS_HANDLER-style idioms provided by the
NSException class, the new exceptions can only be used on Mac
OS X 10.3 (Panther) and later systems, due to additional functionality
needed in the NeXT Objective-C runtime.
@throw an exception
from Objective-C and catch it in C++, or vice versa
(i.e., throw ... @catch).
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GNU Objective-C provides support for synchronized blocks:
@synchronized (ObjCClass *guard) {
...
}
|
Upon entering the @synchronized block, a thread of execution
shall first check whether a lock has been placed on the corresponding
guard object by another thread. If it has, the current thread
shall wait until the other thread relinquishes its lock. Once
guard becomes available, the current thread will place its own
lock on it, execute the code contained in the @synchronized
block, and finally relinquish the lock (thereby making guard
available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be
marked @synchronized. Note that throwing exceptions out of
@synchronized blocks is allowed, and will cause the guarding
object to be unlocked properly.
Because of the interactions between synchronization and exception
handling, you can only use @synchronized when compiling with
exceptions enabled, that is with the command line option
`-fobjc-exceptions'.
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8.9.1 Using fast enumeration 8.9.2 c99-like fast enumeration syntax 8.9.3 Fast enumeration details 8.9.4 Fast enumeration protocol
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GNU Objective-C provides support for the fast enumeration syntax:
id array = ...;
id object;
for (object in array)
{
/* Do something with 'object' */
}
|
array needs to be an Objective-C object (usually a collection
object, for example an array, a dictionary or a set) which implements
the "Fast Enumeration Protocol" (see below). If you are using a
Foundation library such as GNUstep Base or Apple Cocoa Foundation, all
collection objects in the library implement this protocol and can be
used in this way.
The code above would iterate over all objects in array. For
each of them, it assigns it to object, then executes the
Do something with 'object' statements.
Here is a fully worked-out example using a Foundation library (which
provides the implementation of NSArray, NSString and
NSLog):
NSArray *array = [NSArray arrayWithObjects: @"1", @"2", @"3", nil];
NSString *object;
for (object in array)
NSLog (@"Iterating over %@", object);
|
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A c99-like declaration syntax is also allowed:
id array = ...;
for (id object in array)
{
/* Do something with 'object' */
}
|
this is completely equivalent to:
id array = ...;
{
id object;
for (object in array)
{
/* Do something with 'object' */
}
}
|
but can save some typing.
Note that the option `-std=c99' is not required to allow this syntax in Objective-C.
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Here is a more technical description with the gory details. Consider the code
for (object expression in collection expression)
{
statements
}
|
here is what happens when you run it:
collection expression is evaluated exactly once and the
result is used as the collection object to iterate over. This means
it is safe to write code such as for (object in [NSDictionary
keyEnumerator]) ....
object expression is set to nil and the loop
immediately terminates.
object expression
is set to the object, then statements are executed.
statements can contain break and continue
commands, which will abort the iteration or skip to the next loop
iteration as expected.
object expression is set to nil. This allows
you to determine whether the iteration finished because a break
command was used (in which case object expression will remain
set to the last object that was iterated over) or because it iterated
over all the objects (in which case object expression will be
set to nil).
statements must not make any changes to the collection
object; if they do, it is a hard error and the fast enumeration
terminates by invoking objc_enumerationMutation, a runtime
function that normally aborts the program but which can be customized
by Foundation libraries via objc_set_mutation_handler to do
something different, such as raising an exception.
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If you want your own collection object to be usable with fast enumeration, you need to have it implement the method
- (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state
objects: (id *)objects
count: (unsigned long)len;
|
where NSFastEnumerationState must be defined in your code as follows:
typedef struct
{
unsigned long state;
id *itemsPtr;
unsigned long *mutationsPtr;
unsigned long extra[5];
} NSFastEnumerationState;
|
If no NSFastEnumerationState is defined in your code, the
compiler will automatically replace NSFastEnumerationState *
with struct __objcFastEnumerationState *, where that type is
silently defined by the compiler in an identical way. This can be
confusing and we recommend that you define
NSFastEnumerationState (as shown above) instead.
The method is called repeatedly during a fast enumeration to retrieve batches of objects. Each invocation of the method should retrieve the next batch of objects.
The return value of the method is the number of objects in the current
batch; this should not exceed len, which is the maximum size of
a batch as requested by the caller. The batch itself is returned in
the itemsPtr field of the NSFastEnumerationState struct.
To help with returning the objects, the objects array is a C
array preallocated by the caller (on the stack) of size len.
In many cases you can put the objects you want to return in that
objects array, then do itemsPtr = objects. But you
don't have to; if your collection already has the objects to return in
some form of C array, it could return them from there instead.
The state and extra fields of the
NSFastEnumerationState structure allows your collection object
to keep track of the state of the enumeration. In a simple array
implementation, state may keep track of the index of the last
object that was returned, and extra may be unused.
The mutationsPtr field of the NSFastEnumerationState is
used to keep track of mutations. It should point to a number; before
working on each object, the fast enumeration loop will check that this
number has not changed. If it has, a mutation has happened and the
fast enumeration will abort. So, mutationsPtr could be set to
point to some sort of version number of your collection, which is
increased by one every time there is a change (for example when an
object is added or removed). Or, if you are content with less strict
mutation checks, it could point to the number of objects in your
collection or some other value that can be checked to perform an
approximate check that the collection has not been mutated.
Finally, note how we declared the len argument and the return
value to be of type unsigned long. They could also be declared
to be of type unsigned int and everything would still work.
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This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.
The implementation of messaging in the GNU Objective-C runtime is designed to be portable, and so is based on standard C.
Sending a message in the GNU Objective-C runtime is composed of two
separate steps. First, there is a call to the lookup function,
objc_msg_lookup () (or, in the case of messages to super,
objc_msg_lookup_super ()). This runtime function takes as
argument the receiver and the selector of the method to be called; it
returns the IMP, that is a pointer to the function implementing
the method. The second step of method invocation consists of casting
this pointer function to the appropriate function pointer type, and
calling the function pointed to it with the right arguments.
For example, when the compiler encounters a method invocation such as
[object init], it compiles it into a call to
objc_msg_lookup (object, @selector(init)) followed by a cast
of the returned value to the appropriate function pointer type, and
then it calls it.
