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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 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:
@fileRead command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.
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 keywordThis option is supported for HP/UX compatibility. The keyword argument must be one of the strings ‘archive’, ‘shared’, or ‘default’. ‘-aarchive’ is functionally equivalent to ‘-Bstatic’, and the other two keywords are functionally equivalent to ‘-Bdynamic’. This option may be used any number of times.
--audit AUDITLIBAdds AUDITLIB to the 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.
-A architecture--architecture=architectureIn the current release of ld, this option is useful only for the
Intel 960 family of architectures. In that ld configuration, the
architecture argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. See ld and the Intel 960
family, for details.
Future releases of ld may support similar functionality for
other architecture families.
-b input-format--format=input-formatld 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 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 Environment.
You can also define the input format from a script, using the command
TARGET;
see Format Commands.
-c MRI-commandfile--mri-script=MRI-commandfileFor compatibility with linkers produced by MRI, 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-dpThese three options are equivalent; multiple forms are supported for
compatibility with other linkers. They assign space to common symbols
even if a relocatable output file is specified (with ‘-r’). The
script command FORCE_COMMON_ALLOCATION has the same effect.
See Miscellaneous Commands.
--depaudit AUDITLIB-P AUDITLIBAdds AUDITLIB to the 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=entryUse entry as the explicit symbol for beginning execution of your program, rather than the default entry point. If there is no symbol named entry, the linker will try to parse entry as a number, and use that as the entry address (the number will be interpreted in base 10; you may use a leading ‘0x’ for base 16, or a leading ‘0’ for base 8). See Entry Point, for a discussion of defaults and other ways of specifying the entry point.
--exclude-libs lib,lib,...Specifies a list of archive libraries from which symbols should not be automatically
exported. The library names may be delimited by commas or colons. Specifying
--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,...Specifies a list of object files or archive members, from which symbols
should not be automatically exported, but which should be copied wholesale
into the import library being generated during the link. The module names
may be delimited by commas or colons, and must match exactly the filenames
used by 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-dynamicWhen creating a dynamically linked executable, using the -E option or the --export-dynamic option causes the linker to add all symbols to the dynamic symbol table. The dynamic symbol table is the set of symbols which are visible from dynamic objects at run time.
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.
-EBLink big-endian objects. This affects the default output format.
-ELLink little-endian objects. This affects the default output format.
-f name--auxiliary=nameWhen creating an ELF shared object, set the internal DT_AUXILIARY field to the specified name. This tells the dynamic linker that the symbol table of the shared object should be used as an auxiliary filter on the symbol table of the shared object 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=nameWhen creating an ELF shared object, set the internal DT_FILTER field to the specified name. This tells the dynamic linker that the symbol table of the shared object which is being created should be used as a filter on the symbol table of the shared object 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=nameWhen creating an ELF executable or shared object, call NAME when the
executable or shared object is unloaded, by setting DT_FINI to the
address of the function. By default, the linker uses _fini as
the function to call.
-gIgnored. Provided for compatibility with other tools.
-G value--gpsize=valueSet the maximum size of objects to be optimized using the GP register to size. This is only meaningful for object file formats such as MIPS ELF that support putting large and small objects into different sections. This is ignored for other object file formats.
-h name-soname=nameWhen creating an ELF shared object, set the internal DT_SONAME field to the specified name. When an executable is linked with a shared object which has a DT_SONAME field, then when the executable is run the dynamic linker will attempt to load the shared object specified by the DT_SONAME field rather than the using the file name given to the linker.
-iPerform an incremental link (same as option ‘-r’).
-init=nameWhen creating an ELF executable or shared object, call NAME when the
executable or shared object is loaded, by setting DT_INIT to the address
of the function. By default, the linker uses _init as the
function to call.
-l namespec--library=namespecAdd the archive or object file specified by namespec to the
list of files to link. This option may be used any number of times.
If namespec is of the form :filename, 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=searchdirAdd path searchdir to the list of paths that 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 = or $SYSROOT, then this
prefix will be replaced by the sysroot prefix, controlled by the
‘--sysroot’ option, or 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 Environment.
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 emulationEmulate the emulation linker. You can list the available emulations with the ‘--verbose’ or ‘-V’ options.
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-mapPrint a link map to the standard output. A link map provides information about the link, including the following:
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 Expressions for more information about expressions in linker scripts.
-n--nmagicTurn off page alignment of sections, and disable linking against shared
libraries. If the output format supports Unix style magic numbers,
mark the output as NMAGIC.
