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This document is distributed under the terms of the GNU Free Documentation License version 1.3. A copy of the license is included in the section entitled "GNU Free Documentation License".
1. Overview 2. Invocation 3. Linker Scripts 4. Machine Dependent Features
6. Reporting Bugs A. MRI Compatible Script Files B. GNU Free Documentation License LD Index
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ld combines a number of object and archive files, relocates
their data and ties up symbol references. Usually the last step in
compiling a program is to run ld.
ld accepts Linker Command Language files written in
a superset of AT&T's Link Editor Command Language syntax,
to provide explicit and total control over the linking process.
This version of ld uses the general purpose BFD libraries
to operate on object files. This allows ld to read, combine, and
write object files in many different formats--for example, COFF or
a.out. Different formats may be linked together to produce any
available kind of object file. See section 5. BFD, for more information.
Aside from its flexibility, the GNU linker is more helpful than other
linkers in providing diagnostic information. Many linkers abandon
execution immediately upon encountering an error; whenever possible,
ld continues executing, allowing you to identify other errors
(or, in some cases, to get an output file in spite of the error).
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The GNU linker ld is meant to cover a broad range of situations,
and to be as compatible as possible with other linkers. As a result,
you have many choices to control its behavior.
2.1 Command Line Options 2.2 Environment Variables
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The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context.
For instance, a frequent use of ld is to link standard Unix
object files on a standard, supported Unix system. On such a system, to
link a file hello.o:
ld -o output /lib/crt0.o hello.o -lc |
This tells ld to produce a file called output as the
result of linking the file /lib/crt0.o with hello.o and
the library libc.a, which will come from the standard search
directories. (See the discussion of the `-l' option below.)
Some of the command-line options to ld may be specified at any
point in the command line. However, options which refer to files, such
as `-l' or `-T', cause the file to be read at the point at
which the option appears in the command line, relative to the object
files and other file options. Repeating non-file options with a
different argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of that
option. Options which may be meaningfully specified more than once are
noted in the descriptions below.
Non-option arguments are object files or archives which are to be linked together. They may follow, precede, or be mixed in with command-line options, except that an object file argument may not be placed between an option and its argument.
Usually the linker is invoked with at least one object file, but you can specify other forms of binary input files using `-l', `-R', and the script command language. If no binary input files at all are specified, the linker does not produce any output, and issues the message `No input files'.
If the linker cannot recognize the format of an object file, it will
assume that it is a linker script. A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using `-T'). This feature
permits the linker to link against a file which appears to be an object
or an archive, but actually merely defines some symbol values, or uses
INPUT or GROUP to load other objects. Specifying a
script in this way merely augments the main linker script, with the
extra commands placed after the main script; use the `-T' option
to replace the default linker script entirely, but note the effect of
the INSERT command. See section 3. Linker Scripts.
For options whose names are a single letter, option arguments must either follow the option letter without intervening whitespace, or be given as separate arguments immediately following the option that requires them.
For options whose names are multiple letters, either one dash or two can precede the option name; for example, `-trace-symbol' and `--trace-symbol' are equivalent. Note--there is one exception to this rule. Multiple letter options that start with a lower case 'o' can only be preceded by two dashes. This is to reduce confusion with the `-o' option. So for example `-omagic' sets the output file name to `magic' whereas `--omagic' sets the NMAGIC flag on the output.
Arguments to multiple-letter options must either be separated from the option name by an equals sign, or be given as separate arguments immediately following the option that requires them. For example, `--trace-symbol foo' and `--trace-symbol=foo' are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
Note--if the linker is being invoked indirectly, via a compiler driver (e.g. `gcc') then all the linker command line options should be prefixed by `-Wl,' (or whatever is appropriate for the particular compiler driver) like this:
gcc -Wl,--start-group foo.o bar.o -Wl,--end-group |
This is important, because otherwise the compiler driver program may silently drop the linker options, resulting in a bad link. Confusion may also arise when passing options that require values through a driver, as the use of a space between option and argument acts as a separator, and causes the driver to pass only the option to the linker and the argument to the compiler. In this case, it is simplest to use the joined forms of both single- and multiple-letter options, such as:
gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map |
Here is a table of the generic command line switches accepted by the GNU linker:
@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.
-a keyword
--audit AUDITLIB
DT_AUDIT entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the DT_SONAME
specified in the library. If specified multiple times DT_AUDIT
will contain a colon separated list of audit interfaces to use. If the linker
finds an object with an audit entry while searching for shared libraries,
it will add a corresponding DT_DEPAUDIT entry in the output file.
This option is only meaningful on ELF platforms supporting the rtld-audit
interface.
-b input-format
--format=input-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`-b' option to specify the binary format for input object files
that follow this option on the command line. Even when ld is
configured to support alternative object formats, you don't usually need
to specify this, as ld should be configured to expect as a
default input format the most usual format on each machine.
input-format is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with `objdump -i'.)
See section 5. BFD.
You may want to use this option if you are linking files with an unusual binary format. You can also use `-b' to switch formats explicitly (when linking object files of different formats), by including `-b input-format' before each group of object files in a particular format.
The default format is taken from the environment variable
GNUTARGET.
See section 2.2 Environment Variables.
You can also define the input format from a script, using the command
TARGET;
see 3.4.3 Commands Dealing with Object File Formats.
-c MRI-commandfile
--mri-script=MRI-commandfile
ld accepts script
files written in an alternate, restricted command language, described in
MRI Compatible Script Files.
Introduce MRI script files with
the option `-c'; use the `-T' option to run linker
scripts written in the general-purpose ld scripting language.
If MRI-cmdfile does not exist, ld looks for it in the directories
specified by any `-L' options.
-d
-dc
-dp
FORCE_COMMON_ALLOCATION has the same effect.
See section 3.4.5 Other Linker Script Commands.
--depaudit AUDITLIB
-P AUDITLIB
DT_DEPAUDIT entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the DT_SONAME
specified in the library. If specified multiple times DT_DEPAUDIT
will contain a colon separated list of audit interfaces to use. This
option is only meaningful on ELF platforms supporting the rtld-audit interface.
The -P option is provided for Solaris compatibility.
-e entry
--entry=entry
--exclude-libs lib,lib,...
--exclude-libs ALL excludes symbols in all archive libraries from
automatic export. This option is available only for the i386 PE targeted
port of the linker and for ELF targeted ports. For i386 PE, symbols
explicitly listed in a .def file are still exported, regardless of this
option. For ELF targeted ports, symbols affected by this option will
be treated as hidden.
--exclude-modules-for-implib module,module,...
ld to open the files; for archive members, this is simply
the member name, but for object files the name listed must include and
match precisely any path used to specify the input file on the linker's
command-line. This option is available only for the i386 PE targeted port
of the linker. Symbols explicitly listed in a .def file are still exported,
regardless of this option.
-E
--export-dynamic
--no-export-dynamic
If you do not use either of these options (or use the `--no-export-dynamic' option to restore the default behavior), the dynamic symbol table will normally contain only those symbols which are referenced by some dynamic object mentioned in the link.
If you use dlopen to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.
You can also use the dynamic list to control what symbols should be added to the dynamic symbol table if the output format supports it. See the description of `--dynamic-list'.
Note that this option is specific to ELF targeted ports. PE targets support a similar function to export all symbols from a DLL or EXE; see the description of `--export-all-symbols' below.
-EB
-EL
-f name
--auxiliary=name
If you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_AUXILIARY field. If the dynamic linker resolves any symbols from the filter object, it will first check whether there is a definition in the shared object name. If there is one, it will be used instead of the definition in the filter object. The shared object name need not exist. Thus the shared object name may be used to provide an alternative implementation of certain functions, perhaps for debugging or for machine specific performance.
This option may be specified more than once. The DT_AUXILIARY entries will be created in the order in which they appear on the command line.
-F name
--filter=name
If you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_FILTER field. The dynamic linker will resolve symbols according to the symbol table of the filter object as usual, but it will actually link to the definitions found in the shared object name. Thus the filter object can be used to select a subset of the symbols provided by the object name.
Some older linkers used the `-F' option throughout a compilation
toolchain for specifying object-file format for both input and output
object files.
The GNU linker uses other mechanisms for this purpose: the
`-b', `--format', `--oformat' options, the
TARGET command in linker scripts, and the GNUTARGET
environment variable.
The GNU linker will ignore the `-F' option when not
creating an ELF shared object.
-fini=name
_fini as
the function to call.
-g
-G value
--gpsize=value
-h name
-soname=name
-i
-init=name
_init as the
function to call.
-l namespec
--library=namespec
ld
will search the library path for a file called filename, otherwise it
will search the library path for a file called `libnamespec.a'.
On systems which support shared libraries, ld may also search for
files other than `libnamespec.a'. Specifically, on ELF
and SunOS systems, ld will search a directory for a library
called `libnamespec.so' before searching for one called
`libnamespec.a'. (By convention, a .so extension
indicates a shared library.) Note that this behavior does not apply
to `:filename', which always specifies a file called
filename.
The linker will search an archive only once, at the location where it is specified on the command line. If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. However, an undefined symbol in an object appearing later on the command line will not cause the linker to search the archive again.
See the `-(' option for a way to force the linker to search archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers. However,
if you are using ld on AIX, note that it is different from the
behaviour of the AIX linker.
-L searchdir
--library-path=searchdir
ld will search
for archive libraries and ld control scripts. You may use this
option any number of times. The directories are searched in the order
in which they are specified on the command line. Directories specified
on the command line are searched before the default directories. All
`-L' options apply to all `-l' options, regardless of the
order in which the options appear. `-L' options do not affect
how ld searches for a linker script unless `-T'
option is specified.
If searchdir begins with =, then the = will be replaced
by the sysroot prefix, a path specified when the linker is configured.
The default set of paths searched (without being specified with
`-L') depends on which emulation mode ld is using, and in
some cases also on how it was configured. See section 2.2 Environment Variables.
The paths can also be specified in a link script with the
SEARCH_DIR command. Directories specified this way are searched
at the point in which the linker script appears in the command line.
-m emulation
If the `-m' option is not used, the emulation is taken from the
LDEMULATION environment variable, if that is defined.
Otherwise, the default emulation depends upon how the linker was configured.
-M
--print-map
Note - symbols whose values are computed by an expression which involves a reference to a previous value of the same symbol may not have correct result displayed in the link map. This is because the linker discards intermediate results and only retains the final value of an expression. Under such circumstances the linker will display the final value enclosed by square brackets. Thus for example a linker script containing:
foo = 1 foo = foo * 4 foo = foo + 8 |
will produce the following output in the link map if the `-M' option is used:
0x00000001 foo = 0x1 [0x0000000c] foo = (foo * 0x4) [0x0000000c] foo = (foo + 0x8) |
See 3.10 Expressions in Linker Scripts for more information about expressions in linker scripts.
-n
--nmagic
NMAGIC.
-N
--omagic
OMAGIC. Note: Although a writable text section
is allowed for PE-COFF targets, it does not conform to the format
specification published by Microsoft.
--no-omagic
-o output
--output=output
ld; if this
option is not specified, the name `a.out' is used by default. The
script command OUTPUT can also specify the output file name.
-O level
ld optimizes
the output. This might take significantly longer and therefore probably
should only be enabled for the final binary. At the moment this
option only affects ELF shared library generation. Future releases of
the linker may make more use of this option. Also currently there is
no difference in the linker's behaviour for different non-zero values
of this option. Again this may change with future releases.
-q
--emit-relocs
This option is currently only supported on ELF platforms.
--force-dynamic
-r
--relocatable
ld. This is often called partial
linking. As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file's magic number to
OMAGIC.
If this option is not specified, an absolute file is produced. When
linking C++ programs, this option will not resolve references to
constructors; to do that, use `-Ur'.
When an input file does not have the same format as the output file,
partial linking is only supported if that input file does not contain any
relocations. Different output formats can have further restrictions; for
example some a.out-based formats do not support partial linking
with input files in other formats at all.
This option does the same thing as `-i'.
-R filename
--just-symbols=filename
For compatibility with other ELF linkers, if the `-R' option is followed by a directory name, rather than a file name, it is treated as the `-rpath' option.
-s
--strip-all
-S
--strip-debug
-t
--trace
ld processes them.
-T scriptfile
--script=scriptfile
ld's default linker script (rather than adding to it), so
commandfile must specify everything necessary to describe the
output file. See section 3. Linker Scripts. If scriptfile does not exist in
the current directory, ld looks for it in the directories
specified by any preceding `-L' options. Multiple `-T'
options accumulate.
-dT scriptfile
--default-script=scriptfile
This option is similar to the `--script' option except that processing of the script is delayed until after the rest of the command line has been processed. This allows options placed after the `--default-script' option on the command line to affect the behaviour of the linker script, which can be important when the linker command line cannot be directly controlled by the user. (eg because the command line is being constructed by another tool, such as `gcc').
-u symbol
--undefined=symbol
EXTERN linker script command.
-Ur
ld. When linking C++ programs, `-Ur'
does resolve references to constructors, unlike `-r'.
It does not work to use `-Ur' on files that were themselves linked
with `-Ur'; once the constructor table has been built, it cannot
be added to. Use `-Ur' only for the last partial link, and
`-r' for the others.
--unique[=SECTION]
-v
--version
-V
ld. The `-V' option also
lists the supported emulations.
-x
--discard-all
-X
--discard-locals
-y symbol
--trace-symbol=symbol
This option is useful when you have an undefined symbol in your link but don't know where the reference is coming from.
-Y path
-z keyword
dlopen.
dldump.
PT_GNU_RELRO segment header in the object.
PT_GNU_RELRO segment header in the object.
Other keywords are ignored for Solaris compatibility.
-( archives -)
--start-group archives --end-group
The specified archives are searched repeatedly until no new undefined references are created. Normally, an archive is searched only once in the order that it is specified on the command line. If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in an archive that appears later on the command line, the linker would not be able to resolve that reference. By grouping the archives, they all be searched repeatedly until all possible references are resolved.
Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives.
--accept-unknown-input-arch
--no-accept-unknown-input-arch
--as-needed
--no-as-needed
--add-needed
--no-add-needed
-assert keyword
-Bdynamic
-dy
-call_shared
-Bgroup
DF_1_GROUP flag in the DT_FLAGS_1 entry in the dynamic
section. This causes the runtime linker to handle lookups in this
object and its dependencies to be performed only inside the group.
`--unresolved-symbols=report-all' is implied. This option is
only meaningful on ELF platforms which support shared libraries.
-Bstatic
-dn
-non_shared
-static
-Bsymbolic
-Bsymbolic-functions
--dynamic-list=dynamic-list-file
The format of the dynamic list is the same as the version node without scope and node name. See 3.9 VERSION Command for more information.
--dynamic-list-data
--dynamic-list-cpp-new
--dynamic-list-cpp-typeinfo
--check-sections
--no-check-sections
--copy-dt-needed-entries
--no-copy-dt-needed-entries
This option also has an effect on the resolution of symbols in dynamic libraries. With `--copy-dt-needed-entries' dynamic libraries mentioned on the command line will be recursively searched, following their DT_NEEDED tags to other libraries, in order to resolve symbols required by the output binary. With the default setting however the searching of dynamic libraries that follow it will stop with the dynamic library itself. No DT_NEEDED links will be traversed to resolve symbols.
--cref
The format of the table is intentionally simple, so that it may be easily processed by a script if necessary. The symbols are printed out, sorted by name. For each symbol, a list of file names is given. If the symbol is defined, the first file listed is the location of the definition. The remaining files contain references to the symbol.
--no-define-common
INHIBIT_COMMON_ALLOCATION has the same effect.
See section 3.4.5 Other Linker Script Commands.
The `--no-define-common' option allows decoupling the decision to assign addresses to Common symbols from the choice of the output file type; otherwise a non-Relocatable output type forces assigning addresses to Common symbols. Using `--no-define-common' allows Common symbols that are referenced from a shared library to be assigned addresses only in the main program. This eliminates the unused duplicate space in the shared library, and also prevents any possible confusion over resolving to the wrong duplicate when there are many dynamic modules with specialized search paths for runtime symbol resolution.
--defsym=symbol=expression
+ and - to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (see section Assignment: Symbol Definitions). Note: there should be no white
space between symbol, the equals sign ("="), and
expression.
--demangle[=style]
--no-demangle
-Ifile
--dynamic-linker=file
--fatal-warnings
--no-fatal-warnings
--force-exe-suffix
If a successfully built fully linked output file does not have a
.exe or .dll suffix, this option forces the linker to copy
the output file to one of the same name with a .exe suffix. This
option is useful when using unmodified Unix makefiles on a Microsoft
Windows host, since some versions of Windows won't run an image unless
it ends in a .exe suffix.
--gc-sections
--no-gc-sections
`--gc-sections' decides which input sections are used by examining symbols and relocations. The section containing the entry symbol and all sections containing symbols undefined on the command-line will be kept, as will sections containing symbols referenced by dynamic objects. Note that when building shared libraries, the linker must assume that any visible symbol is referenced. Once this initial set of sections has been determined, the linker recursively marks as used any section referenced by their relocations. See `--entry' and `--undefined'.
This option can be set when doing a partial link (enabled with option
`-r'). In this case the root of symbols kept must be explicitly
specified either by an `--entry' or `--undefined' option or by
a ENTRY command in the linker script.
--print-gc-sections
--no-print-gc-sections
--print-output-format
OUTPUT_FORMAT linker script command (see section 3.4.2 Commands Dealing with Files).
--help
--target-help
-Map=mapfile
--no-keep-memory
ld normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells ld to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if ld runs out of memory space
while linking a large executable.
--no-undefined
-z defs
--allow-multiple-definition
-z muldefs
--allow-shlib-undefined
--no-allow-shlib-undefined
The default behaviour is to report errors for any undefined symbols referenced in shared libraries if the linker is being used to create an executable, but to allow them if the linker is being used to create a shared library.
The reasons for allowing undefined symbol references in shared libraries specified at link time are that:
The BeOS kernel for example patches shared libraries at load time to select whichever function is most appropriate for the current architecture. This is used, for example, to dynamically select an appropriate memset function.
--no-undefined-version
--default-symver
--default-imported-symver
--no-warn-mismatch
ld will give an error if you try to link together input
files that are mismatched for some reason, perhaps because they have
been compiled for different processors or for different endiannesses.
This option tells ld that it should silently permit such possible
errors. This option should only be used with care, in cases when you
have taken some special action that ensures that the linker errors are
inappropriate.
--no-warn-search-mismatch
ld will give a warning if it finds an incompatible
library during a library search. This option silences the warning.
--no-whole-archive
--noinhibit-exec
-nostdlib
--oformat=output-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`--oformat' option to specify the binary format for the output
object file. Even when ld is configured to support alternative
object formats, you don't usually need to specify this, as ld
should be configured to produce as a default output format the most
usual format on each machine. output-format is a text string, the
name of a particular format supported by the BFD libraries. (You can
list the available binary formats with `objdump -i'.) The script
command OUTPUT_FORMAT can also specify the output format, but
this option overrides it. See section 5. BFD.
-pie
--pic-executable
-qmagic
-Qy
--relax
--no-relax
ld and the H8/300.
On some platforms the `--relax' option performs target specific, global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes, synthesizing new instructions, selecting shorter version of current instructions, and combinig constant values.
On some platforms these link time global optimizations may make symbolic debugging of the resulting executable impossible. This is known to be the case for the Matsushita MN10200 and MN10300 family of processors.
On platforms where this is not supported, `--relax' is accepted, but ignored.
On platforms where `--relax' is accepted the option `--no-relax' can be used to disable the feature.
--retain-symbols-file=filename
`--retain-symbols-file' does not discard undefined symbols, or symbols needed for relocations.
You may only specify `--retain-symbols-file' once in the command line. It overrides `-s' and `-S'.
-rpath=dir
LD_RUN_PATH will be used if it is defined.
The `-rpath' option may also be used on SunOS. By default, on SunOS, the linker will form a runtime search patch out of all the `-L' options it is given. If a `-rpath' option is used, the runtime search path will be formed exclusively using the `-rpath' options, ignoring the `-L' options. This can be useful when using gcc, which adds many `-L' options which may be on NFS mounted file systems.
For compatibility with other ELF linkers, if the `-R' option is followed by a directory name, rather than a file name, it is treated as the `-rpath' option.
-rpath-link=dir
ld -shared link includes a shared library as one
of the input files.
When the linker encounters such a dependency when doing a non-shared, non-relocatable link, it will automatically try to locate the required shared library and include it in the link, if it is not included explicitly. In such a case, the `-rpath-link' option specifies the first set of directories to search. The `-rpath-link' option may specify a sequence of directory names either by specifying a list of names separated by colons, or by appearing multiple times.
This option should be used with caution as it overrides the search path that may have been hard compiled into a shared library. In such a case it is possible to use unintentionally a different search path than the runtime linker would do.
The linker uses the following search paths to locate required shared libraries:
LD_RUN_PATH.
LD_LIBRARY_PATH.
DT_RUNPATH or
DT_RPATH of a shared library are searched for shared
libraries needed by it. The DT_RPATH entries are ignored if
DT_RUNPATH entries exist.
If the required shared library is not found, the linker will issue a warning and continue with the link.
-shared
-Bshareable
--sort-common
--sort-common=ascending
--sort-common=descending
ld to sort the common symbols by alignment in
ascending or descending order when it places them in the appropriate output
sections. The symbol alignments considered are sixteen-byte or larger,
eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
between symbols due to alignment constraints. If no sorting order is
specified, then descending order is assumed.
--sort-section=name
SORT_BY_NAME to all wildcard section
patterns in the linker script.
--sort-section=alignment
SORT_BY_ALIGNMENT to all wildcard section
patterns in the linker script.
--split-by-file[=size]
--split-by-reloc[=count]
--stats
--sysroot=directory
--traditional-format
ld is different in some ways from
the output of some existing linker. This switch requests ld to
use the traditional format instead.
For example, on SunOS, ld combines duplicate entries in the
symbol string table. This can reduce the size of an output file with
full debugging information by over 30 percent. Unfortunately, the SunOS
dbx program can not read the resulting program (gdb has no
trouble). The `--traditional-format' switch tells ld to not
combine duplicate entries.
--section-start=sectionname=org
-Tbss=org
-Tdata=org
-Ttext=org
.bss, .data or
.text as the sectionname.
-Ttext-segment=org
--unresolved-symbols=method
The behaviour for shared libraries on their own can also be controlled by the `--[no-]allow-shlib-undefined' option.
Normally the linker will generate an error message for each reported unresolved symbol but the option `--warn-unresolved-symbols' can change this to a warning.
--dll-verbose
--verbose[=NUMBER]
ld and list the linker emulations
supported. Display which input files can and cannot be opened. Display
the linker script being used by the linker. If the optional NUMBER
argument > 1, plugin symbol status will also be displayed.
--version-script=version-scriptfile
ld and WIN32 (cygwin/mingw).
--warn-common
There are three kinds of global symbols, illustrated here by C examples:
The `--warn-common' option can produce five kinds of warnings. Each warning consists of a pair of lines: the first describes the symbol just encountered, and the second describes the previous symbol encountered with the same name. One or both of the two symbols will be a common symbol.
file(section): warning: common of `symbol' overridden by definition file(section): warning: defined here |
file(section): warning: definition of `symbol' overriding common file(section): warning: common is here |
file(section): warning: multiple common of `symbol' file(section): warning: previous common is here |
file(section): warning: common of `symbol' overridden by larger common file(section): warning: larger common is here |
file(section): warning: common of `symbol' overriding smaller common file(section): warning: smaller common is here |
--warn-constructors
--warn-multiple-gp
--warn-once
--warn-section-align
SECTIONS command does not specify a start address for
the section (see section 3.6 SECTIONS Command).
--warn-shared-textrel
--warn-alternate-em
--warn-unresolved-symbols
--error-unresolved-symbols
--whole-archive
Two notes when using this option from gcc: First, gcc doesn't know about this option, so you have to use `-Wl,-whole-archive'. Second, don't forget to use `-Wl,-no-whole-archive' after your list of archives, because gcc will add its own list of archives to your link and you may not want this flag to affect those as well.
--wrap=symbol
__wrap_symbol. Any
undefined reference to __real_symbol will be resolved to
symbol.
This can be used to provide a wrapper for a system function. The
wrapper function should be called __wrap_symbol. If it
wishes to call the system function, it should call
__real_symbol.
Here is a trivial example:
void *
__wrap_malloc (size_t c)
{
printf ("malloc called with %zu\n", c);
return __real_malloc (c);
}
|
If you link other code with this file using `--wrap malloc', then
all calls to malloc will call the function __wrap_malloc
instead. The call to __real_malloc in __wrap_malloc will
call the real malloc function.
You may wish to provide a __real_malloc function as well, so that
links without the `--wrap' option will succeed. If you do this,
you should not put the definition of __real_malloc in the same
file as __wrap_malloc; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to malloc.
--eh-frame-hdr
.eh_frame_hdr section and ELF
PT_GNU_EH_FRAME segment header.
--no-ld-generated-unwind-info
.eh_frame unwind info for linker
generated code sections like PLT. This option is on by default
if linker generated unwind info is supported.
--enable-new-dtags
--disable-new-dtags
--hash-size=number
--hash-style=style
sysv for classic ELF .hash section, gnu for
new style GNU .gnu.hash section or both for both
the classic ELF .hash and new style GNU .gnu.hash
hash tables. The default is sysv.
--renesas
--reduce-memory-overheads
Another effect of the switch is to set the default hash table size to 1021, which again saves memory at the cost of lengthening the linker's run time. This is not done however if the `--hash-size' switch has been used.
The `--reduce-memory-overheads' switch may be also be used to enable other tradeoffs in future versions of the linker.
--build-id
--build-id=style
.note.gnu.build-id ELF note section.
The contents of the note are unique bits identifying this linked
file. style can be uuid to use 128 random bits,
sha1 to use a 160-bit SHA1 hash on the normative
parts of the output contents, md5 to use a 128-bit
MD5 hash on the normative parts of the output contents, or
0xhexstring to use a chosen bit string specified as
an even number of hexadecimal digits (- and :
characters between digit pairs are ignored). If style is
omitted, sha1 is used.
The md5 and sha1 styles produces an identifier
that is always the same in an identical output file, but will be
unique among all nonidentical output files. It is not intended
to be compared as a checksum for the file's contents. A linked
file may be changed later by other tools, but the build ID bit
string identifying the original linked file does not change.
Passing none for style disables the setting from any
--build-id options earlier on the command line.
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You can change the behaviour of ld with the environment variables
GNUTARGET,
LDEMULATION and COLLECT_NO_DEMANGLE.
GNUTARGET determines the input-file object format if you don't
use `-b' (or its synonym `--format'). Its value should be one
of the BFD names for an input format (see section 5. BFD). If there is no
GNUTARGET in the environment, ld uses the natural format
of the target. If GNUTARGET is set to default then BFD
attempts to discover the input format by examining binary input files;
this method often succeeds, but there are potential ambiguities, since
there is no method of ensuring that the magic number used to specify
object-file formats is unique. However, the configuration procedure for
BFD on each system places the conventional format for that system first
in the search-list, so ambiguities are resolved in favor of convention.
LDEMULATION determines the default emulation if you don't use the
`-m' option. The emulation can affect various aspects of linker
behaviour, particularly the default linker script. You can list the
available emulations with the `--verbose' or `-V' options. If
the `-m' option is not used, and the LDEMULATION environment
variable is not defined, the default emulation depends upon how the
linker was configured.