8.10.1 Dynamically registering methods 8.10.2 Forwarding hook
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If objc_msg_lookup() does not find a suitable method
implementation, because the receiver does not implement the required
method, it tries to see if the class can dynamically register the
method.
To do so, the runtime checks if the class of the receiver implements the method
+ (BOOL) resolveInstanceMethod: (SEL)selector; |
in the case of an instance method, or
+ (BOOL) resolveClassMethod: (SEL)selector; |
in the case of a class method. If the class implements it, the
runtime invokes it, passing as argument the selector of the original
method, and if it returns YES, the runtime tries the lookup
again, which could now succeed if a matching method was added
dynamically by +resolveInstanceMethod: or
+resolveClassMethod:.
This allows classes to dynamically register methods (by adding them to
the class using class_addMethod) when they are first called.
To do so, a class should implement +resolveInstanceMethod: (or,
depending on the case, +resolveClassMethod:) and have it
recognize the selectors of methods that can be registered dynamically
at runtime, register them, and return YES. It should return
NO for methods that it does not dynamically registered at
runtime.
If +resolveInstanceMethod: (or +resolveClassMethod:) is
not implemented or returns NO, the runtime then tries the
forwarding hook.
Support for +resolveInstanceMethod: and
resolveClassMethod: was added to the GNU Objective-C runtime in
GCC version 4.6.
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The GNU Objective-C runtime provides a hook, called
__objc_msg_forward2, which is called by
objc_msg_lookup() when it can't find a method implementation in
the runtime tables and after calling +resolveInstanceMethod:
and +resolveClassMethod: has been attempted and did not succeed
in dynamically registering the method.
To configure the hook, you set the global variable
__objc_msg_foward2 to a function with the same argument and
return types of objc_msg_lookup(). When
objc_msg_lookup() can not find a method implementation, it
invokes the hook function you provided to get a method implementation
to return. So, in practice __objc_msg_forward2 allows you to
extend objc_msg_lookup() by adding some custom code that is
called to do a further lookup when no standard method implementation
can be found using the normal lookup.
This hook is generally reserved for "Foundation" libraries such as
GNUstep Base, which use it to implement their high-level method
forwarding API, typically based around the forwardInvocation:
method. So, unless you are implementing your own "Foundation"
library, you should not set this hook.
In a typical forwarding implementation, the __objc_msg_forward2
hook function determines the argument and return type of the method
that is being looked up, and then creates a function that takes these
arguments and has that return type, and returns it to the caller.
Creating this function is non-trivial and is typically performed using
a dedicated library such as libffi.
The forwarding method implementation thus created is returned by
objc_msg_lookup() and is executed as if it was a normal method
implementation. When the forwarding method implementation is called,
it is usually expected to pack all arguments into some sort of object
(typically, an NSInvocation in a "Foundation" library), and
hand it over to the programmer (forwardInvocation:) who is then
allowed to manipulate the method invocation using a high-level API
provided by the "Foundation" library. For example, the programmer
may want to examine the method invocation arguments and name and
potentially change them before forwarding the method invocation to one
or more local objects (performInvocation:) or even to remote
objects (by using Distributed Objects or some other mechanism). When
all this completes, the return value is passed back and must be
returned correctly to the original caller.
Note that the GNU Objective-C runtime currently provides no support
for method forwarding or method invocations other than the
__objc_msg_forward2 hook.
If the forwarding hook does not exist or returns NULL, the
runtime currently attempts forwarding using an older, deprecated API,
and if that fails, it aborts the program. In future versions of the
GNU Objective-C runtime, the runtime will immediately abort.
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Binary compatibility encompasses several related concepts:
The application binary interface implemented by a C or C++ compiler affects code generation and runtime support for:
In addition, the application binary interface implemented by a C++ compiler affects code generation and runtime support for:
Some GCC compilation options cause the compiler to generate code that does not conform to the platform's default ABI. Other options cause different program behavior for implementation-defined features that are not covered by an ABI. These options are provided for consistency with other compilers that do not follow the platform's default ABI or the usual behavior of implementation-defined features for the platform. Be very careful about using such options.
Most platforms have a well-defined ABI that covers C code, but ABIs that cover C++ functionality are not yet common.
Starting with GCC 3.2, GCC binary conventions for C++ are based on a written, vendor-neutral C++ ABI that was designed to be specific to 64-bit Itanium but also includes generic specifications that apply to any platform. This C++ ABI is also implemented by other compiler vendors on some platforms, notably GNU/Linux and BSD systems. We have tried hard to provide a stable ABI that will be compatible with future GCC releases, but it is possible that we will encounter problems that make this difficult. Such problems could include different interpretations of the C++ ABI by different vendors, bugs in the ABI, or bugs in the implementation of the ABI in different compilers. GCC's `-Wabi' switch warns when G++ generates code that is probably not compatible with the C++ ABI.
The C++ library used with a C++ compiler includes the Standard C++ Library, with functionality defined in the C++ Standard, plus language runtime support. The runtime support is included in a C++ ABI, but there is no formal ABI for the Standard C++ Library. Two implementations of that library are interoperable if one follows the de-facto ABI of the other and if they are both built with the same compiler, or with compilers that conform to the same ABI for C++ compiler and runtime support.
When G++ and another C++ compiler conform to the same C++ ABI, but the implementations of the Standard C++ Library that they normally use do not follow the same ABI for the Standard C++ Library, object files built with those compilers can be used in the same program only if they use the same C++ library. This requires specifying the location of the C++ library header files when invoking the compiler whose usual library is not being used. The location of GCC's C++ header files depends on how the GCC build was configured, but can be seen by using the G++ `-v' option. With default configuration options for G++ 3.3 the compile line for a different C++ compiler needs to include
-Igcc_install_directory/include/c++/3.3 |
Similarly, compiling code with G++ that must use a C++ library other than the GNU C++ library requires specifying the location of the header files for that other library.
The most straightforward way to link a program to use a particular
C++ library is to use a C++ driver that specifies that C++ library by
default. The g++ driver, for example, tells the linker where
to find GCC's C++ library (`libstdc++') plus the other libraries
and startup files it needs, in the proper order.
If a program must use a different C++ library and it's not possible
to do the final link using a C++ driver that uses that library by default,
it is necessary to tell g++ the location and name of that
library. It might also be necessary to specify different startup files
and other runtime support libraries, and to suppress the use of GCC's
support libraries with one or more of the options `-nostdlib',
`-nostartfiles', and `-nodefaultlibs'.