-N--omagicSet the text and data sections to be readable and writable. Also, do
not page-align the data segment, and disable linking against shared
libraries. If the output format supports Unix style magic numbers,
mark the output as 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-omagicThis option negates most of the effects of the -N option. It sets the text section to be read-only, and forces the data segment to be page-aligned. Note - this option does not enable linking against shared libraries. Use -Bdynamic for this.
-o output--output=outputUse output as the name for the program produced by 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 levelIf level is a numeric values greater than zero 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.
-plugin nameInvolve a plugin in the linking process. The name parameter is the absolute filename of the plugin. Usually this parameter is automatically added by the complier, when using link time optimization, but users can also add their own plugins if they so wish.
Note that the location of the compiler originated plugins is different
from the place where the ar, nm and
ranlib programs search for their plugins. In order for
those commands to make use of a compiler based plugin it must first be
copied into the ${libdir}/bfd-plugins directory. All gcc
based linker plugins are backward compatible, so it is sufficient to
just copy in the newest one.
--push-stateThe --push-state allows to preserve the current state of the flags which govern the input file handling so that they can all be restored with one corresponding --pop-state option.
The option which are covered are: -Bdynamic, -Bstatic, -dn, -dy, -call_shared, -non_shared, -static, -N, -n, --whole-archive, --no-whole-archive, -r, -Ur, --copy-dt-needed-entries, --no-copy-dt-needed-entries, --as-needed, --no-as-needed, and -a.
One target for this option are specifications for pkg-config. When used with the --libs option all possibly needed libraries are listed and then possibly linked with all the time. It is better to return something as follows:
-Wl,--push-state,--as-needed -libone -libtwo -Wl,--pop-state
--pop-stateUndoes the effect of –push-state, restores the previous values of the flags governing input file handling.
-q--emit-relocsLeave relocation sections and contents in fully linked executables. Post link analysis and optimization tools may need this information in order to perform correct modifications of executables. This results in larger executables.
This option is currently only supported on ELF platforms.
--force-dynamicForce the output file to have dynamic sections. This option is specific to VxWorks targets.
-r--relocatableGenerate relocatable output—i.e., generate an output file that can in
turn serve as input to 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=filenameRead symbol names and their addresses from filename, but do not relocate it or include it in the output. This allows your output file to refer symbolically to absolute locations of memory defined in other programs. You may use this option more than once.
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-allOmit all symbol information from the output file.
-S--strip-debugOmit debugger symbol information (but not all symbols) from the output file.
--strip-discarded--no-strip-discardedOmit (or do not omit) global symbols defined in discarded sections. Enabled by default.
-t--tracePrint the names of the input files as ld processes them.
-T scriptfile--script=scriptfileUse scriptfile as the linker script. This script replaces
ld’s default linker script (rather than adding to it), so
commandfile must specify everything necessary to describe the
output file. See 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=scriptfileUse scriptfile as the default linker script. See Scripts.
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=symbolForce symbol to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of additional
modules from standard libraries. ‘-u’ may be repeated with
different option arguments to enter additional undefined symbols. This
option is equivalent to the EXTERN linker script command.
If this option is being used to force additional modules to be pulled into the link, and if it is an error for the symbol to remain undefined, then the option --require-defined should be used instead.
--require-defined=symbolRequire that symbol is defined in the output file. This option
is the same as option --undefined except that if symbol
is not defined in the output file then the linker will issue an error
and exit. The same effect can be achieved in a linker script by using
EXTERN, ASSERT and DEFINED together. This option
can be used multiple times to require additional symbols.
-UrFor anything other than C++ programs, this option is equivalent to
‘-r’: it generates relocatable output—i.e., an output file that can in
turn serve as input to 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.
--orphan-handling=MODEControl how orphan sections are handled. An orphan section is one not specifically mentioned in a linker script. See Orphan Sections.
MODE can have any of the following values:
placeOrphan sections are placed into a suitable output section following the strategy described in Orphan Sections. The option ‘--unique’ also affects how sections are placed.
discardAll orphan sections are discarded, by placing them in the ‘/DISCARD/’ section (see Output Section Discarding).
warnThe linker will place the orphan section as for place and also
issue a warning.
errorThe linker will exit with an error if any orphan section is found.
The default if ‘--orphan-handling’ is not given is place.
--unique[=SECTION]Creates a separate output section for every input section matching SECTION, or if the optional wildcard SECTION argument is missing, for every orphan input section. An orphan section is one not specifically mentioned in a linker script. You may use this option multiple times on the command line; It prevents the normal merging of input sections with the same name, overriding output section assignments in a linker script.