Normally, the linker will default to demangling symbols. However, if
COLLECT_NO_DEMANGLE is set in the environment, then it will
default to not demangling symbols. This environment variable is used in
a similar fashion by the gcc linker wrapper program. The default
may be overridden by the `--demangle' and `--no-demangle'
options.
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Every link is controlled by a linker script. This script is written in the linker command language.
The main purpose of the linker script is to describe how the sections in the input files should be mapped into the output file, and to control the memory layout of the output file. Most linker scripts do nothing more than this. However, when necessary, the linker script can also direct the linker to perform many other operations, using the commands described below.
The linker always uses a linker script. If you do not supply one yourself, the linker will use a default script that is compiled into the linker executable. You can use the `--verbose' command line option to display the default linker script. Certain command line options, such as `-r' or `-N', will affect the default linker script.
You may supply your own linker script by using the `-T' command line option. When you do this, your linker script will replace the default linker script.
You may also use linker scripts implicitly by naming them as input files to the linker, as though they were files to be linked. See section 3.11 Implicit Linker Scripts.
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The linker combines input files into a single output file. The output file and each input file are in a special data format known as an object file format. Each file is called an object file. The output file is often called an executable, but for our purposes we will also call it an object file. Each object file has, among other things, a list of sections. We sometimes refer to a section in an input file as an input section; similarly, a section in the output file is an output section.
Each section in an object file has a name and a size. Most sections also have an associated block of data, known as the section contents. A section may be marked as loadable, which mean that the contents should be loaded into memory when the output file is run. A section with no contents may be allocatable, which means that an area in memory should be set aside, but nothing in particular should be loaded there (in some cases this memory must be zeroed out). A section which is neither loadable nor allocatable typically contains some sort of debugging information.
Every loadable or allocatable output section has two addresses. The first is the VMA, or virtual memory address. This is the address the section will have when the output file is run. The second is the LMA, or load memory address. This is the address at which the section will be loaded. In most cases the two addresses will be the same. An example of when they might be different is when a data section is loaded into ROM, and then copied into RAM when the program starts up (this technique is often used to initialize global variables in a ROM based system). In this case the ROM address would be the LMA, and the RAM address would be the VMA.
You can see the sections in an object file by using the objdump
program with the `-h' option.
Every object file also has a list of symbols, known as the symbol table. A symbol may be defined or undefined. Each symbol has a name, and each defined symbol has an address, among other information. If you compile a C or C++ program into an object file, you will get a defined symbol for every defined function and global or static variable. Every undefined function or global variable which is referenced in the input file will become an undefined symbol.
You can see the symbols in an object file by using the nm
program, or by using the objdump program with the `-t'
option.
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You write a linker script as a series of commands. Each command is either a keyword, possibly followed by arguments, or an assignment to a symbol. You may separate commands using semicolons. Whitespace is generally ignored.
Strings such as file or format names can normally be entered directly. If the file name contains a character such as a comma which would otherwise serve to separate file names, you may put the file name in double quotes. There is no way to use a double quote character in a file name.
You may include comments in linker scripts just as in C, delimited by `/*' and `*/'. As in C, comments are syntactically equivalent to whitespace.
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The simplest possible linker script has just one command: `SECTIONS'. You use the `SECTIONS' command to describe the memory layout of the output file.
The `SECTIONS' command is a powerful command. Here we will describe a simple use of it. Let's assume your program consists only of code, initialized data, and uninitialized data. These will be in the `.text', `.data', and `.bss' sections, respectively. Let's assume further that these are the only sections which appear in your input files.
For this example, let's say that the code should be loaded at address 0x10000, and that the data should start at address 0x8000000. Here is a linker script which will do that:
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
. = 0x8000000;
.data : { *(.data) }
.bss : { *(.bss) }
}
|
You write the `SECTIONS' command as the keyword `SECTIONS', followed by a series of symbol assignments and output section descriptions enclosed in curly braces.
The first line inside the `SECTIONS' command of the above example sets the value of the special symbol `.', which is the location counter. If you do not specify the address of an output section in some other way (other ways are described later), the address is set from the current value of the location counter. The location counter is then incremented by the size of the output section. At the start of the `SECTIONS' command, the location counter has the value `0'.
The second line defines an output section, `.text'. The colon is required syntax which may be ignored for now. Within the curly braces after the output section name, you list the names of the input sections which should be placed into this output section. The `*' is a wildcard which matches any file name. The expression `*(.text)' means all `.text' input sections in all input files.
Since the location counter is `0x10000' when the output section `.text' is defined, the linker will set the address of the `.text' section in the output file to be `0x10000'.
The remaining lines define the `.data' and `.bss' sections in the output file. The linker will place the `.data' output section at address `0x8000000'. After the linker places the `.data' output section, the value of the location counter will be `0x8000000' plus the size of the `.data' output section. The effect is that the linker will place the `.bss' output section immediately after the `.data' output section in memory.
The linker will ensure that each output section has the required alignment, by increasing the location counter if necessary. In this example, the specified addresses for the `.text' and `.data' sections will probably satisfy any alignment constraints, but the linker may have to create a small gap between the `.data' and `.bss' sections.
That's it! That's a simple and complete linker script.
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3.4.1 Setting the Entry Point Setting the entry point 3.4.2 Commands Dealing with Files Commands dealing with files 3.4.3 Commands Dealing with Object File Formats Commands dealing with object file formats
3.4.4 Assign alias names to memory regions 3.4.5 Other Linker Script Commands Other linker script commands
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ENTRY linker script command to set the
entry point. The argument is a symbol name:
ENTRY(symbol) |
There are several ways to set the entry point. The linker will set the entry point by trying each of the following methods in order, and stopping when one of them succeeds:
ENTRY(symbol) command in a linker script;
start, but PE and BeOS based systems for example
check a list of possible entry symbols, matching the first one found.
0.
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INCLUDE filename
INCLUDE up to
10 levels deep.
You can place INCLUDE directives at the top level, in MEMORY or
SECTIONS commands, or in output section descriptions.
INPUT(file, file, ...)
INPUT(file file ...)
INPUT command directs the linker to include the named files
in the link, as though they were named on the command line.
For example, if you always want to include `subr.o' any time you do a link, but you can't be bothered to put it on every link command line, then you can put `INPUT (subr.o)' in your linker script.
In fact, if you like, you can list all of your input files in the linker script, and then invoke the linker with nothing but a `-T' option.
In case a sysroot prefix is configured, and the filename starts with the `/' character, and the script being processed was located inside the sysroot prefix, the filename will be looked for in the sysroot prefix. Otherwise, the linker will try to open the file in the current directory. If it is not found, the linker will search through the archive library search path. See the description of `-L' in Command Line Options.
If you use `INPUT (-lfile)', ld will transform the
name to libfile.a, as with the command line argument
`-l'.
When you use the INPUT command in an implicit linker script, the
files will be included in the link at the point at which the linker
script file is included. This can affect archive searching.
GROUP(file, file, ...)
GROUP(file file ...)
GROUP command is like INPUT, except that the named
files should all be archives, and they are searched repeatedly until no
new undefined references are created. See the description of `-('
in Command Line Options.
AS_NEEDED(file, file, ...)
AS_NEEDED(file file ...)
INPUT or GROUP
commands, among other filenames. The files listed will be handled
as if they appear directly in the INPUT or GROUP commands,
with the exception of ELF shared libraries, that will be added only
when they are actually needed. This construct essentially enables
`--as-needed' option for all the files listed inside of it
and restores previous `--as-needed' resp. `--no-as-needed'
setting afterwards.
OUTPUT(filename)
OUTPUT command names the output file. Using
OUTPUT(filename) in the linker script is exactly like using
`-o filename' on the command line (see section Command Line Options). If both are used, the command line option takes
precedence.
You can use the OUTPUT command to define a default name for the
output file other than the usual default of `a.out'.
SEARCH_DIR(path)
SEARCH_DIR command adds path to the list of paths where
ld looks for archive libraries. Using
SEARCH_DIR(path) is exactly like using `-L path'
on the command line (see section Command Line Options). If both
are used, then the linker will search both paths. Paths specified using
the command line option are searched first.
STARTUP(filename)
STARTUP command is just like the INPUT command, except
that filename will become the first input file to be linked, as
though it were specified first on the command line. This may be useful
when using a system in which the entry point is always the start of the
first file.
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OUTPUT_FORMAT(bfdname)
OUTPUT_FORMAT(default, big, little)
OUTPUT_FORMAT command names the BFD format to use for the
output file (see section 5. BFD). Using OUTPUT_FORMAT(bfdname) is
exactly like using `--oformat bfdname' on the command line
(see section Command Line Options). If both are used, the command
line option takes precedence.
You can use OUTPUT_FORMAT with three arguments to use different
formats based on the `-EB' and `-EL' command line options.
This permits the linker script to set the output format based on the
desired endianness.
If neither `-EB' nor `-EL' are used, then the output format will be the first argument, default. If `-EB' is used, the output format will be the second argument, big. If `-EL' is used, the output format will be the third argument, little.
For example, the default linker script for the MIPS ELF target uses this command:
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) |
TARGET(bfdname)
TARGET command names the BFD format to use when reading input
files. It affects subsequent INPUT and GROUP commands.
This command is like using `-b bfdname' on the command line
(see section Command Line Options). If the TARGET command
is used but OUTPUT_FORMAT is not, then the last TARGET
command is also used to set the format for the output file. See section 5. BFD.
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Alias names can be added to existing memory regions created with the 3.7 MEMORY Command command. Each name corresponds to at most one memory region.
REGION_ALIAS(alias, region) |
The REGION_ALIAS function creates an alias name alias for the
memory region region. This allows a flexible mapping of output sections
to memory regions. An example follows.
Suppose we have an application for embedded systems which come with various
memory storage devices. All have a general purpose, volatile memory RAM
that allows code execution or data storage. Some may have a read-only,
non-volatile memory ROM that allows code execution and read-only data
access. The last variant is a read-only, non-volatile memory ROM2 with
read-only data access and no code execution capability. We have four output
sections:
.text program code;
.rodata read-only data;
.data read-write initialized data;
.bss read-write zero initialized data.
The goal is to provide a linker command file that contains a system independent
part defining the output sections and a system dependent part mapping the
output sections to the memory regions available on the system. Our embedded
systems come with three different memory setups A, B and
C:
| Section | Variant A | Variant B | Variant C |
| .text | RAM | ROM | ROM |
| .rodata | RAM | ROM | ROM2 |
| .data | RAM | RAM/ROM | RAM/ROM2 |
| .bss | RAM | RAM | RAM |
RAM/ROM or RAM/ROM2 means that this section is
loaded into region ROM or ROM2 respectively. Please note that
the load address of the .data section starts in all three variants at
the end of the .rodata section.
The base linker script that deals with the output sections follows. It
includes the system dependent linkcmds.memory file that describes the
memory layout:
INCLUDE linkcmds.memory
SECTIONS
{
.text :
{
*(.text)
} > REGION_TEXT
.rodata :
{
*(.rodata)
rodata_end = .;
} > REGION_RODATA
.data : AT (rodata_end)
{
data_start = .;
*(.data)
} > REGION_DATA
data_size = SIZEOF(.data);
data_load_start = LOADADDR(.data);
.bss :
{
*(.bss)
} > REGION_BSS
}
|
Now we need three different linkcmds.memory files to define memory
regions and alias names. The content of linkcmds.memory for the three
variants A, B and C:
A
RAM.
MEMORY
{
RAM : ORIGIN = 0, LENGTH = 4M
}
REGION_ALIAS("REGION_TEXT", RAM);
REGION_ALIAS("REGION_RODATA", RAM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
|
B
ROM. Read-write data goes
into the RAM. An image of the initialized data is loaded into the
ROM and will be copied during system start into the RAM.
MEMORY
{
ROM : ORIGIN = 0, LENGTH = 3M
RAM : ORIGIN = 0x10000000, LENGTH = 1M
}
REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
|
C
ROM. Read-only data goes into the
ROM2. Read-write data goes into the RAM. An image of the
initialized data is loaded into the ROM2 and will be copied during
system start into the RAM.
MEMORY
{
ROM : ORIGIN = 0, LENGTH = 2M
ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
RAM : ORIGIN = 0x20000000, LENGTH = 1M
}
REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM2);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
|
It is possible to write a common system initialization routine to copy the
.data section from ROM or ROM2 into the RAM if
necessary:
#include <string.h>
extern char data_start [];
extern char data_size [];
extern char data_load_start [];
void copy_data(void)
{
if (data_start != data_load_start)
{
memcpy(data_start, data_load_start, (size_t) data_size);
}
}
|
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ASSERT(exp, message)
EXTERN(symbol symbol ...)
EXTERN, and you may use EXTERN multiple times. This
command has the same effect as the `-u' command-line option.
FORCE_COMMON_ALLOCATION
ld assign space to common symbols even if a relocatable
output file is specified (`-r').
INHIBIT_COMMON_ALLOCATION
ld omit the assignment of addresses
to common symbols even for a non-relocatable output file.
INSERT [ AFTER | BEFORE ] output_section
SECTIONS with, for example, overlays. It
inserts all prior linker script statements after (or before)
output_section, and also causes `-T' to not override the
default linker script. The exact insertion point is as for orphan
sections. See section 3.10.5 The Location Counter. The insertion happens after the
linker has mapped input sections to output sections. Prior to the
insertion, since `-T' scripts are parsed before the default
linker script, statements in the `-T' script occur before the
default linker script statements in the internal linker representation
of the script. In particular, input section assignments will be made
to `-T' output sections before those in the default script. Here
is an example of how a `-T' script using INSERT might look:
SECTIONS
{
OVERLAY :
{
.ov1 { ov1*(.text) }
.ov2 { ov2*(.text) }
}
}
INSERT AFTER .text;
|
NOCROSSREFS(section section ...)
ld to issue an error about any
references among certain output sections.