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gcov---a Test Coverage Program
gcov is a tool you can use in conjunction with GCC to
test code coverage in your programs.
10.1 Introduction to gcovIntroduction to gcov. 10.2 Invoking gcovHow to use gcov. 10.3 Using gcovwith GCC OptimizationUsing gcov with GCC optimization. 10.4 Brief description of gcovdata filesThe files used by gcov. 10.5 Data file relocation to support cross-profiling Data file relocation.
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gcov
gcov is a test coverage program. Use it in concert with GCC
to analyze your programs to help create more efficient, faster running
code and to discover untested parts of your program. You can use
gcov as a profiling tool to help discover where your
optimization efforts will best affect your code. You can also use
gcov along with the other profiling tool, gprof, to
assess which parts of your code use the greatest amount of computing
time.
Profiling tools help you analyze your code's performance. Using a
profiler such as gcov or gprof, you can find out some
basic performance statistics, such as:
Once you know these things about how your code works when compiled, you
can look at each module to see which modules should be optimized.
gcov helps you determine where to work on optimization.
Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product.
You should compile your code without optimization if you plan to use
gcov because the optimization, by combining some lines of code
into one function, may not give you as much information as you need to
look for `hot spots' where the code is using a great deal of computer
time. Likewise, because gcov accumulates statistics by line (at
the lowest resolution), it works best with a programming style that
places only one statement on each line. If you use complicated macros
that expand to loops or to other control structures, the statistics are
less helpful--they only report on the line where the macro call
appears. If your complex macros behave like functions, you can replace
them with inline functions to solve this problem.
gcov creates a logfile called `sourcefile.gcov' which
indicates how many times each line of a source file `sourcefile.c'
has executed. You can use these logfiles along with gprof to aid
in fine-tuning the performance of your programs. gprof gives
timing information you can use along with the information you get from
gcov.
gcov works only on code compiled with GCC. It is not
compatible with any other profiling or test coverage mechanism.
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gcov
gcov [options] files |
gcov accepts the following options:
-h
--help
gcov (on the standard output), and
exit without doing any further processing.
-v
--version
gcov version number (on the standard output),
and exit without doing any further processing.
-a
--all-blocks
-b
--branch-probabilities
-c
--branch-counts
-n
--no-output
gcov output file.
-l
--long-file-names
gcov on the file `a.c' will
produce an output file called `a.c##x.h.gcov' instead of
`x.h.gcov'. This can be useful if `x.h' is included in
multiple source files and you want to see the individual
contributions. If you use the `-p' option, both the including
and included file names will be complete path names.
-p
--preserve-paths
-r
--relative-only
-f
--function-summaries
-o directory|file
--object-directory directory
--object-file file
-s directory
--source-prefix directory
-u
--unconditional-branches
-d
--display-progress
gcov should be run with the current directory the same as that
when you invoked the compiler. Otherwise it will not be able to locate
the source files. gcov produces files called
`mangledname.gcov' in the current directory. These contain
the coverage information of the source file they correspond to.
One `.gcov' file is produced for each source (or header) file
containing code,
which was compiled to produce the data files. The mangledname part
of the output file name is usually simply the source file name, but can
be something more complicated if the `-l' or `-p' options are
given. Refer to those options for details.
If you invoke gcov with multiple input files, the
contributions from each input file are summed. Typically you would
invoke it with the same list of files as the final link of your executable.
The `.gcov' files contain the `:' separated fields along with program source code. The format is
execution_count:line_number:source line text |
Additional block information may succeed each line, when requested by command line option. The execution_count is `-' for lines containing no code. Unexecuted lines are marked `#####' or `====', depending on whether they are reachable by non-exceptional paths or only exceptional paths such as C++ exception handlers, respectively.
Some lines of information at the start have line_number of zero. These preamble lines are of the form
-:0:tag:value |
The ordering and number of these preamble lines will be augmented as
gcov development progresses -- do not rely on them remaining
unchanged. Use tag to locate a particular preamble line.
The additional block information is of the form
tag information |
The information is human readable, but designed to be simple enough for machine parsing too.
When printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively. Other values which would conventionally be rounded to 0% or 100% are instead printed as the nearest non-boundary value.
When using gcov, you must first compile your program with two
special GCC options: `-fprofile-arcs -ftest-coverage'.
This tells the compiler to generate additional information needed by
gcov (basically a flow graph of the program) and also includes
additional code in the object files for generating the extra profiling
information needed by gcov. These additional files are placed in the
directory where the object file is located.
Running the program will cause profile output to be generated. For each source file compiled with `-fprofile-arcs', an accompanying `.gcda' file will be placed in the object file directory.
Running gcov with your program's source file names as arguments
will now produce a listing of the code along with frequency of execution
for each line. For example, if your program is called `tmp.c', this
is what you see when you use the basic gcov facility:
$ gcc -fprofile-arcs -ftest-coverage tmp.c $ a.out $ gcov tmp.c 90.00% of 10 source lines executed in file tmp.c Creating tmp.c.gcov. |
The file `tmp.c.gcov' contains output from gcov.
Here is a sample:
-: 0:Source:tmp.c
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
1: 4:{
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
10: 10: total += i;
-: 11:
1: 12: if (total != 45)
#####: 13: printf ("Failure\n");
-: 14: else
1: 15: printf ("Success\n");
1: 16: return 0;
-: 17:}
|
When you use the `-a' option, you will get individual block counts, and the output looks like this:
-: 0:Source:tmp.c
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
1: 4:{
1: 4-block 0
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
11: 9-block 0
10: 10: total += i;
10: 10-block 0
-: 11:
1: 12: if (total != 45)
1: 12-block 0
#####: 13: printf ("Failure\n");
$$$$$: 13-block 0
-: 14: else
1: 15: printf ("Success\n");
1: 15-block 0
1: 16: return 0;
1: 16-block 0
-: 17:}
|
In this mode, each basic block is only shown on one line -- the last line of the block. A multi-line block will only contribute to the execution count of that last line, and other lines will not be shown to contain code, unless previous blocks end on those lines. The total execution count of a line is shown and subsequent lines show the execution counts for individual blocks that end on that line. After each block, the branch and call counts of the block will be shown, if the `-b' option is given.
Because of the way GCC instruments calls, a call count can be shown after a line with no individual blocks. As you can see, line 13 contains a basic block that was not executed.