-v--version-VDisplay the version number for ld. The -V option also
lists the supported emulations.
-x--discard-allDelete all local symbols.
-X--discard-localsDelete all temporary local symbols. (These symbols start with system-specific local label prefixes, typically ‘.L’ for ELF systems or ‘L’ for traditional a.out systems.)
-y symbol--trace-symbol=symbolPrint the name of each linked file in which symbol appears. This option may be given any number of times. On many systems it is necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your link but don’t know where the reference is coming from.
-Y pathAdd path to the default library search path. This option exists for Solaris compatibility.
-z keywordThe recognized keywords are:
Always generate BND prefix in PLT entries. Supported for Linux/x86_64.
Specify the 1-byte NOP padding when transforming indirect call
to a locally defined function, foo, via its GOT slot.
call-nop=prefix-addr generates 0x67 call foo.
call-nop=suffix-nop generates call foo 0x90.
call-nop=prefix-byte generates byte call foo.
call-nop=suffix-byte generates call foo byte.
Supported for i386 and x86_64.
Combine multiple dynamic relocation sections and sort to improve dynamic symbol lookup caching. Do not do this if ‘nocombreloc’.
Generate common symbols with STT_COMMON type during a relocatable link. Use STT_OBJECT type if ‘nocommon’.
Set the page size most commonly used to value. Memory image layout will be optimized to minimize memory pages if the system is using pages of this size.
Report unresolved symbol references from regular object files. This is done even if the linker is creating a non-symbolic shared library. This option is the inverse of ‘-z undefs’.
Make undefined weak symbols dynamic when building a dynamic object, if they are referenced from a regular object file and not forced local by symbol visibility or versioning. Do not make them dynamic if ‘nodynamic-undefined-weak’. If neither option is given, a target may default to either option being in force, or make some other selection of undefined weak symbols dynamic. Not all targets support these options.
Marks the object as requiring executable stack.
This option is only meaningful when building a shared object. It makes the symbols defined by this shared object available for symbol resolution of subsequently loaded libraries.
This option is only meaningful when building a dynamic executable.
This option marks the executable as requiring global auditing by
setting the DF_1_GLOBAUDIT bit in the DT_FLAGS_1 dynamic
tag. Global auditing requires that any auditing library defined via
the --depaudit or -P command line options be run for
all dynamic objects loaded by the application.
Generate Intel Indirect Branch Tracking (IBT) enabled PLT entries. Supported for Linux/i386 and Linux/x86_64.
Generate GNU_PROPERTY_X86_FEATURE_1_IBT in .note.gnu.property section to indicate compatibility with IBT. This also implies ibtplt. Supported for Linux/i386 and Linux/x86_64.
This option is only meaningful when building a shared object. It marks the object so that its runtime initialization will occur before the runtime initialization of any other objects brought into the process at the same time. Similarly the runtime finalization of the object will occur after the runtime finalization of any other objects.
Specify that the dynamic loader should modify its symbol search order so that symbols in this shared library interpose all other shared libraries not so marked.
When generating an executable or shared library, mark it to tell the dynamic linker to defer function call resolution to the point when the function is called (lazy binding), rather than at load time. Lazy binding is the default.
Specify that the object’s filters be processed immediately at runtime.
Set the maximum memory page size supported to value.
Allow multiple definitions.
Disable linker generated .dynbss variables used in place of variables defined in shared libraries. May result in dynamic text relocations.
Specify that the dynamic loader search for dependencies of this object should ignore any default library search paths.
Specify that the object shouldn’t be unloaded at runtime.
Specify that the object is not available to dlopen.
Specify that the object can not be dumped by dldump.
Marks the object as not requiring executable stack.
Don’t treat protected data symbols as external when building a shared library. This option overrides the linker backend default. It can be used to work around incorrect relocations against protected data symbols generated by compiler. Updates on protected data symbols by another module aren’t visible to the resulting shared library. Supported for i386 and x86-64.
Disable relocation overflow check. This can be used to disable relocation overflow check if there will be no dynamic relocation overflow at run-time. Supported for x86_64.
When generating an executable or shared library, mark it to tell the dynamic linker to resolve all symbols when the program is started, or when the shared library is loaded by dlopen, instead of deferring function call resolution to the point when the function is first called.
Specify that the object requires ‘$ORIGIN’ handling in paths.
Create an ELF PT_GNU_RELRO segment header in the object. This
specifies a memory segment that should be made read-only after
relocation, if supported. Specifying ‘common-page-size’ smaller
than the system page size will render this protection ineffective.
Don’t create an ELF PT_GNU_RELRO segment if ‘norelro’.