In certain types of programs, particularly on embedded systems when using overlays, when one section is loaded into memory, another section will not be. Any direct references between the two sections would be errors. For example, it would be an error if code in one section called a function defined in the other section.
The NOCROSSREFS command takes a list of output section names. If
ld detects any cross references between the sections, it reports
an error and returns a non-zero exit status. Note that the
NOCROSSREFS command uses output section names, not input section
names.
OUTPUT_ARCH(bfdarch)
objdump program with
the `-f' option.
LD_FEATURE(string)
ld behavior. If
string is "SANE_EXPR" then absolute symbols and numbers
in a script are simply treated as numbers everywhere.
See section 3.10.8 The Section of an Expression.
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3.5.1 Simple Assignments 3.5.2 PROVIDE 3.5.3 PROVIDE_HIDDEN 3.5.4 Source Code Reference How to use a linker script defined symbol in source code
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You may assign to a symbol using any of the C assignment operators:
symbol = expression ;
symbol += expression ;
symbol -= expression ;
symbol *= expression ;
symbol /= expression ;
symbol <<= expression ;
symbol >>= expression ;
symbol &= expression ;
symbol |= expression ;
The first case will define symbol to the value of expression. In the other cases, symbol must already be defined, and the value will be adjusted accordingly.
The special symbol name `.' indicates the location counter. You
may only use this within a SECTIONS command. See section 3.10.5 The Location Counter.
The semicolon after expression is required.
Expressions are defined below; see 3.10 Expressions in Linker Scripts.
You may write symbol assignments as commands in their own right, or as
statements within a SECTIONS command, or as part of an output
section description in a SECTIONS command.
The section of the symbol will be set from the section of the expression; for more information, see 3.10.8 The Section of an Expression.
Here is an example showing the three different places that symbol assignments may be used:
floating_point = 0;
SECTIONS
{
.text :
{
*(.text)
_etext = .;
}
_bdata = (. + 3) & ~ 3;
.data : { *(.data) }
}
|
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PROVIDE keyword may be used to define a symbol, such as
`etext', only if it is referenced but not defined. The syntax is
PROVIDE(symbol = expression).
Here is an example of using PROVIDE to define `etext':
SECTIONS
{
.text :
{
*(.text)
_etext = .;
PROVIDE(etext = .);
}
}
|
In this example, if the program defines `_etext' (with a leading underscore), the linker will give a multiple definition error. If, on the other hand, the program defines `etext' (with no leading underscore), the linker will silently use the definition in the program. If the program references `etext' but does not define it, the linker will use the definition in the linker script.
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PROVIDE. For ELF targeted ports, the symbol will be
hidden and won't be exported.
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Accessing a linker script defined variable from source code is not intuitive. In particular a linker script symbol is not equivalent to a variable declaration in a high level language, it is instead a symbol that does not have a value.
Before going further, it is important to note that compilers often transform names in the source code into different names when they are stored in the symbol table. For example, Fortran compilers commonly prepend or append an underscore, and C++ performs extensive `name mangling'. Therefore there might be a discrepancy between the name of a variable as it is used in source code and the name of the same variable as it is defined in a linker script. For example in C a linker script variable might be referred to as:
extern int foo; |
But in the linker script it might be defined as:
_foo = 1000; |
In the remaining examples however it is assumed that no name transformation has taken place.
When a symbol is declared in a high level language such as C, two things happen. The first is that the compiler reserves enough space in the program's memory to hold the value of the symbol. The second is that the compiler creates an entry in the program's symbol table which holds the symbol's address. ie the symbol table contains the address of the block of memory holding the symbol's value. So for example the following C declaration, at file scope:
int foo = 1000; |
creates a entry called `foo' in the symbol table. This entry holds the address of an `int' sized block of memory where the number 1000 is initially stored.
When a program references a symbol the compiler generates code that first accesses the symbol table to find the address of the symbol's memory block and then code to read the value from that memory block. So:
foo = 1; |
looks up the symbol `foo' in the symbol table, gets the address associated with this symbol and then writes the value 1 into that address. Whereas:
int * a = & foo; |
looks up the symbol `foo' in the symbol table, gets it address and then copies this address into the block of memory associated with the variable `a'.
Linker scripts symbol declarations, by contrast, create an entry in the symbol table but do not assign any memory to them. Thus they are an address without a value. So for example the linker script definition:
foo = 1000; |
creates an entry in the symbol table called `foo' which holds the address of memory location 1000, but nothing special is stored at address 1000. This means that you cannot access the value of a linker script defined symbol - it has no value - all you can do is access the address of a linker script defined symbol.
Hence when you are using a linker script defined symbol in source code you should always take the address of the symbol, and never attempt to use its value. For example suppose you want to copy the contents of a section of memory called .ROM into a section called .FLASH and the linker script contains these declarations:
start_of_ROM = .ROM; end_of_ROM = .ROM + sizeof (.ROM) - 1; start_of_FLASH = .FLASH; |
Then the C source code to perform the copy would be:
extern char start_of_ROM, end_of_ROM, start_of_FLASH; memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM); |
Note the use of the `&' operators. These are correct.
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SECTIONS command tells the linker how to map input sections
into output sections, and how to place the output sections in memory.
The format of the SECTIONS command is:
SECTIONS
{
sections-command
sections-command
...
}
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Each sections-command may of be one of the following:
ENTRY command (see section Entry command)
The ENTRY command and symbol assignments are permitted inside the
SECTIONS command for convenience in using the location counter in
those commands. This can also make the linker script easier to
understand because you can use those commands at meaningful points in
the layout of the output file.
Output section descriptions and overlay descriptions are described below.
If you do not use a SECTIONS command in your linker script, the
linker will place each input section into an identically named output
section in the order that the sections are first encountered in the
input files. If all input sections are present in the first file, for
example, the order of sections in the output file will match the order
in the first input file. The first section will be at address zero.
3.6.1 Output Section Description Output section description 3.6.2 Output Section Name Output section name 3.6.3 Output Section Address Output section address 3.6.4 Input Section Description Input section description 3.6.5 Output Section Data Output section data 3.6.6 Output Section Keywords Output section keywords 3.6.7 Output Section Discarding Output section discarding 3.6.8 Output Section Attributes Output section attributes 3.6.9 Overlay Description Overlay description
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section [address] [(type)] :
[AT(lma)]
[ALIGN(section_align)]
[SUBALIGN(subsection_align)]
[constraint]
{
output-section-command
output-section-command
...
} [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp]
|
Most output sections do not use most of the optional section attributes.
The whitespace around section is required, so that the section name is unambiguous. The colon and the curly braces are also required. The line breaks and other white space are optional.
Each output-section-command may be one of the following:
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a.out, the name
must be one of the names supported by the format (a.out, for
example, allows only `.text', `.data' or `.bss'). If the
output format supports any number of sections, but with numbers and not
names (as is the case for Oasys), the name should be supplied as a
quoted numeric string. A section name may consist of any sequence of
characters, but a name which contains any unusual characters such as
commas must be quoted.
The output section name `/DISCARD/' is special; 3.6.7 Output Section Discarding.
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If the output address is not specified then one will be chosen for the section, based on the heuristic below. This address will be adjusted to fit the alignment requirement of the output section. The alignment requirement is the strictest alignment of any input section contained within the output section.
The output section address heuristic is as follows:
For example:
.text . : { *(.text) }
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and
.text : { *(.text) }
|
are subtly different. The first will set the address of the `.text' output section to the current value of the location counter. The second will set it to the current value of the location counter aligned to the strictest alignment of any of the `.text' input sections.
The address may be an arbitrary expression; 3.10 Expressions in Linker Scripts. For example, if you want to align the section on a 0x10 byte boundary, so that the lowest four bits of the section address are zero, you could do something like this:
.text ALIGN(0x10) : { *(.text) }
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ALIGN returns the current location counter
aligned upward to the specified value.
Specifying address for a section will change the value of the location counter, provided that the section is non-empty. (Empty sections are ignored).
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The input section description is the most basic linker script operation. You use output sections to tell the linker how to lay out your program in memory. You use input section descriptions to tell the linker how to map the input files into your memory layout.
3.6.4.1 Input Section Basics Input section basics 3.6.4.2 Input Section Wildcard Patterns Input section wildcard patterns 3.6.4.3 Input Section for Common Symbols Input section for common symbols 3.6.4.4 Input Section and Garbage Collection Input section and garbage collection 3.6.4.5 Input Section Example Input section example
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The file name and the section name may be wildcard patterns, which we describe further below (see section 3.6.4.2 Input Section Wildcard Patterns).
The most common input section description is to include all input sections with a particular name in the output section. For example, to include all input `.text' sections, you would write:
*(.text) |
*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors) |
There are two ways to include more than one section:
*(.text .rdata) *(.text) *(.rdata) |
You can specify a file name to include sections from a particular file. You would do this if one or more of your files contain special data that needs to be at a particular location in memory. For example:
data.o(.data) |
To refine the sections that are included based on the section flags of an input section, INPUT_SECTION_FLAGS may be used.
Here is a simple example for using Section header flags for ELF sections:
SECTIONS {
.text : { INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) }
.text2 : { INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) }
}
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In this example, the output section `.text' will be comprised of any
input section matching the name *(.text) whose section header flags
SHF_MERGE and SHF_STRINGS are set. The output section
`.text2' will be comprised of any input section matching the name *(.text)
whose section header flag SHF_WRITE is clear.
You can also specify files within archives by writing a pattern matching the archive, a colon, then the pattern matching the file, with no whitespace around the colon.
Either one or both of `archive' and `file' can contain shell
wildcards. On DOS based file systems, the linker will assume that a
single letter followed by a colon is a drive specifier, so
`c:myfile.o' is a simple file specification, not `myfile.o'
within an archive called `c'. `archive:file' filespecs may
also be used within an EXCLUDE_FILE list, but may not appear in
other linker script contexts. For instance, you cannot extract a file
from an archive by using `archive:file' in an INPUT
command.
If you use a file name without a list of sections, then all sections in the input file will be included in the output section. This is not commonly done, but it may by useful on occasion. For example:
data.o |
When you use a file name which is not an `archive:file' specifier
and does not contain any wild card
characters, the linker will first see if you also specified the file
name on the linker command line or in an INPUT command. If you
did not, the linker will attempt to open the file as an input file, as
though it appeared on the command line. Note that this differs from an
INPUT command, because the linker will not search for the file in
the archive search path.
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The file name of `*' seen in many examples is a simple wildcard pattern for the file name.
The wildcard patterns are like those used by the Unix shell.
When a file name is matched with a wildcard, the wildcard characters will not match a `/' character (used to separate directory names on Unix). A pattern consisting of a single `*' character is an exception; it will always match any file name, whether it contains a `/' or not. In a section name, the wildcard characters will match a `/' character.
File name wildcard patterns only match files which are explicitly
specified on the command line or in an INPUT command. The linker
does not search directories to expand wildcards.
If a file name matches more than one wildcard pattern, or if a file name appears explicitly and is also matched by a wildcard pattern, the linker will use the first match in the linker script. For example, this sequence of input section descriptions is probably in error, because the `data.o' rule will not be used:
.data : { *(.data) }
.data1 : { data.o(.data) }
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Normally, the linker will place files and sections matched by wildcards
in the order in which they are seen during the link. You can change
this by using the SORT_BY_NAME keyword, which appears before a wildcard
pattern in parentheses (e.g., SORT_BY_NAME(.text*)). When the
SORT_BY_NAME keyword is used, the linker will sort the files or sections
into ascending order by name before placing them in the output file.
SORT_BY_ALIGNMENT is very similar to SORT_BY_NAME. The
difference is SORT_BY_ALIGNMENT will sort sections into
ascending order by alignment before placing them in the output file.
SORT_BY_INIT_PRIORITY is very similar to SORT_BY_NAME. The
difference is SORT_BY_INIT_PRIORITY will sort sections into
ascending order by numerical value of the GCC init_priority attribute
encoded in the section name before placing them in the output file.
SORT is an alias for SORT_BY_NAME.
When there are nested section sorting commands in linker script, there can be at most 1 level of nesting for section sorting commands.
SORT_BY_NAME (SORT_BY_ALIGNMENT (wildcard section pattern)).
It will sort the input sections by name first, then by alignment if 2
sections have the same name.
SORT_BY_ALIGNMENT (SORT_BY_NAME (wildcard section pattern)).
It will sort the input sections by alignment first, then by name if 2
sections have the same alignment.
SORT_BY_NAME (SORT_BY_NAME (wildcard section pattern)) is
treated the same as SORT_BY_NAME (wildcard section pattern).
SORT_BY_ALIGNMENT (SORT_BY_ALIGNMENT (wildcard section pattern))
is treated the same as SORT_BY_ALIGNMENT (wildcard section pattern).
When both command line section sorting option and linker script section sorting command are used, section sorting command always takes precedence over the command line option.
If the section sorting command in linker script isn't nested, the command line option will make the section sorting command to be treated as nested sorting command.
SORT_BY_NAME (wildcard section pattern ) with
`--sort-sections alignment' is equivalent to
SORT_BY_NAME (SORT_BY_ALIGNMENT (wildcard section pattern)).
SORT_BY_ALIGNMENT (wildcard section pattern) with
`--sort-section name' is equivalent to
SORT_BY_ALIGNMENT (SORT_BY_NAME (wildcard section pattern)).
If the section sorting command in linker script is nested, the command line option will be ignored.
If you ever get confused about where input sections are going, use the `-M' linker option to generate a map file. The map file shows precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition files. This linker script directs the linker to place all `.text' sections in `.text' and all `.bss' sections in `.bss'. The linker will place the `.data' section from all files beginning with an upper case character in `.DATA'; for all other files, the linker will place the `.data' section in `.data'.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}
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You may use file names with the `COMMON' section just as with any other input sections. You can use this to place common symbols from a particular input file in one section while common symbols from other input files are placed in another section.