When you use the `-b' option, your output looks like this:
$ gcov -b tmp.c 90.00% of 10 source lines executed in file tmp.c 80.00% of 5 branches executed in file tmp.c 80.00% of 5 branches taken at least once in file tmp.c 50.00% of 2 calls executed in file tmp.c Creating tmp.c.gcov. |
Here is a sample of a resulting `tmp.c.gcov' file:
-: 0:Source:tmp.c
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
function main called 1 returned 1 blocks executed 75%
1: 4:{
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
branch 0 taken 91% (fallthrough)
branch 1 taken 9%
10: 10: total += i;
-: 11:
1: 12: if (total != 45)
branch 0 taken 0% (fallthrough)
branch 1 taken 100%
#####: 13: printf ("Failure\n");
call 0 never executed
-: 14: else
1: 15: printf ("Success\n");
call 0 called 1 returned 100%
1: 16: return 0;
-: 17:}
|
For each function, a line is printed showing how many times the function is called, how many times it returns and what percentage of the function's blocks were executed.
For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.
For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message "never executed" is printed.
For a call, if it was executed at least once, then a percentage
indicating the number of times the call returned divided by the number
of times the call was executed will be printed. This will usually be
100%, but may be less for functions that call exit or longjmp,
and thus may not return every time they are called.
The execution counts are cumulative. If the example program were executed again without removing the `.gcda' file, the count for the number of times each line in the source was executed would be added to the results of the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate data over a number of program runs as part of a test verification suite, or to provide more accurate long-term information over a large number of program runs.
The data in the `.gcda' files is saved immediately before the program exits. For each source file compiled with `-fprofile-arcs', the profiling code first attempts to read in an existing `.gcda' file; if the file doesn't match the executable (differing number of basic block counts) it will ignore the contents of the file. It then adds in the new execution counts and finally writes the data to the file.
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gcov with GCC Optimization
If you plan to use gcov to help optimize your code, you must
first compile your program with two special GCC options:
`-fprofile-arcs -ftest-coverage'. Aside from that, you can use any
other GCC options; but if you want to prove that every single line
in your program was executed, you should not compile with optimization
at the same time. On some machines the optimizer can eliminate some
simple code lines by combining them with other lines. For example, code
like this:
if (a != b) c = 1; else c = 0; |
can be compiled into one instruction on some machines. In this case,
there is no way for gcov to calculate separate execution counts
for each line because there isn't separate code for each line. Hence
the gcov output looks like this if you compiled the program with
optimization:
100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;
|
The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1.
Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at all.
If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file that needs it. If `fileA.o' and `fileB.o' both contain out of line bodies of a particular inlineable function, they will also both contain coverage counts for that function. When `fileA.o' and `fileB.o' are linked together, the linker will, on many systems, select one of those out of line bodies for all calls to that function, and remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function body. Hence when instrumented, all but one use of that function will show zero counts.
If the function is inlined in several places, the block structure in each location might not be the same. For instance, a condition might now be calculable at compile time in some instances. Because the coverage of all the uses of the inline function will be shown for the same source lines, the line counts themselves might seem inconsistent.
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gcov data files
gcov uses two files for profiling. The names of these files
are derived from the original object file by substituting the
file suffix with either `.gcno', or `.gcda'. All of these files
are placed in the same directory as the object file, and contain data
stored in a platform-independent format.
The `.gcno' file is generated when the source file is compiled with the GCC `-ftest-coverage' option. It contains information to reconstruct the basic block graphs and assign source line numbers to blocks.
The `.gcda' file is generated when a program containing object files built with the GCC `-fprofile-arcs' option is executed. A separate `.gcda' file is created for each object file compiled with this option. It contains arc transition counts, and some summary information.
The full details of the file format is specified in `gcov-io.h', and functions provided in that header file should be used to access the coverage files.
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Running the program will cause profile output to be generated. For each source file compiled with `-fprofile-arcs', an accompanying `.gcda' file will be placed in the object file directory. That implicitly requires running the program on the same system as it was built or having the same absolute directory structure on the target system. The program will try to create the needed directory structure, if it is not already present.
To support cross-profiling, a program compiled with `-fprofile-arcs' can relocate the data files based on two environment variables:
Note: If GCOV_PREFIX_STRIP is set without GCOV_PREFIX is undefined, then a relative path is made out of the hardwired absolute paths.
For example, if the object file `/user/build/foo.o' was built with `-fprofile-arcs', the final executable will try to create the data file `/user/build/foo.gcda' when running on the target system. This will fail if the corresponding directory does not exist and it is unable to create it. This can be overcome by, for example, setting the environment as `GCOV_PREFIX=/target/run' and `GCOV_PREFIX_STRIP=1'. Such a setting will name the data file `/target/run/build/foo.gcda'.
You must move the data files to the expected directory tree in order to
use them for profile directed optimizations (`--use-profile'), or to
use the gcov tool.
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This section describes known problems that affect users of GCC. Most of these are not GCC bugs per se--if they were, we would fix them. But the result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best.
11.1 Actual Bugs We Haven't Fixed Yet Bugs we will fix later. 11.2 Cross-Compiler Problems Common problems of cross compiling with GCC. 11.3 Interoperation Problems using GCC with other compilers, and with certain linkers, assemblers and debuggers. 11.4 Incompatibilities of GCC GCC is incompatible with traditional C. 11.5 Fixed Header Files GCC uses corrected versions of system header files. This is necessary, but doesn't always work smoothly. 11.6 Standard Libraries GCC uses the system C library, which might not be compliant with the ISO C standard. 11.7 Disappointments and Misunderstandings Regrettable things we can't change, but not quite bugs. 11.8 Common Misunderstandings with GNU C++ Common misunderstandings with GNU C++. 11.9 Certain Changes We Don't Want to Make Things we think are right, but some others disagree. 11.10 Warning Messages and Error Messages Which problems in your code get warnings, and which get errors.
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fixincludes script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while fixincludes is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
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You may run into problems with cross compilation on certain machines, for several reasons.
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This section lists various difficulties encountered in using GCC together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.
An area where the difference is most apparent is name mangling. The use of different name mangling is intentional, to protect you from more subtle problems. Compilers differ as to many internal details of C++ implementation, including: how class instances are laid out, how multiple inheritance is implemented, and how virtual function calls are handled. If the name encoding were made the same, your programs would link against libraries provided from other compilers--but the programs would then crash when run. Incompatible libraries are then detected at link time, rather than at run time.
double on an 8-byte
boundary, and it expects every double to be so aligned. The Sun
compiler usually gives double values 8-byte alignment, with one
exception: function arguments of type double may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type double and passes this pointer of type
double * to a function compiled with GCC, dereferencing the
pointer may cause a fatal signal.
One way to solve this problem is to compile your entire program with GCC.
Another solution is to modify the function that is compiled with
Sun CC to copy the argument into a local variable; local variables
are always properly aligned. A third solution is to modify the function
that uses the pointer to dereference it via the following function
access_double instead of directly with `*':
inline double
access_double (double *unaligned_ptr)
{
union d2i { double d; int i[2]; };
union d2i *p = (union d2i *) unaligned_ptr;
union d2i u;
u.i[0] = p->i[0];
u.i[1] = p->i[1];
return u.d;
}
|
Storing into the pointer can be done likewise with the same union.
malloc function in the `libmalloc.a' library
may allocate memory that is only 4 byte aligned. Since GCC on the
SPARC assumes that doubles are 8 byte aligned, this may result in a
fatal signal if doubles are stored in memory allocated by the
`libmalloc.a' library.
The solution is to not use the `libmalloc.a' library. Use instead
malloc and related functions from `libc.a'; they do not have
this problem.
alloca or variable-size arrays. This is because GCC doesn't
generate HP-UX unwind descriptors for such functions. It may even be
impossible to generate them.
(warning) Use of GR3 when frame >= 8192 may cause conflict. |
These warnings are harmless and can be safely ignored.
LANG
environment variable to `C' or `En_US'.
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There are several noteworthy incompatibilities between GNU C and K&R (non-ISO) versions of C.
One consequence is that you cannot call mktemp with a string
constant argument. The function mktemp always alters the
string its argument points to.
Another consequence is that sscanf does not work on some very
old systems when passed a string constant as its format control string
or input. This is because sscanf incorrectly tries to write
into the string constant. Likewise fscanf and scanf.
The solution to these problems is to change the program to use
char-array variables with initialization strings for these
purposes instead of string constants.
-2147483648 is positive.
This is because 2147483648 cannot fit in the type int, so
(following the ISO C rules) its data type is unsigned long int.
Negating this value yields 2147483648 again.
#define foo(a) "a" |
will produce output "a" regardless of what the argument a is.
setjmp and longjmp, the only automatic
variables guaranteed to remain valid are those declared
volatile. This is a consequence of automatic register
allocation. Consider this function:
jmp_buf j;
foo ()
{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* |
Here a may or may not be restored to its first value when the
longjmp occurs. If a is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
If you use the `-W' option with the `-O' option, you will get a warning when GCC thinks such a problem might be possible.
foobar (
#define luser
hack)
|
ISO C does not permit such a construct.
In some other C compilers, an extern declaration affects all the
rest of the file even if it happens within a block.
long, etc., with a typedef name,
as shown here:
typedef int foo; typedef long foo bar; |
In ISO C, this is not allowed: long and other type modifiers
require an explicit int.
typedef int foo; typedef foo foo; |
#if 0 You can't expect this to work. #endif |
The best solution to such a problem is to put the text into an actual C comment delimited by `/*...*/'.
time, so it did not matter what type your program declared it to
return. But in systems with ISO C headers, time is declared to
return time_t, and if that is not the same as long, then
`long time ();' is erroneous.
The solution is to change your program to use appropriate system headers
(<time.h> on systems with ISO C headers) and not to declare
time if the system header files declare it, or failing that to
use time_t as the return type of time.
float, PCC converts it to
a double. GCC actually returns a float. If you are concerned
with PCC compatibility, you should declare your functions to return
double; you might as well say what you mean.
The method used by GCC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The target hook TARGET_STRUCT_VALUE_RTX
tells GCC where to pass this address.
By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GCC does not use this method because it is slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all structure and union returning. GCC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers.
You can tell GCC to use a compatible convention for all structure and union returning with the option `-fpcc-struct-return'.
A preprocessing token is a preprocessing number if it begins with a digit and is followed by letters, underscores, digits, periods and `e+', `e-', `E+', `E-', `p+', `p-', `P+', or `P-' character sequences. (In strict C90 mode, the sequences `p+', `p-', `P+' and `P-' cannot appear in preprocessing numbers.)
To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number.
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GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GCC unless they are changed. Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers.
Installing GCC automatically creates and installs the fixed header
files, by running a program called fixincludes. Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
mkheaders script
installed in
`libexecdir/gcc/target/version/install-tools/'.
The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it.
It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you'll have to do this by hand.
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GCC by itself attempts to be a conforming freestanding implementation. See section Language Standards Supported by GCC, for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using `-Wall') that you don't expect.
For example, the sprintf function on SunOS 4.1.3 returns
char * while the C standard says that sprintf returns an
int. The fixincludes program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return char *.
If you need a Standard compliant library, then you need to find one, as
GCC does not provide one. The GNU C library (called glibc)
provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for
GNU/Linux and HURD-based GNU systems; no recent version of it supports
other systems, though some very old versions did. Version 2.2 of the
GNU C library includes nearly complete C99 support. You could also ask
your operating system vendor if newer libraries are available.
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These problems are perhaps regrettable, but we don't know any practical way around them.
This occurs because sometimes GCC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable "would have had", and it is not clear that would be desirable anyway. So GCC simply does not mention the eliminated variable when it writes debugging information.
You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization.
int foo (struct mumble *);
struct mumble { ... };
int foo (struct mumble *x)
{ ... }
|
This code really is erroneous, because the scope of struct
mumble in the prototype is limited to the argument list containing it.
It does not refer to the struct mumble defined with file scope
immediately below--they are two unrelated types with similar names in
different scopes.
But in the definition of foo, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ISO standard specifies.
It is easy enough for you to make your code work by moving the
definition of struct mumble above the prototype. It's not worth
being incompatible with ISO C just to avoid an error for the example
shown above.
If you care about controlling the amount of memory that is accessed, use volatile but do not use bit-fields.
If new system header files are installed, nothing automatically arranges
to update the corrected header files. They can be updated using the
mkheaders script installed in
`libexecdir/gcc/target/version/install-tools/'.
double in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the `-ffloat-store' option (see section 3.10 Options That Control Optimization).