In most cases, common symbols in input files will be placed in the `.bss' section in the output file. For example:
.bss { *(.bss) *(COMMON) }
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Some object file formats have more than one type of common symbol. For example, the MIPS ELF object file format distinguishes standard common symbols and small common symbols. In this case, the linker will use a different special section name for other types of common symbols. In the case of MIPS ELF, the linker uses `COMMON' for standard common symbols and `.scommon' for small common symbols. This permits you to map the different types of common symbols into memory at different locations.
You will sometimes see `[COMMON]' in old linker scripts. This notation is now considered obsolete. It is equivalent to `*(COMMON)'.
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KEEP(), as in KEEP(*(.init)) or
KEEP(SORT_BY_NAME(*)(.ctors)).
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SECTIONS {
outputa 0x10000 :
{
all.o
foo.o (.input1)
}
outputb :
{
foo.o (.input2)
foo1.o (.input1)
}
outputc :
{
*(.input1)
*(.input2)
}
}
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BYTE, SHORT, LONG, QUAD, or SQUAD as
an output section command. Each keyword is followed by an expression in
parentheses providing the value to store (see section 3.10 Expressions in Linker Scripts). The
value of the expression is stored at the current value of the location
counter.
The BYTE, SHORT, LONG, and QUAD commands
store one, two, four, and eight bytes (respectively). After storing the
bytes, the location counter is incremented by the number of bytes
stored.
For example, this will store the byte 1 followed by the four byte value of the symbol `addr':
BYTE(1) LONG(addr) |
When using a 64 bit host or target, QUAD and SQUAD are the
same; they both store an 8 byte, or 64 bit, value. When both host and
target are 32 bits, an expression is computed as 32 bits. In this case
QUAD stores a 32 bit value zero extended to 64 bits, and
SQUAD stores a 32 bit value sign extended to 64 bits.
If the object file format of the output file has an explicit endianness, which is the normal case, the value will be stored in that endianness. When the object file format does not have an explicit endianness, as is true of, for example, S-records, the value will be stored in the endianness of the first input object file.
Note--these commands only work inside a section description and not between them, so the following will produce an error from the linker:
SECTIONS { .text : { *(.text) } LONG(1) .data : { *(.data) } }
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SECTIONS { .text : { *(.text) ; LONG(1) } .data : { *(.data) } }
|
You may use the FILL command to set the fill pattern for the
current section. It is followed by an expression in parentheses. Any
otherwise unspecified regions of memory within the section (for example,
gaps left due to the required alignment of input sections) are filled
with the value of the expression, repeated as
necessary. A FILL statement covers memory locations after the
point at which it occurs in the section definition; by including more
than one FILL statement, you can have different fill patterns in
different parts of an output section.
This example shows how to fill unspecified regions of memory with the value `0x90':
FILL(0x90909090) |
The FILL command is similar to the `=fillexp' output
section attribute, but it only affects the
part of the section following the FILL command, rather than the
entire section. If both are used, the FILL command takes
precedence. See section 3.6.8.8 Output Section Fill, for details on the fill
expression.
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CREATE_OBJECT_SYMBOLS
CREATE_OBJECT_SYMBOLS command appears.
This is conventional for the a.out object file format. It is not normally used for any other object file format.
CONSTRUCTORS
CONSTRUCTORS command tells the
linker to place constructor information in the output section where the
CONSTRUCTORS command appears. The CONSTRUCTORS command is
ignored for other object file formats.
The symbol __CTOR_LIST__ marks the start of the global
constructors, and the symbol __CTOR_END__ marks the end.
Similarly, __DTOR_LIST__ and __DTOR_END__ mark
the start and end of the global destructors. The
first word in the list is the number of entries, followed by the address
of each constructor or destructor, followed by a zero word. The
compiler must arrange to actually run the code. For these object file
formats GNU C++ normally calls constructors from a subroutine
__main; a call to __main is automatically inserted into
the startup code for main. GNU C++ normally runs
destructors either by using atexit, or directly from the function
exit.
For object file formats such as COFF or ELF which support
arbitrary section names, GNU C++ will normally arrange to put the
addresses of global constructors and destructors into the .ctors
and .dtors sections. Placing the following sequence into your
linker script will build the sort of table which the GNU C++
runtime code expects to see.
__CTOR_LIST__ = .;
LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
*(.ctors)
LONG(0)
__CTOR_END__ = .;
__DTOR_LIST__ = .;
LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
*(.dtors)
LONG(0)
__DTOR_END__ = .;
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If you are using the GNU C++ support for initialization priority,
which provides some control over the order in which global constructors
are run, you must sort the constructors at link time to ensure that they
are executed in the correct order. When using the CONSTRUCTORS
command, use `SORT_BY_NAME(CONSTRUCTORS)' instead. When using the
.ctors and .dtors sections, use `*(SORT_BY_NAME(.ctors))' and
`*(SORT_BY_NAME(.dtors))' instead of just `*(.ctors)' and
`*(.dtors)'.
Normally the compiler and linker will handle these issues automatically, and you will not need to concern yourself with them. However, you may need to consider this if you are using C++ and writing your own linker scripts.
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.foo : { *(.foo) }
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The linker will ignore address assignments (see section 3.6.3 Output Section Address) on discarded output sections, except when the linker script defines symbols in the output section. In that case the linker will obey the address assignments, possibly advancing dot even though the section is discarded.
The special output section name `/DISCARD/' may be used to discard input sections. Any input sections which are assigned to an output section named `/DISCARD/' are not included in the output file.
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section [address] [(type)] :
[AT(lma)]
[ALIGN(section_align)]
[SUBALIGN(subsection_align)]
[constraint]
{
output-section-command
output-section-command
...
} [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp]
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We've already described section, address, and output-section-command. In this section we will describe the remaining section attributes.
3.6.8.1 Output Section Type Output section type 3.6.8.2 Output Section LMA Output section LMA 3.6.8.3 Forced Output Alignment 3.6.8.4 Forced Input Alignment 3.6.8.5 Output Section Constraint Output section constraint 3.6.8.6 Output Section Region Output section region 3.6.8.7 Output Section Phdr Output section phdr 3.6.8.8 Output Section Fill Output section fill
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NOLOAD
DSECT
COPY
INFO
OVERLAY
The linker normally sets the attributes of an output section based on the input sections which map into it. You can override this by using the section type. For example, in the script sample below, the `ROM' section is addressed at memory location `0' and does not need to be loaded when the program is run.
SECTIONS {
ROM 0 (NOLOAD) : { ... }
...
}
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AT or AT> keywords. Specifying a load
address is optional.
The AT keyword takes an expression as an argument. This
specifies the exact load address of the section. The AT> keyword
takes the name of a memory region as an argument. See section 3.7 MEMORY Command. The
load address of the section is set to the next free address in the
region, aligned to the section's alignment requirements.
If neither AT nor AT> is specified for an allocatable
section, the linker will use the following heuristic to determine the
load address:
This feature is designed to make it easy to build a ROM image. For
example, the following linker script creates three output sections: one
called `.text', which starts at 0x1000, one called
`.mdata', which is loaded at the end of the `.text' section
even though its VMA is 0x2000, and one called `.bss' to hold
uninitialized data at address 0x3000. The symbol _data is
defined with the value 0x2000, which shows that the location
counter holds the VMA value, not the LMA value.
SECTIONS
{
.text 0x1000 : { *(.text) _etext = . ; }
.mdata 0x2000 :
AT ( ADDR (.text) + SIZEOF (.text) )
{ _data = . ; *(.data); _edata = . ; }
.bss 0x3000 :
{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;}
}
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The run-time initialization code for use with a program generated with this linker script would include something like the following, to copy the initialized data from the ROM image to its runtime address. Notice how this code takes advantage of the symbols defined by the linker script.
extern char _etext, _data, _edata, _bstart, _bend; char *src = &_etext; char *dst = &_data; /* ROM has data at end of text; copy it. */ while (dst < &_edata) *dst++ = *src++; /* Zero bss. */ for (dst = &_bstart; dst< &_bend; dst++) *dst = 0; |
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ONLY_IF_RO and
ONLY_IF_RW respectively.
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Here is a simple example:
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
SECTIONS { ROM : { *(.text) } >rom }
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:phdr modifier. You can use :NONE to tell the
linker to not put the section in any segment at all.
Here is a simple example:
PHDRS { text PT_LOAD ; }
SECTIONS { .text : { *(.text) } :text }
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+, the fill
pattern is the four least significant bytes of the value of the
expression. In all cases, the number is big-endian.
You can also change the fill value with a FILL command in the
output section commands; (see section 3.6.5 Output Section Data).
Here is a simple example:
SECTIONS { .text : { *(.text) } =0x90909090 }
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Overlays are described using the OVERLAY command. The
OVERLAY command is used within a SECTIONS command, like an
output section description. The full syntax of the OVERLAY
command is as follows:
OVERLAY [start] : [NOCROSSREFS] [AT ( ldaddr )]
{
secname1
{
output-section-command
output-section-command
...
} [:phdr...] [=fill]
secname2
{
output-section-command
output-section-command
...
} [:phdr...] [=fill]
...
} [>region] [:phdr...] [=fill]
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Everything is optional except OVERLAY (a keyword), and each
section must have a name (secname1 and secname2 above). The
section definitions within the OVERLAY construct are identical to
those within the general SECTIONS contruct (see section 3.6 SECTIONS Command),
except that no addresses and no memory regions may be defined for
sections within an OVERLAY.
The sections are all defined with the same starting address. The load
addresses of the sections are arranged such that they are consecutive in
memory starting at the load address used for the OVERLAY as a
whole (as with normal section definitions, the load address is optional,
and defaults to the start address; the start address is also optional,
and defaults to the current value of the location counter).
If the NOCROSSREFS keyword is used, and there any references
among the sections, the linker will report an error. Since the sections
all run at the same address, it normally does not make sense for one
section to refer directly to another. See section NOCROSSREFS.
For each section within the OVERLAY, the linker automatically
provides two symbols. The symbol __load_start_secname is
defined as the starting load address of the section. The symbol
__load_stop_secname is defined as the final load address of
the section. Any characters within secname which are not legal
within C identifiers are removed. C (or assembler) code may use these
symbols to move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set to the start address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a
SECTIONS construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
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__load_start_text0,
__load_stop_text0, __load_start_text1,
__load_stop_text1.
C code to copy overlay .text1 into the overlay area might look
like the following.
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
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Note that the OVERLAY command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
PROVIDE (__load_start_text0 = LOADADDR (.text0));
PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
PROVIDE (__load_start_text1 = LOADADDR (.text1));
PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
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MEMORY command.
The MEMORY command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
A linker script may contain at most one use of the MEMORY
command. However, you can define as many blocks of memory within it as
you wish. The syntax is:
MEMORY
{
name [(attr)] : ORIGIN = origin, LENGTH = len
...
}
|
The name is a name used in the linker script to refer to the
region. The region name has no meaning outside of the linker script.
Region names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each memory region
must have a distinct name within the MEMORY command. However you can
add later alias names to existing memory regions with the 3.4.4 Assign alias names to memory regions
command.
The attr string is an optional list of attributes that specify whether to use a particular memory region for an input section which is not explicitly mapped in the linker script. As described in 3.6 SECTIONS Command, if you do not specify an output section for some input section, the linker will create an output section with the same name as the input section. If you define region attributes, the linker will use them to select the memory region for the output section that it creates.
The attr string must consist only of the following characters:
If a unmapped section matches any of the listed attributes other than `!', it will be placed in the memory region. The `!' attribute reverses this test, so that an unmapped section will be placed in the memory region only if it does not match any of the listed attributes.
The origin is an numerical expression for the start address of
the memory region. The expression must evaluate to a constant and it
cannot involve any symbols. The keyword ORIGIN may be
abbreviated to org or o (but not, for example,
ORG).
The len is an expression for the size in bytes of the memory
region. As with the origin expression, the expression must
be numerical only and must evaluate to a constant. The keyword
LENGTH may be abbreviated to len or l.
In the following example, we specify that there are two memory regions available for allocation: one starting at `0' for 256 kilobytes, and the other starting at `0x40000000' for four megabytes. The linker will place into the `rom' memory region every section which is not explicitly mapped into a memory region, and is either read-only or executable. The linker will place other sections which are not explicitly mapped into a memory region into the `ram' memory region.
MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
|
Once you define a memory region, you can direct the linker to place specific output sections into that memory region by using the `>region' output section attribute. For example, if you have a memory region named `mem', you would use `>mem' in the output section definition. See section 3.6.8.6 Output Section Region. If no address was specified for the output section, the linker will set the address to the next available address within the memory region. If the combined output sections directed to a memory region are too large for the region, the linker will issue an error message.
It is possible to access the origin and length of a memory in an
expression via the ORIGIN(memory) and
LENGTH(memory) functions:
_fstack = ORIGIN(ram) + LENGTH(ram) - 4; |
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objdump
program with the `-p' option.
When you run an ELF program on a native ELF system, the system loader reads the program headers in order to figure out how to load the program. This will only work if the program headers are set correctly. This manual does not describe the details of how the system loader interprets program headers; for more information, see the ELF ABI.
The linker will create reasonable program headers by default. However,
in some cases, you may need to specify the program headers more
precisely. You may use the PHDRS command for this purpose. When
the linker sees the PHDRS command in the linker script, it will
not create any program headers other than the ones specified.
The linker only pays attention to the PHDRS command when
generating an ELF output file. In other cases, the linker will simply
ignore PHDRS.
This is the syntax of the PHDRS command. The words PHDRS,
FILEHDR, AT, and FLAGS are keywords.