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C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.
11.8.1 Declare and Define Static Members Static member declarations are not definitions 11.8.2 Name lookup, templates, and accessing members of base classes 11.8.3 Temporaries May Vanish Before You Expect Temporaries may vanish before you expect 11.8.4 Implicit Copy-Assignment for Virtual Bases Copy Assignment operators copy virtual bases twice
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When a class has static data members, it is not enough to declare the static member; you must also define it. For example:
class Foo
{
...
void method();
static int bar;
};
|
This declaration only establishes that the class Foo has an
int named Foo::bar, and a member function named
Foo::method. But you still need to define both
method and bar elsewhere. According to the ISO
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0; |
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to g++ from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: g++ reports as undefined
symbols any static data members that lack definitions.
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The C++ standard prescribes that all names that are not dependent on template parameters are bound to their present definitions when parsing a template function or class.(5) Only names that are dependent are looked up at the point of instantiation. For example, consider
void foo(double);
struct A {
template <typename T>
void f () {
foo (1); // 1
int i = N; // 2
T t;
t.bar(); // 3
foo (t); // 4
}
static const int N;
};
|
Here, the names foo and N appear in a context that does
not depend on the type of T. The compiler will thus require that
they are defined in the context of use in the template, not only before
the point of instantiation, and will here use ::foo(double) and
A::N, respectively. In particular, it will convert the integer
value to a double when passing it to ::foo(double).
Conversely, bar and the call to foo in the fourth marked
line are used in contexts that do depend on the type of T, so
they are only looked up at the point of instantiation, and you can
provide declarations for them after declaring the template, but before
instantiating it. In particular, if you instantiate A::f<int>,
the last line will call an overloaded ::foo(int) if one was
provided, even if after the declaration of struct A.
This distinction between lookup of dependent and non-dependent names is called two-stage (or dependent) name lookup. G++ implements it since version 3.4.
Two-stage name lookup sometimes leads to situations with behavior different from non-template codes. The most common is probably this:
template <typename T> struct Base {
int i;
};
template <typename T> struct Derived : public Base<T> {
int get_i() { return i; }
};
|
In get_i(), i is not used in a dependent context, so the
compiler will look for a name declared at the enclosing namespace scope
(which is the global scope here). It will not look into the base class,
since that is dependent and you may declare specializations of
Base even after declaring Derived, so the compiler can't
really know what i would refer to. If there is no global
variable i, then you will get an error message.
In order to make it clear that you want the member of the base class,
you need to defer lookup until instantiation time, at which the base
class is known. For this, you need to access i in a dependent
context, by either using this->i (remember that this is of
type Derived<T>*, so is obviously dependent), or using
Base<T>::i. Alternatively, Base<T>::i might be brought
into scope by a using-declaration.
Another, similar example involves calling member functions of a base class:
template <typename T> struct Base {
int f();
};
template <typename T> struct Derived : Base<T> {
int g() { return f(); };
};
|
Again, the call to f() is not dependent on template arguments
(there are no arguments that depend on the type T, and it is also
not otherwise specified that the call should be in a dependent context).
Thus a global declaration of such a function must be available, since
the one in the base class is not visible until instantiation time. The
compiler will consequently produce the following error message:
x.cc: In member function `int Derived<T>::g()':
x.cc:6: error: there are no arguments to `f' that depend on a template
parameter, so a declaration of `f' must be available
x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but
allowing the use of an undeclared name is deprecated)
|
To make the code valid either use this->f(), or
Base<T>::f(). Using the `-fpermissive' flag will also let
the compiler accept the code, by marking all function calls for which no
declaration is visible at the time of definition of the template for
later lookup at instantiation time, as if it were a dependent call.
We do not recommend using `-fpermissive' to work around invalid
code, and it will also only catch cases where functions in base classes
are called, not where variables in base classes are used (as in the
example above).
Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above code without an error. Those compilers do not implement two-stage name lookup correctly.
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It is dangerous to use pointers or references to portions of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type char *
or const char *---which is one reason why the standard
string class requires you to call the c_str member
function. However, any class that returns a pointer to some internal
structure is potentially subject to this problem.
For example, a program may use a function strfunc that returns
string objects, and another function charfunc that
operates on pointers to char:
string strfunc ();
void charfunc (const char *);
void
f ()
{
const char *p = strfunc().c_str();
...
charfunc (p);
...
charfunc (p);
}
|
In this situation, it may seem reasonable to save a pointer to the C
string returned by the c_str member function and use that rather
than call c_str repeatedly. However, the temporary string
created by the call to strfunc is destroyed after p is
initialized, at which point p is left pointing to freed memory.
Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.
The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example:
const string& tmp = strfunc (); charfunc (tmp.c_str ()); |
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When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example:
struct Base{
char *name;
Base(char *n) : name(strdup(n)){}
Base& operator= (const Base& other){
free (name);
name = strdup (other.name);
}
};
struct A:virtual Base{
int val;
A():Base("A"){}
};
struct B:virtual Base{
int bval;
B():Base("B"){}
};
struct Derived:public A, public B{
Derived():Base("Derived"){}
};
void func(Derived &d1, Derived &d2)
{
d1 = d2;
}
|
The C++ standard specifies that `Base::Base' is only called once when constructing or copy-constructing a Derived object. It is unspecified whether `Base::operator=' is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside `func' in the example).
G++ implements the "intuitive" algorithm for copy-assignment: assign all
direct bases, then assign all members. In that algorithm, the virtual
base subobject can be encountered more than once. In the example, copying
proceeds in the following order: `val', `name' (via
strdup), `bval', and `name' again.
If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.
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This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.
Such a feature would work only occasionally--only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile.
Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good.
Such assignments must be very common; warning about them would cause more annoyance than good.
C contains many standard functions that return a value that most
programs choose to ignore. One obvious example is printf.
Warning about this practice only leads the defensive programmer to
clutter programs with dozens of casts to void. Such casts are
required so frequently that they become visual noise. Writing those
casts becomes so automatic that they no longer convey useful
information about the intentions of the programmer. For functions
where the return value should never be ignored, use the
warn_unused_result function attribute (see section 6.30 Declaring Attributes of Functions).
This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly.
The ISO C standard leaves it up to the implementation whether a bit-field
declared plain int is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed dialect with `-fsigned-bitfields' and the unsigned dialect with `-funsigned-bitfields'. However, this leaves open the question of which dialect to use by default.