PHDRS
{
name type [ FILEHDR ] [ PHDRS ] [ AT ( address ) ]
[ FLAGS ( flags ) ] ;
}
|
The name is used only for reference in the SECTIONS command
of the linker script. It is not put into the output file. Program
header names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each program header
must have a distinct name. The headers are processed in order and it
is usual for them to map to sections in ascending load address order.
Certain program header types describe segments of memory which the system loader will load from the file. In the linker script, you specify the contents of these segments by placing allocatable output sections in the segments. You use the `:phdr' output section attribute to place a section in a particular segment. See section 3.6.8.7 Output Section Phdr.
It is normal to put certain sections in more than one segment. This merely implies that one segment of memory contains another. You may repeat `:phdr', using it once for each segment which should contain the section.
If you place a section in one or more segments using `:phdr',
then the linker will place all subsequent allocatable sections which do
not specify `:phdr' in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
placed in a single segment. You can use :NONE to override the
default segment and tell the linker to not put the section in any
segment at all.
You may use the FILEHDR and PHDRS keywords after
the program header type to further describe the contents of the segment.
The FILEHDR keyword means that the segment should include the ELF
file header. The PHDRS keyword means that the segment should
include the ELF program headers themselves. If applied to a loadable
segment (PT_LOAD), all prior loadable segments must have one of
these keywords.
The type may be one of the following. The numbers indicate the value of the keyword.
PT_NULL (0)
PT_LOAD (1)
PT_DYNAMIC (2)
PT_INTERP (3)
PT_NOTE (4)
PT_SHLIB (5)
PT_PHDR (6)
You can specify that a segment should be loaded at a particular address
in memory by using an AT expression. This is identical to the
AT command used as an output section attribute (see section 3.6.8.2 Output Section LMA). The AT command for a program header overrides the
output section attribute.
The linker will normally set the segment flags based on the sections
which comprise the segment. You may use the FLAGS keyword to
explicitly specify the segment flags. The value of flags must be
an integer. It is used to set the p_flags field of the program
header.
Here is an example of PHDRS. This shows a typical set of program
headers used on a native ELF system.
PHDRS
{
headers PT_PHDR PHDRS ;
interp PT_INTERP ;
text PT_LOAD FILEHDR PHDRS ;
data PT_LOAD ;
dynamic PT_DYNAMIC ;
}
SECTIONS
{
. = SIZEOF_HEADERS;
.interp : { *(.interp) } :text :interp
.text : { *(.text) } :text
.rodata : { *(.rodata) } /* defaults to :text */
...
. = . + 0x1000; /* move to a new page in memory */
.data : { *(.data) } :data
.dynamic : { *(.dynamic) } :data :dynamic
...
}
|
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You can include a version script directly in the main linker script, or you can supply the version script as an implicit linker script. You can also use the `--version-script' linker option.
The syntax of the VERSION command is simply
VERSION { version-script-commands }
|
The format of the version script commands is identical to that used by Sun's linker in Solaris 2.5. The version script defines a tree of version nodes. You specify the node names and interdependencies in the version script. You can specify which symbols are bound to which version nodes, and you can reduce a specified set of symbols to local scope so that they are not globally visible outside of the shared library.
The easiest way to demonstrate the version script language is with a few examples.
VERS_1.1 {
global:
foo1;
local:
old*;
original*;
new*;
};
VERS_1.2 {
foo2;
} VERS_1.1;
VERS_2.0 {
bar1; bar2;
extern "C++" {
ns::*;
"f(int, double)";
};
} VERS_1.2;
|
This example version script defines three version nodes. The first version node defined is `VERS_1.1'; it has no other dependencies. The script binds the symbol `foo1' to `VERS_1.1'. It reduces a number of symbols to local scope so that they are not visible outside of the shared library; this is done using wildcard patterns, so that any symbol whose name begins with `old', `original', or `new' is matched. The wildcard patterns available are the same as those used in the shell when matching filenames (also known as "globbing"). However, if you specify the symbol name inside double quotes, then the name is treated as literal, rather than as a glob pattern.
Next, the version script defines node `VERS_1.2'. This node depends upon `VERS_1.1'. The script binds the symbol `foo2' to the version node `VERS_1.2'.
Finally, the version script defines node `VERS_2.0'. This node depends upon `VERS_1.2'. The scripts binds the symbols `bar1' and `bar2' are bound to the version node `VERS_2.0'.
When the linker finds a symbol defined in a library which is not specifically bound to a version node, it will effectively bind it to an unspecified base version of the library. You can bind all otherwise unspecified symbols to a given version node by using `global: *;' somewhere in the version script. Note that it's slightly crazy to use wildcards in a global spec except on the last version node. Global wildcards elsewhere run the risk of accidentally adding symbols to the set exported for an old version. That's wrong since older versions ought to have a fixed set of symbols.
The names of the version nodes have no specific meaning other than what they might suggest to the person reading them. The `2.0' version could just as well have appeared in between `1.1' and `1.2'. However, this would be a confusing way to write a version script.
Node name can be omitted, provided it is the only version node in the version script. Such version script doesn't assign any versions to symbols, only selects which symbols will be globally visible out and which won't.
{ global: foo; bar; local: *; };
|
When you link an application against a shared library that has versioned symbols, the application itself knows which version of each symbol it requires, and it also knows which version nodes it needs from each shared library it is linked against. Thus at runtime, the dynamic loader can make a quick check to make sure that the libraries you have linked against do in fact supply all of the version nodes that the application will need to resolve all of the dynamic symbols. In this way it is possible for the dynamic linker to know with certainty that all external symbols that it needs will be resolvable without having to search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of doing minor version checking that SunOS does. The fundamental problem that is being addressed here is that typically references to external functions are bound on an as-needed basis, and are not all bound when the application starts up. If a shared library is out of date, a required interface may be missing; when the application tries to use that interface, it may suddenly and unexpectedly fail. With symbol versioning, the user will get a warning when they start their program if the libraries being used with the application are too old.
There are several GNU extensions to Sun's versioning approach. The first of these is the ability to bind a symbol to a version node in the source file where the symbol is defined instead of in the versioning script. This was done mainly to reduce the burden on the library maintainer. You can do this by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
|
The second GNU extension is to allow multiple versions of the same function to appear in a given shared library. In this way you can make an incompatible change to an interface without increasing the major version number of the shared library, while still allowing applications linked against the old interface to continue to function.
To do this, you must use multiple `.symver' directives in the source file. Here is an example:
__asm__(".symver original_foo,foo@");
__asm__(".symver old_foo,foo@VERS_1.1");
__asm__(".symver old_foo1,foo@VERS_1.2");
__asm__(".symver new_foo,foo@@VERS_2.0");
|
In this example, `foo@' represents the symbol `foo' bound to the unspecified base version of the symbol. The source file that contains this example would define 4 C functions: `original_foo', `old_foo', `old_foo1', and `new_foo'.
When you have multiple definitions of a given symbol, there needs to be some way to specify a default version to which external references to this symbol will be bound. You can do this with the `foo@@VERS_2.0' type of `.symver' directive. You can only declare one version of a symbol as the default in this manner; otherwise you would effectively have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol within the shared library, you can use the aliases of convenience (i.e., `old_foo'), or you can use the `.symver' directive to specifically bind to an external version of the function in question.
You can also specify the language in the version script:
VERSION extern "lang" { version-script-commands }
|
The supported `lang's are `C', `C++', and `Java'. The linker will iterate over the list of symbols at the link time and demangle them according to `lang' before matching them to the patterns specified in `version-script-commands'. The default `lang' is `C'.
Demangled names may contains spaces and other special characters. As described above, you can use a glob pattern to match demangled names, or you can use a double-quoted string to match the string exactly. In the latter case, be aware that minor differences (such as differing whitespace) between the version script and the demangler output will cause a mismatch. As the exact string generated by the demangler might change in the future, even if the mangled name does not, you should check that all of your version directives are behaving as you expect when you upgrade.
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You can use and set symbol values in expressions.
The linker defines several special purpose builtin functions for use in expressions.
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As in C, the linker considers an integer beginning with `0' to be octal, and an integer beginning with `0x' or `0X' to be hexadecimal. Alternatively the linker accepts suffixes of `h' or `H' for hexadeciaml, `o' or `O' for octal, `b' or `B' for binary and `d' or `D' for decimal. Any integer value without a prefix or a suffix is considered to be decimal.
In addition, you can use the suffixes K and M to scale a
constant by
1024 or 1024*1024
respectively. For example, the following
all refer to the same quantity:
_fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000; _fourk_4 = 10000o; |
Note - the K and M suffixes cannot be used in
conjunction with the base suffixes mentioned above.
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CONSTANT(name) operator, where name is one of:
MAXPAGESIZE
COMMONPAGESIZE
So for example:
.text ALIGN (CONSTANT (MAXPAGESIZE)) : { *(.text) }
|
will create a text section aligned to the largest page boundary supported by the target.
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"SECTION" = 9; "with a space" = "also with a space" + 10; |
Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, `A-B' is one symbol, whereas `A - B' is an expression involving subtraction.
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For ELF targets, the attribute of the section includes section type as well as section flag.
If an orphaned section's name is representable as a C identifier then the linker will automatically see section 3.5.2 PROVIDE two symbols: __start_SECNAME and __end_SECNAME, where SECNAME is the name of the section. These indicate the start address and end address of the orphaned section respectively. Note: most section names are not representable as C identifiers because they contain a `.' character.
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. always refers to a
location in an output section, it may only appear in an expression
within a SECTIONS command. The . symbol may appear
anywhere that an ordinary symbol is allowed in an expression.
Assigning a value to . will cause the location counter to be
moved. This may be used to create holes in the output section. The
location counter may not be moved backwards inside an output section,
and may not be moved backwards outside of an output section if so
doing creates areas with overlapping LMAs.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x12345678;
}
|
Note: . actually refers to the byte offset from the start of the
current containing object. Normally this is the SECTIONS
statement, whose start address is 0, hence . can be used as an
absolute address. If . is used inside a section description
however, it refers to the byte offset from the start of that section,
not an absolute address. Thus in a script like this:
SECTIONS
{
. = 0x100
.text: {
*(.text)
. = 0x200
}
. = 0x500
.data: {
*(.data)
. += 0x600
}
}
|
The `.text' section will be assigned a starting address of 0x100
and a size of exactly 0x200 bytes, even if there is not enough data in
the `.text' input sections to fill this area. (If there is too
much data, an error will be produced because this would be an attempt to
move . backwards). The `.data' section will start at 0x500
and it will have an extra 0x600 bytes worth of space after the end of
the values from the `.data' input sections and before the end of
the `.data' output section itself.
Setting symbols to the value of the location counter outside of an output section statement can result in unexpected values if the linker needs to place orphan sections. For example, given the following:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
start_of_data = . ;
.data: { *(.data) }
end_of_data = . ;
}
|
If the linker needs to place some input section, e.g. .rodata,
not mentioned in the script, it might choose to place that section
between .text and .data. You might think the linker
should place .rodata on the blank line in the above script, but
blank lines are of no particular significance to the linker. As well,
the linker doesn't associate the above symbol names with their
sections. Instead, it assumes that all assignments or other
statements belong to the previous output section, except for the
special case of an assignment to .. I.e., the linker will
place the orphan .rodata section as if the script was written
as follows:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
start_of_data = . ;
.rodata: { *(.rodata) }
.data: { *(.data) }
end_of_data = . ;
}
|
This may or may not be the script author's intention for the value of
start_of_data. One way to influence the orphan section
placement is to assign the location counter to itself, as the linker
assumes that an assignment to . is setting the start address of
a following output section and thus should be grouped with that
section. So you could write:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
. = . ;
start_of_data = . ;
.data: { *(.data) }
end_of_data = . ;
}
|
Now, the orphan .rodata section will be placed between
end_of_text and start_of_data.
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precedence associativity Operators Notes (highest) 1 left ! - ~ (1) 2 left * / % 3 left + - 4 left >> << 5 left == != > < <= >= 6 left & 7 left | 8 left && 9 left || 10 right ? : 11 right &= += -= *= /= (2) (lowest) |
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The linker needs some information, such as the value of the start address of the first section, and the origins and lengths of memory regions, in order to do any linking at all. These values are computed as soon as possible when the linker reads in the linker script.
However, other values (such as symbol values) are not known or needed until after storage allocation. Such values are evaluated later, when other information (such as the sizes of output sections) is available for use in the symbol assignment expression.
The sizes of sections cannot be known until after allocation, so assignments dependent upon these are not performed until after allocation.
Some expressions, such as those depending upon the location counter `.', must be evaluated during section allocation.
If the result of an expression is required, but the value is not available, then an error results. For example, a script like the following
SECTIONS
{
.text 9+this_isnt_constant :
{ *(.text) }
}
|
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Some terms in linker expressions are addresses. This is true of
section relative symbols and for builtin functions that return an
address, such as ADDR, LOADADDR, ORIGIN and
SEGMENT_START. Other terms are simply numbers, or are builtin
functions that return a non-address value, such as LENGTH.
One complication is that unless you set LD_FEATURE ("SANE_EXPR")
(see section 3.4.5 Other Linker Script Commands), numbers and absolute symbols are treated
differently depending on their location, for compatibility with older
versions of ld. Expressions appearing outside an output
section definition treat all numbers as absolute addresses.
Expressions appearing inside an output section definition treat
absolute symbols as numbers. If LD_FEATURE ("SANE_EXPR") is
given, then absolute symbols and numbers are simply treated as numbers
everywhere.
In the following simple example,
SECTIONS
{
. = 0x100;
__executable_start = 0x100;
.data :
{
. = 0x10;
__data_start = 0x10;
*(.data)
}
...
}
|
both . and __executable_start are set to the absolute
address 0x100 in the first two assignments, then both . and
__data_start are set to 0x10 relative to the .data
section in the second two assignments.