Currently, the preferred dialect makes plain bit-fields signed, because
this is simplest. Since int is the same as signed int in
every other context, it is cleanest for them to be the same in bit-fields
as well.
Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures.
A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines.
Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility.
This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default).
There are some arguments for making bit-fields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future.
(Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.)
__STDC__ when `-ansi' is not used.
Currently, GCC defines __STDC__ unconditionally. This provides
good results in practice.
Programmers normally use conditionals on __STDC__ to ask whether
it is safe to use certain features of ISO C, such as function
prototypes or ISO token concatenation. Since plain gcc supports
all the features of ISO C, the correct answer to these questions is
"yes".
Some users try to use __STDC__ to check for the availability of
certain library facilities. This is actually incorrect usage in an ISO
C program, because the ISO C standard says that a conforming
freestanding implementation should define __STDC__ even though it
does not have the library facilities. `gcc -ansi -pedantic' is a
conforming freestanding implementation, and it is therefore required to
define __STDC__, even though it does not come with an ISO C
library.
Sometimes people say that defining __STDC__ in a compiler that
does not completely conform to the ISO C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ISO C, such as `gcc -ansi'---not for other
compilers such as plain gcc. Whatever the ISO C standard says
is relevant to the design of plain gcc without `-ansi' only
for pragmatic reasons, not as a requirement.
GCC normally defines __STDC__ to be 1, and in addition
defines __STRICT_ANSI__ if you specify the `-ansi' option,
or a `-std' option for strict conformance to some version of ISO C.
On some hosts, system include files use a different convention, where
__STDC__ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GCC follows the host convention when
processing system include files, but when processing user files it follows
the usual GNU C convention.
__STDC__ in C++.
Programs written to compile with C++-to-C translators get the
value of __STDC__ that goes with the C compiler that is
subsequently used. These programs must test __STDC__
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ISO C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if __STDC__ is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ISO
C but not in traditional C. Many of these header files can work without
change in C++ provided __STDC__ is defined. If __STDC__
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
Historically, GCC has not deleted "empty" loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.
However, the rationale here is that optimization of a nonempty loop cannot produce an empty one. This held for carefully written C compiled with less powerful optimizers but is not always the case for carefully written C++ or with more powerful optimizers. Thus GCC will remove operations from loops whenever it can determine those operations are not externally visible (apart from the time taken to execute them, of course). In case the loop can be proved to be finite, GCC will also remove the loop itself.
Be aware of this when performing timing tests, for instance the
following loop can be completely removed, provided
some_expression can provably not change any global state.
{
int sum = 0;
int ix;
for (ix = 0; ix != 10000; ix++)
sum += some_expression;
}
|
Even though sum is accumulated in the loop, no use is made of
that summation, so the accumulation can be removed.
It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another:
void func (int, int); int i = 2; func (i++, i++); |
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any particular
order. Either increment might happen first. func might get the
arguments `2, 3', or it might get `3, 2', or even `2, 2'.
Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program.
ISO C requires a "diagnostic" message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If testsuites call this "failure", they should be run with the GCC option `-pedantic-errors', which will turn these warnings into errors.
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The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the `-W' options (for instance, `-Wall' requests a variety of useful warnings).
GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The `-pedantic' option tells GCC to issue warnings in such cases; `-pedantic-errors' says to make them errors instead. This does not mean that all non-ISO constructs get warnings or errors.
See section Options to Request or Suppress Warnings, for more detail on these and related command-line options.
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Your bug reports play an essential role in making GCC reliable.
When you encounter a problem, the first thing to do is to see if it is already known. See section 11. Known Causes of Trouble with GCC. If it isn't known, then you should report the problem.
12.1 Have You Found a Bug? Have you really found a bug? 12.2 How and where to Report Bugs How to report a bug effectively. 11. Known Causes of Trouble with GCC Known problems. 13. How To Get Help with GCC Where to ask for help.
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If you are not sure whether you have found a bug, here are some guidelines:
asm statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.
However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write `x;'
at the end of a function instead of `return x;', with the same
results. But the value of the function is undefined if return
is omitted; it is not a bug when GCC produces different results.
Problems often result from expressions with two increment operators,
as in f (*p++, *p++). Your previous compiler might have
interpreted that expression the way you intended; GCC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.
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Bugs should be reported to the bug database at http://gcc.gnu.org/bugs.html.
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If you need help installing, using or changing GCC, there are two ways to find it:
For further information, see http://gcc.gnu.org/faq.html#support.
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If you would like to help pretest GCC releases to assure they work well, current development sources are available by SVN (see http://gcc.gnu.org/svn.html). Source and binary snapshots are also available for FTP; see http://gcc.gnu.org/snapshots.html.
If you would like to work on improvements to GCC, please read the advice at these URLs:
http://gcc.gnu.org/contribute.html http://gcc.gnu.org/contributewhy.html |
for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at http://gcc.gnu.org/projects/.
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If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.
Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers--the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can compare, such as, "We will donate ten dollars to the Frobnitz project for each disk sold." Don't be satisfied with a vague promise, such as "A portion of the profits are donated," since it doesn't give a basis for comparison.
Even a precise fraction "of the profits from this disk" is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.
By establishing the idea that supporting further development is "the proper thing to do" when distributing free software for a fee, we can assure a steady flow of resources into making more free software.
Copyright (C) 1994 Free Software Foundation, Inc. Verbatim copying and redistribution of this section is permitted without royalty; alteration is not permitted. |
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The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for "GNU's Not Unix"; it is pronounced "guh-NEW".) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as "Linux", they are more accurately called GNU/Linux systems.
For more information, see:
http://www.gnu.org/ http://www.gnu.org/gnu/linux-and-gnu.html |
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Copyright (C) 2007 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. |
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/. |
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:
program Copyright (C) year name of author This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. |
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an "about box".
You should also get your employer (if you work as a programmer) or school, if any, to sign a "copyright disclaimer" for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.