For expressions involving numbers, relative addresses and absolute addresses, ld follows these rules to evaluate terms:
The result section of each sub-expression is as follows:
You can use the builtin function ABSOLUTE to force an expression
to be absolute when it would otherwise be relative. For example, to
create an absolute symbol set to the address of the end of the output
section `.data':
SECTIONS
{
.data : { *(.data) _edata = ABSOLUTE(.); }
}
|
Using LOADADDR also forces an expression absolute, since this
particular builtin function returns an absolute address.
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ABSOLUTE(exp)
ADDR(section)
start_of_output_1, symbol_1 and
symbol_2 are assigned equivalent values, except that
symbol_1 will be relative to the .output1 section while
the other two will be absolute:
SECTIONS { ...
.output1 :
{
start_of_output_1 = ABSOLUTE(.);
...
}
.output :
{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
}
... }
|
ALIGN(align)
ALIGN(exp,align)
.) or arbitrary expression aligned
to the next align boundary. The single operand ALIGN
doesn't change the value of the location counter--it just does
arithmetic on it. The two operand ALIGN allows an arbitrary
expression to be aligned upwards (ALIGN(align) is
equivalent to ALIGN(., align)).
Here is an example which aligns the output .data section to the
next 0x2000 byte boundary after the preceding section and sets a
variable within the section to the next 0x8000 boundary after the
input sections:
SECTIONS { ...
.data ALIGN(0x2000): {
*(.data)
variable = ALIGN(0x8000);
}
... }
|
ALIGN in this example specifies the location of
a section because it is used as the optional address attribute of
a section definition (see section 3.6.3 Output Section Address). The second use
of ALIGN is used to defines the value of a symbol.
The builtin function NEXT is closely related to ALIGN.
ALIGNOF(section)
.output section is stored as the first
value in that section.
SECTIONS{ ...
.output {
LONG (ALIGNOF (.output))
...
}
... }
|
BLOCK(exp)
ALIGN, for compatibility with older linker
scripts. It is most often seen when setting the address of an output
section.
DATA_SEGMENT_ALIGN(maxpagesize, commonpagesize)
(ALIGN(maxpagesize) + (. & (maxpagesize - 1))) |
(ALIGN(maxpagesize) + (. & (maxpagesize - commonpagesize))) |
DATA_SEGMENT_END) than the former or not.
If the latter form is used, it means commonpagesize bytes of runtime
memory will be saved at the expense of up to commonpagesize wasted
bytes in the on-disk file.
This expression can only be used directly in SECTIONS commands, not in
any output section descriptions and only once in the linker script.
commonpagesize should be less or equal to maxpagesize and should
be the system page size the object wants to be optimized for (while still
working on system page sizes up to maxpagesize).
Example:
. = DATA_SEGMENT_ALIGN(0x10000, 0x2000); |
DATA_SEGMENT_END(exp)
DATA_SEGMENT_ALIGN
evaluation purposes.
. = DATA_SEGMENT_END(.); |
DATA_SEGMENT_RELRO_END(offset, exp)
PT_GNU_RELRO segment when
`-z relro' option is used. Second argument is returned.
When `-z relro' option is not present, DATA_SEGMENT_RELRO_END
does nothing, otherwise DATA_SEGMENT_ALIGN is padded so that
exp + offset is aligned to the most commonly used page
boundary for particular target. If present in the linker script,
it must always come in between DATA_SEGMENT_ALIGN and
DATA_SEGMENT_END.
. = DATA_SEGMENT_RELRO_END(24, .); |
DEFINED(symbol)
SECTIONS { ...
.text : {
begin = DEFINED(begin) ? begin : . ;
...
}
...
}
|
LENGTH(memory)
LOADADDR(section)
MAX(exp1, exp2)
MIN(exp1, exp2)
NEXT(exp)
ALIGN(exp); unless you
use the MEMORY command to define discontinuous memory for the
output file, the two functions are equivalent.
ORIGIN(memory)
SEGMENT_START(segment, default)
SEGMENT_START with any segment
name.
SIZEOF(section)
symbol_1 and symbol_2 are assigned identical values:
SECTIONS{ ...
.output {
.start = . ;
...
.end = . ;
}
symbol_1 = .end - .start ;
symbol_2 = SIZEOF(.output);
... }
|
SIZEOF_HEADERS
sizeof_headers
When producing an ELF output file, if the linker script uses the
SIZEOF_HEADERS builtin function, the linker must compute the
number of program headers before it has determined all the section
addresses and sizes. If the linker later discovers that it needs
additional program headers, it will report an error `not enough
room for program headers'. To avoid this error, you must avoid using
the SIZEOF_HEADERS function, or you must rework your linker
script to avoid forcing the linker to use additional program headers, or
you must define the program headers yourself using the PHDRS
command (see section 3.8 PHDRS Command).
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An implicit linker script will not replace the default linker script.
Typically an implicit linker script would contain only symbol
assignments, or the INPUT, GROUP, or VERSION
commands.
Any input files read because of an implicit linker script will be read at the position in the command line where the implicit linker script was read. This can affect archive searching.
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ld has additional features on some platforms; the following
sections describe them. Machines where ld has no additional
functionality are not listed.
4.1 ldand the H8/3004.2 ldand WIN32 (cygwin/mingw)
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ld and the H8/300
For the H8/300, ld can perform these global optimizations when
you specify the `--relax' command-line option.
ld finds all jsr and jmp instructions whose
targets are within eight bits, and turns them into eight-bit
program-counter relative bsr and bra instructions,
respectively.
ld finds all mov.b instructions which use the
sixteen-bit absolute address form, but refer to the top
page of memory, and changes them to use the eight-bit address form.
(That is: the linker turns `mov.b @aa:16' into
`mov.b @aa:8' whenever the address aa is in the
top page of memory).
ld also finds all mov.b/w/l instructions which use the
register indirect with 32 bit displacement address form, but refer to the
top page of memory, and changes them to use register indirect with 16 bit
displacement address form.
(Example: The linker turns `mov.w @(d:32,ERs),Rd' into
`mov.w @(d:16,ERs),Rd' and `mov.w Rs,@(d:32,ERd)' into
`mov.w Rs,@(d:16,ERd)' whenever the address d is in
the top page of memory).
ld finds all bit manipulation instructions like band, bclr,
biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor
which use 32 bit and 16 bit absolute address form, but refer to the top
page of memory, and changes them to use the 8 bit address form.
(That is: the linker turns `bset #xx:3,@aa:32' into
`bset #xx:3,@aa:8' whenever the address aa is in
the top page of memory).
ld finds all ldc.w, stc.w instructions which use the
32 bit absolute address form, but refer to the top page of memory, and
changes them to use 16 bit address form.
(That is: the linker turns `ldc.w @aa:32,ccr' into
`ldc.w @aa:16,ccr' whenever the address aa is in
the top page of memory).
ld also finds all ldc.w, stc.w instructions which use the
register indirect with 32 bit displacement address form, but refer to the
top page of memory, and changes them to use register indirect with 16 bit
displacement address form.
(That is: the linker turns `ldc.w @(d:32,ERn),ccr' into
`ldc.w @(d:16,ERn),ccr' whenever the address d is in
the top page of memory).
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ld and WIN32 (cygwin/mingw)
This section describes some of the win32 specific ld issues.
See Command Line Options for detailed description of the
command line options mentioned here.
ld have specific
support for creating such libraries provided with the
`--out-implib' command line option.
ld has several ways to export symbols for dll's.
ld exports symbols with the auto-export functionality,
which is controlled by the following command line options:
When auto-export is in operation, ld will export all the non-local
(global and common) symbols it finds in a DLL, with the exception of a few
symbols known to belong to the system's runtime and libraries. As it will
often not be desirable to export all of a DLL's symbols, which may include
private functions that are not part of any public interface, the command-line
options listed above may be used to filter symbols out from the list for
exporting. The `--output-def' option can be used in order to see the
final list of exported symbols with all exclusions taken into effect.
If `--export-all-symbols' is not given explicitly on the command line, then the default auto-export behavior will be disabled if either of the following are true:
gcc -o <output> <objectfiles> <dll name>.def |
Using a DEF file turns off the normal auto-export behavior, unless the `--export-all-symbols' option is also used.
Here is an example of a DEF file for a shared library called `xyz.dll':
LIBRARY "xyz.dll" BASE=0x20000000 EXPORTS foo bar _bar = bar another_foo = abc.dll.afoo var1 DATA doo = foo == foo2 eoo DATA == var1 |
This example defines a DLL with a non-default base address and seven
symbols in the export table. The third exported symbol _bar is an
alias for the second. The fourth symbol, another_foo is resolved
by "forwarding" to another module and treating it as an alias for
afoo exported from the DLL `abc.dll'. The final symbol
var1 is declared to be a data object. The `doo' symbol in
export library is an alias of `foo', which gets the string name
in export table `foo2'. The `eoo' symbol is an data export
symbol, which gets in export table the name `var1'.
The optional LIBRARY <name> command indicates the internal
name of the output DLL. If `<name>' does not include a suffix,
the default library suffix, `.DLL' is appended.
When the .DEF file is used to build an application, rather than a
library, the NAME <name> command should be used instead of
LIBRARY. If `<name>' does not include a suffix, the default
executable suffix, `.EXE' is appended.
With either LIBRARY <name> or NAME <name> the optional
specification BASE = <number> may be used to specify a
non-default base address for the image.
If neither LIBRARY <name> nor NAME <name> is specified,
or they specify an empty string, the internal name is the same as the
filename specified on the command line.
The complete specification of an export symbol is:
EXPORTS
( ( ( <name1> [ = <name2> ] )
| ( <name1> = <module-name> . <external-name>))
[ @ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
|
Declares `<name1>' as an exported symbol from the DLL, or declares `<name1>' as an exported alias for `<name2>'; or declares `<name1>' as a "forward" alias for the symbol `<external-name>' in the DLL `<module-name>'. Optionally, the symbol may be exported by the specified ordinal `<integer>' alias. The optional `<name3>' is the to be used string in import/export table for the symbol.
The optional keywords that follow the declaration indicate:
NONAME: Do not put the symbol name in the DLL's export table. It
will still be exported by its ordinal alias (either the value specified
by the .def specification or, otherwise, the value assigned by the
linker). The symbol name, however, does remain visible in the import
library (if any), unless PRIVATE is also specified.
DATA: The symbol is a variable or object, rather than a function.
The import lib will export only an indirect reference to foo as
the symbol _imp__foo (ie, foo must be resolved as
*_imp__foo).
CONSTANT: Like DATA, but put the undecorated foo as
well as _imp__foo into the import library. Both refer to the
read-only import address table's pointer to the variable, not to the
variable itself. This can be dangerous. If the user code fails to add
the dllimport attribute and also fails to explicitly add the
extra indirection that the use of the attribute enforces, the
application will behave unexpectedly.
PRIVATE: Put the symbol in the DLL's export table, but do not put
it into the static import library used to resolve imports at link time. The
symbol can still be imported using the LoadLibrary/GetProcAddress
API at runtime or by by using the GNU ld extension of linking directly to
the DLL without an import library.
See ld/deffilep.y in the binutils sources for the full specification of other DEF file statements
While linking a shared dll, ld is able to create a DEF file
with the `--output-def <file>' command line option.
__declspec(dllexport) int a_variable __declspec(dllexport) void a_function(int with_args) |
All such symbols will be exported from the DLL. If, however, any of the object files in the DLL contain symbols decorated in this way, then the normal auto-export behavior is disabled, unless the `--export-all-symbols' option is also used.
Note that object files that wish to access these symbols must not decorate them with dllexport. Instead, they should use dllimport, instead:
__declspec(dllimport) int a_variable __declspec(dllimport) void a_function(int with_args) |
This complicates the structure of library header files, because when included by the library itself the header must declare the variables and functions as dllexport, but when included by client code the header must declare them as dllimport. There are a number of idioms that are typically used to do this; often client code can omit the __declspec() declaration completely. See `--enable-auto-import' and `automatic data imports' for more information.
auto-import of variables does not always work flawlessly without additional assistance. Sometimes, you will see this message
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's --enable-auto-import for details."
The `--enable-auto-import' documentation explains why this error occurs, and several methods that can be used to overcome this difficulty. One of these methods is the runtime pseudo-relocs feature, described below.
For complex variables imported from DLLs (such as structs or classes), object files typically contain a base address for the variable and an offset (addend) within the variable--to specify a particular field or public member, for instance. Unfortunately, the runtime loader used in win32 environments is incapable of fixing these references at runtime without the additional information supplied by dllimport/dllexport decorations. The standard auto-import feature described above is unable to resolve these references.
The `--enable-runtime-pseudo-relocs' switch allows these references to be resolved without error, while leaving the task of adjusting the references themselves (with their non-zero addends) to specialized code provided by the runtime environment. Recent versions of the cygwin and mingw environments and compilers provide this runtime support; older versions do not. However, the support is only necessary on the developer's platform; the compiled result will run without error on an older system.
`--enable-runtime-pseudo-relocs' is not the default; it must be explicitly enabled as needed.
ld support the direct linking,
including data symbols, to a dll without the usage of any import
libraries. This is much faster and uses much less memory than does the
traditional import library method, especially when linking large
libraries or applications. When ld creates an import lib, each
function or variable exported from the dll is stored in its own bfd, even
though a single bfd could contain many exports. The overhead involved in
storing, loading, and processing so many bfd's is quite large, and explains the
tremendous time, memory, and storage needed to link against particularly
large or complex libraries when using import libs.
Linking directly to a dll uses no extra command-line switches other than
`-L' and `-l', because ld already searches for a number
of names to match each library. All that is needed from the developer's
perspective is an understanding of this search, in order to force ld to
select the dll instead of an import library.