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Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. |
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
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Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. |
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The GCC project would like to thank its many contributors. Without them the project would not have been nearly as successful as it has been. Any omissions in this list are accidental. Feel free to contact law@redhat.com or gerald@pfeifer.com if you have been left out or some of your contributions are not listed. Please keep this list in alphabetical order.
valarray<>, complex<>, maintaining the numerics library
(including that pesky <limits> :-) and keeping up-to-date anything
to do with numbers.
complex<>, sanity checking and disbursement, configuration
architecture, libio maintenance, and early math work.
protoize and unprotoize
tools, the support for Dwarf symbolic debugging information, and much of
the support for System V Release 4. He has also worked heavily on the
Intel 386 and 860 support.
restrict support, and serving as release manager for GCC 3.x.
INTEGER*1, INTEGER*2, and
LOGICAL*1.
The following people are recognized for their contributions to GNAT, the Ada front end of GCC:
The following people are recognized for their contributions of new features, bug reports, testing and integration of classpath/libgcj for GCC version 4.1:
JTree implementation and lots Free Swing
additions and bug fixes.
GapContent bug fixes.
JList, Free Swing 1.5 updates and mouse event
fixes, lots of Free Swing work including JTable editing.
HTTPURLConnection fixes.
MessageFormat fixes.
Serialization fixes.
StAX
and DOM xml:id support.
TreePath and TreeSelection fixes.
URLClassLoader updates.
SocketTimeoutException.
BitSet bug fixes, HttpURLConnection
rewrite and improvements.
ClassLoader and nio cleanups, serialization fixes,
better Proxy support, bug fixes and IKVM integration.
AccessControlContext fixes.
VMClassLoader and AccessController
improvements.
basic and metal icon and plaf support
and lots of documenting, Lots of Free Swing and metal theme
additions. MetalIconFactory implementation.
MIDI framework, ALSA and DSSI
providers.
Serialization and URLClassLoader fixes,
gcj build speedups.
JFileChooser implementation.
Locale and net fixes, URI RFC2986
updates, Serialization fixes, Properties XML support and
generic branch work, VMIntegration guide update.
TimeZone bug fixing.
NetworkInterface implementation and updates.
BoxLayout, GrayFilter and
SplitPane, plus bug fixes all over. Lots of Free Swing work
including styled text.
String cleanups and optimization suggestions.
Locale updates, bug and
build fixes.
Pointer updates. Logger bug fixes.
Graphics2D upgraded to Cairo 0.5 and new regex
features.
TextLayout
fixes. GtkImage rewrite, 2D, awt, free swing and date/time fixes and
implementing the Qt4 peers.
FileChannel lock,
SystemLogger and FileHandler rotate implementations, NIO
FileChannel.map support, security and policy updates.
File locking fixes.
Image, Logger and URLClassLoader
updates.
MenuSelectionManager implementation.
BasicTreeUI and JTree fixes.
TreeNode enumerations and ActionCommand and various
fixes, XML and URL, AWT and Free Swing bug fixes.
CACAO integration, fdlibm updates.
VMClassLoader boot packages support suggestions.
Qt4
support for Darwin/OS X, Graphics2D support, gtk+
updates.
DEBUG support, build cleanups and
Kaffe integration. Qt4 build infrastructure, SHA1PRNG
and GdkPixbugDecoder updates.
Clipboard implementation, system call interrupts and network
timeouts and GdkPixpufDecoder fixes.
In addition to the above, all of which also contributed time and energy in testing GCC, we would like to thank the following for their contributions to testing:
And finally we'd like to thank everyone who uses the compiler, provides feedback and generally reminds us why we're doing this work in the first place.
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GCC's command line options are indexed here without any initial `-' or `--'. Where an option has both positive and negative forms (such as `-foption' and `-fno-option'), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms.
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On some systems, `gcc -shared' needs to build supplementary stub code for constructors to work. On multi-libbed systems, `gcc -shared' must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.
Future versions of GCC may zero-extend, or use
a target-defined ptr_extend pattern. Do not rely on sign extension.
The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.
A file's basename was the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'.
The C++ standard just uses the term "dependent" for names that depend on the type or value of template parameters. This shorter term will also be used in the rest of this section.
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typeof
6.17 Arrays of Length Zero
6.18 Structures With No Members
6.19 Arrays of Variable Length
6.20 Macros with a Variable Number of Arguments.
6.21 Slightly Looser Rules for Escaped Newlines
6.22 Non-Lvalue Arrays May Have Subscripts
6.23 Arithmetic on void- and Function-Pointers
6.24 Non-Constant Initializers
6.25 Compound Literals
6.26 Designated Initializers
6.27 Case Ranges
6.28 Cast to a Union Type
6.29 Mixed Declarations and Code
6.30 Declaring Attributes of Functions
6.31 Attribute Syntax
6.32 Prototypes and Old-Style Function Definitions
6.33 C++ Style Comments
6.34 Dollar Signs in Identifier Names
6.35 The Character ESC in Constants
6.36 Specifying Attributes of Variables
6.37 Specifying Attributes of Types
6.38 Inquiring on Alignment of Types or Variables
6.39 An Inline Function is As Fast As a Macro
6.40 When is a Volatile Object Accessed?
6.41 Assembler Instructions with C Expression Operands
asm Operands
enum Types
+load: Executing code before main
gcov---a Test Coverage Program
gcov
gcov
gcov with GCC Optimization
gcov data files
| [Top] | [Contents] | [Index] | [ ? ] |
1. Programming Languages Supported by GCC
2. Language Standards Supported by GCC
3. GCC Command Options
4. C Implementation-defined behavior
5. C++ Implementation-defined behavior
6. Extensions to the C Language Family
7. Extensions to the C++ Language
8. GNU Objective-C features
9. Binary Compatibility
10.gcov---a Test Coverage Program
11. Known Causes of Trouble with GCC
12. Reporting Bugs
13. How To Get Help with GCC
14. Contributing to GCC Development
Funding Free Software
The GNU Project and GNU/Linux
GNU General Public License
GNU Free Documentation License
Contributors to GCC
Option Index
Keyword Index
| [Top] | [Contents] | [Index] | [ ? ] |
| Button | Name | Go to | From 1.2.3 go to |
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| [ < ] | Back | previous section in reading order | 1.2.2 |
| [ > ] | Forward | next section in reading order | 1.2.4 |
| [ << ] | FastBack | previous or up-and-previous section | 1.1 |
| [ Up ] | Up | up section | 1.2 |
| [ >> ] | FastForward | next or up-and-next section | 1.3 |
| [Top] | Top | cover (top) of document | |
| [Contents] | Contents | table of contents | |
| [Index] | Index | concept index | |
| [ ? ] | About | this page |