For instance, when ld is called with the argument `-lxxx' it will attempt to find, in the first directory of its search path,
libxxx.dll.a xxx.dll.a libxxx.a xxx.lib cygxxx.dll (*) libxxx.dll xxx.dll |
before moving on to the next directory in the search path.
(*) Actually, this is not `cygxxx.dll' but in fact is `<prefix>xxx.dll',
where `<prefix>' is set by the ld option
`--dll-search-prefix=<prefix>'. In the case of cygwin, the standard gcc spec
file includes `--dll-search-prefix=cyg', so in effect we actually search for
`cygxxx.dll'.
Other win32-based unix environments, such as mingw or pw32, may use other `<prefix>'es, although at present only cygwin makes use of this feature. It was originally intended to help avoid name conflicts among dll's built for the various win32/un*x environments, so that (for example) two versions of a zlib dll could coexist on the same machine.
The generic cygwin/mingw path layout uses a `bin' directory for applications and dll's and a `lib' directory for the import libraries (using cygwin nomenclature):
bin/ cygxxx.dll lib/ libxxx.dll.a (in case of dll's) libxxx.a (in case of static archive) |
Linking directly to a dll without using the import library can be done two ways:
1. Use the dll directly by adding the `bin' path to the link line
gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx |
However, as the dll's often have version numbers appended to their names (`cygncurses-5.dll') this will often fail, unless one specifies `-L../bin -lncurses-5' to include the version. Import libs are generally not versioned, and do not have this difficulty.
2. Create a symbolic link from the dll to a file in the `lib' directory according to the above mentioned search pattern. This should be used to avoid unwanted changes in the tools needed for making the app/dll.
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a] |
Then you can link without any make environment changes.
gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx |
This technique also avoids the version number problems, because the following is perfectly legal
bin/ cygxxx-5.dll lib/ libxxx.dll.a -> ../bin/cygxxx-5.dll |
Linking directly to a dll without using an import lib will work even when auto-import features are exercised, and even when `--enable-runtime-pseudo-relocs' is used.
Given the improvements in speed and memory usage, one might justifiably wonder why import libraries are used at all. There are three reasons:
1. Until recently, the link-directly-to-dll functionality did not work with auto-imported data.
2. Sometimes it is necessary to include pure static objects within the import library (which otherwise contains only bfd's for indirection symbols that point to the exports of a dll). Again, the import lib for the cygwin kernel makes use of this ability, and it is not possible to do this without an import lib.
3. Symbol aliases can only be resolved using an import lib. This is critical when linking against OS-supplied dll's (eg, the win32 API) in which symbols are usually exported as undecorated aliases of their stdcall-decorated assembly names.
So, import libs are not going away. But the ability to replace true import libs with a simple symbolic link to (or a copy of) a dll, in many cases, is a useful addition to the suite of tools binutils makes available to the win32 developer. Given the massive improvements in memory requirements during linking, storage requirements, and linking speed, we expect that many developers will soon begin to use this feature whenever possible.
LIBRARY "xyz.dll" BASE=0x61000000 EXPORTS foo _foo = foo |
The line `_foo = foo' maps the symbol `foo' to `_foo'.
Another method for creating a symbol alias is to create it in the source code using the "weak" attribute:
void foo () { /* Do something. */; }
void _foo () __attribute__ ((weak, alias ("foo")));
|
See the gcc manual for more information about attributes and weak symbols.
LIBRARY "xyz.dll" BASE=0x61000000 EXPORTS _foo = foo |
The line `_foo = foo' maps the exported symbol `foo' to `_foo'.
Note: using a DEF file disables the default auto-export behavior, unless the `--export-all-symbols' command line option is used. If, however, you are trying to rename symbols, then you should list all desired exports in the DEF file, including the symbols that are not being renamed, and do not use the `--export-all-symbols' option. If you list only the renamed symbols in the DEF file, and use `--export-all-symbols' to handle the other symbols, then the both the new names and the original names for the renamed symbols will be exported. In effect, you'd be aliasing those symbols, not renaming them, which is probably not what you wanted.
ld and respected when laying out the common symbols. Native
tools will be able to process object files employing this GNU extension,
but will fail to respect the alignment instructions, and may issue noisy
warnings about unknown linker directives.
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The linker accesses object and archive files using the BFD libraries.
These libraries allow the linker to use the same routines to operate on
object files whatever the object file format. A different object file
format can be supported simply by creating a new BFD back end and adding
it to the library. To conserve runtime memory, however, the linker and
associated tools are usually configured to support only a subset of the
object file formats available. You can use objdump -i
(see section `objdump' in The GNU Binary Utilities) to
list all the formats available for your configuration.
As with most implementations, BFD is a compromise between several conflicting requirements. The major factor influencing BFD design was efficiency: any time used converting between formats is time which would not have been spent had BFD not been involved. This is partly offset by abstraction payback; since BFD simplifies applications and back ends, more time and care may be spent optimizing algorithms for a greater speed.
One minor artifact of the BFD solution which you should bear in mind is the potential for information loss. There are two places where useful information can be lost using the BFD mechanism: during conversion and during output. See section 5.1.1 Information Loss.
5.1 How It Works: An Outline of BFD How it works: an outline of BFD
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As different information from the object files is required, BFD reads from different sections of the file and processes them. For example, a very common operation for the linker is processing symbol tables. Each BFD back end provides a routine for converting between the object file's representation of symbols and an internal canonical format. When the linker asks for the symbol table of an object file, it calls through a memory pointer to the routine from the relevant BFD back end which reads and converts the table into a canonical form. The linker then operates upon the canonical form. When the link is finished and the linker writes the output file's symbol table, another BFD back end routine is called to take the newly created symbol table and convert it into the chosen output format.
5.1.1 Information Loss 5.1.2 The BFD canonical object-file format
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Information can be lost during output. The output formats
supported by BFD do not provide identical facilities, and
information which can be described in one form has nowhere to go in
another format. One example of this is alignment information in
b.out. There is nowhere in an a.out format file to store
alignment information on the contained data, so when a file is linked
from b.out and an a.out image is produced, alignment
information will not propagate to the output file. (The linker will
still use the alignment information internally, so the link is performed
correctly).
Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections (e.g.,
a.out) or has sections without names (e.g., the Oasys format), the
link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker command
language.
Information can be lost during canonicalization. The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation internally. This means that the BFD back ends cannot maintain all possible data richness through the transformation between external to internal and back to external formats.
This limitation is only a problem when an application reads one
format and writes another. Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD
canonical form has structures which are opaque to the BFD core,
and exported only to the back ends. When a file is read in one format,
the canonical form is generated for BFD and the application. At the
same time, the back end saves away any information which may otherwise
be lost. If the data is then written back in the same format, the back
end routine will be able to use the canonical form provided by the
BFD core as well as the information it prepared earlier. Since
there is a great deal of commonality between back ends,
there is no information lost when
linking or copying big endian COFF to little endian COFF, or a.out to
b.out. When a mixture of formats is linked, the information is
only lost from the files whose format differs from the destination.
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The greatest potential for loss of information occurs when there is the least overlap between the information provided by the source format, that stored by the canonical format, and that needed by the destination format. A brief description of the canonical form may help you understand which kinds of data you can count on preserving across conversions.
ZMAGIC
file would have both the demand pageable bit and the write protected
text bit set. The byte order of the target is stored on a per-file
basis, so that big- and little-endian object files may be used with one
another.
ld can
operate on a collection of symbols of wildly different formats without
problems.
Normal global and simple local symbols are maintained on output, so an
output file (no matter its format) will retain symbols pointing to
functions and to global, static, and common variables. Some symbol
information is not worth retaining; in a.out, type information is
stored in the symbol table as long symbol names. This information would
be useless to most COFF debuggers; the linker has command line switches
to allow users to throw it away.
There is one word of type information within the symbol, so if the format supports symbol type information within symbols (for example, COFF, IEEE, Oasys) and the type is simple enough to fit within one word (nearly everything but aggregates), the information will be preserved.
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Your bug reports play an essential role in making ld reliable.
Reporting a bug may help you by bringing a solution to your problem, or
it may not. But in any case the principal function of a bug report is
to help the entire community by making the next version of ld
work better. Bug reports are your contribution to the maintenance of
ld.
In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.
6.1 Have You Found a Bug? Have you found a bug? 6.2 How to Report Bugs How to report bugs
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If you are not sure whether you have found a bug, here are some guidelines:
ld bug. Reliable linkers never crash.
ld produces an error message for valid input, that is a bug.
ld does not produce an error message for invalid input, that
may be a bug. In the general case, the linker can not verify that
object files are correct.
ld are welcome in any case.
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A number of companies and individuals offer support for GNU
products. If you obtained ld from a support organization, we
recommend you contact that organization first.
You can find contact information for many support companies and individuals in the file `etc/SERVICE' in the GNU Emacs distribution.
Otherwise, send bug reports for ld to
http://www.sourceware.org/bugzilla/.
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of a symbol you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the linker into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.
To enable us to fix the bug, you should include all these things:
ld. ld announces it if you start it with
the `--version' argument.
Without this, we will not know whether there is any point in looking for
the bug in the current version of ld.
ld source, including any
patches made to the BFD library.
ld---e.g.
"gcc-2.7".
If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.
If the source files were assembled using gas or compiled using
gcc, then it may be OK to send the source files rather than the
object files. In this case, be sure to say exactly what version of
gas or gcc was used to produce the object files. Also say
how gas or gcc were configured.
Of course, if the bug is that ld gets a fatal signal, then we
will certainly notice it. But if the bug is incorrect output, we might
not notice unless it is glaringly wrong. You might as well not give us
a chance to make a mistake.
Even if the problem you experience is a fatal signal, you should still
say so explicitly. Suppose something strange is going on, such as, your
copy of ld is out of sync, or you have encountered a bug in the
C library on your system. (This has happened!) Your copy might crash
and ours would not. If you told us to expect a crash, then when ours
fails to crash, we would know that the bug was not happening for us. If
you had not told us to expect a crash, then we would not be able to draw
any conclusion from our observations.
ld source, send us context
diffs, as generated by diff with the `-u', `-c', or
`-p' option. Always send diffs from the old file to the new file.
If you even discuss something in the ld source, refer to it by
context, not by line number.
The line numbers in our development sources will not match those in your sources. Your line numbers would convey no useful information to us.
Here are some things that are not necessary:
Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less time, and so on.
However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.
A patch for the bug does help us if it is a good one. But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as ld it is very hard to
construct an example that will make the program follow a certain path
through the code. If you do not send us the example, we will not be
able to construct one, so we will not be able to verify that the bug is
fixed.
And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it. A test case will help us to understand.
Such guesses are usually wrong. Even we cannot guess right about such things without first using the debugger to find the facts.
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ld from the MRI
linker, ld can use MRI compatible linker scripts as an
alternative to the more general-purpose linker scripting language
described in 3. Linker Scripts. MRI compatible linker scripts have a much
simpler command set than the scripting language otherwise used with
ld. GNU ld supports the most commonly used MRI
linker commands; these commands are described here.
In general, MRI scripts aren't of much use with the a.out object
file format, since it only has three sections and MRI scripts lack some
features to make use of them.
You can specify a file containing an MRI-compatible script using the `-c' command-line option.
Each command in an MRI-compatible script occupies its own line; each
command line starts with the keyword that identifies the command (though
blank lines are also allowed for punctuation). If a line of an
MRI-compatible script begins with an unrecognized keyword, ld
issues a warning message, but continues processing the script.
Lines beginning with `*' are comments.
You can write these commands using all upper-case letters, or all lower case; for example, `chip' is the same as `CHIP'. The following list shows only the upper-case form of each command.
ABSOLUTE secname
ABSOLUTE secname, secname, ... secname
ld includes in the output file all sections from all
the input files. However, in an MRI-compatible script, you can use the
ABSOLUTE command to restrict the sections that will be present in
your output program. If the ABSOLUTE command is used at all in a
script, then only the sections named explicitly in ABSOLUTE
commands will appear in the linker output. You can still use other
input sections (whatever you select on the command line, or using
LOAD) to resolve addresses in the output file.
ALIAS out-secname, in-secname
in-secname may be an integer.
ALIGN secname = expression
BASE expression
CHIP expression
CHIP expression, expression
END
FORMAT output-format
OUTPUT_FORMAT command in the more general linker
language, but restricted to one of these output formats:
LIST anything...
ld command-line option `-M'.
The keyword LIST may be followed by anything on the
same line, with no change in its effect.
LOAD filename
LOAD filename, filename, ... filename
ld
command line.
NAME output-name
ld; the
MRI-compatible command NAME is equivalent to the command-line
option `-o' or the general script language command OUTPUT.
ORDER secname, secname, ... secname
ORDER secname secname secname
ld orders the sections in its output file in the
order in which they first appear in the input files. In an MRI-compatible
script, you can override this ordering with the ORDER command. The
sections you list with ORDER will appear first in your output
file, in the order specified.
PUBLIC name=expression
PUBLIC name,expression
PUBLIC name expression
SECT secname, expression
SECT secname=expression
SECT secname expression
SECT command to
specify the start address (expression) for section secname.
If you have more than one SECT statement for the same
secname, only the first sets the start address.
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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.
This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
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The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
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You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.
You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
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You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements."
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
"Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A "Massive Multiauthor Collaboration" (or "MMC") contained in the site means any set of copyrightable works thus published on the MMC site.
"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
"Incorporate" means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
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''. |
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the "with...Texts." line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
|
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
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ld and the H8/300
ld and WIN32 (cygwin/mingw)
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1. Overview
2. Invocation
3. Linker Scripts
4. Machine Dependent Features
5. BFD
6. Reporting Bugs
A. MRI Compatible Script Files
B. GNU Free Documentation License
LD Index
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| Button | Name | Go to | From 1.2.3 go to |
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| [Top] | Top | cover (top) of document | |
| [Contents] | Contents | table of contents | |
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| [ ? ] | About | this page |