Create separate code PT_LOAD segment header in the object. This
specifies a memory segment that should contain only instructions and must
be in wholly disjoint pages from any other data. Don’t create separate
code PT_LOAD segment if ‘noseparate-code’ is used.
Generate GNU_PROPERTY_X86_FEATURE_1_SHSTK in .note.gnu.property section to indicate compatibility with Intel Shadow Stack. Supported for Linux/i386 and Linux/x86_64.
Specify a stack size for an ELF PT_GNU_STACK segment.
Specifying zero will override any default non-zero sized
PT_GNU_STACK segment creation.
Report an error if DT_TEXTREL is set, i.e., if the binary has dynamic relocations in read-only sections. Don’t report an error if ‘notext’ or ‘textoff’.
Do not report unresolved symbol references from regular object files, either when creating an executable, or when creating a shared library. This option is the inverse of ‘-z defs’.
Other keywords are ignored for Solaris compatibility.
-( archives -)--start-group archives --end-groupThe archives should be a list of archive files. They may be either explicit file names, or ‘-l’ options.
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-archTells the linker to accept input files whose architecture cannot be recognised. The assumption is that the user knows what they are doing and deliberately wants to link in these unknown input files. This was the default behaviour of the linker, before release 2.14. The default behaviour from release 2.14 onwards is to reject such input files, and so the ‘--accept-unknown-input-arch’ option has been added to restore the old behaviour.
--as-needed--no-as-neededThis option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the --as-needed option. Normally the linker will add a DT_NEEDED tag for each dynamic library mentioned on the command line, regardless of whether the library is actually needed or not. --as-needed causes a DT_NEEDED tag to only be emitted for a library that at that point in the link satisfies a non-weak undefined symbol reference from a regular object file or, if the library is not found in the DT_NEEDED lists of other needed libraries, a non-weak undefined symbol reference from another needed dynamic library. Object files or libraries appearing on the command line after the library in question do not affect whether the library is seen as needed. This is similar to the rules for extraction of object files from archives. --no-as-needed restores the default behaviour.
--add-needed--no-add-neededThese two options have been deprecated because of the similarity of their names to the --as-needed and --no-as-needed options. They have been replaced by --copy-dt-needed-entries and --no-copy-dt-needed-entries.
-assert keywordThis option is ignored for SunOS compatibility.
-Bdynamic-dy-call_sharedLink against dynamic libraries. This is only meaningful on platforms for which shared libraries are supported. This option is normally the default on such platforms. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for -l options which follow it.
-BgroupSet the 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-staticDo not link against shared libraries. This is only meaningful on platforms for which shared libraries are supported. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for -l options which follow it. This option also implies --unresolved-symbols=report-all. This option can be used with -shared. Doing so means that a shared library is being created but that all of the library’s external references must be resolved by pulling in entries from static libraries.
-BsymbolicWhen creating a shared library, bind references to global symbols to the definition within the shared library, if any. Normally, it is possible for a program linked against a shared library to override the definition within the shared library. This option can also be used with the --export-dynamic option, when creating a position independent executable, to bind references to global symbols to the definition within the executable. This option is only meaningful on ELF platforms which support shared libraries and position independent executables.
-Bsymbolic-functionsWhen creating a shared library, bind references to global function symbols to the definition within the shared library, if any. This option can also be used with the --export-dynamic option, when creating a position independent executable, to bind references to global function symbols to the definition within the executable. This option is only meaningful on ELF platforms which support shared libraries and position independent executables.
--dynamic-list=dynamic-list-fileSpecify the name of a dynamic list file to the linker. This is typically used when creating shared libraries to specify a list of global symbols whose references shouldn’t be bound to the definition within the shared library, or creating dynamically linked executables to specify a list of symbols which should be added to the symbol table in the executable. This option is only meaningful on ELF platforms which support shared libraries.
The format of the dynamic list is the same as the version node without scope and node name. See VERSION for more information.
--dynamic-list-dataInclude all global data symbols to the dynamic list.
--dynamic-list-cpp-newProvide the builtin dynamic list for C++ operator new and delete. It is mainly useful for building shared libstdc++.
--dynamic-list-cpp-typeinfoProvide the builtin dynamic list for C++ runtime type identification.
--check-sections--no-check-sectionsAsks the linker not to check section addresses after they have been assigned to see if there are any overlaps. Normally the linker will perform this check, and if it finds any overlaps it will produce suitable error messages. The linker does know about, and does make allowances for sections in overlays. The default behaviour can be restored by using the command line switch --check-sections. Section overlap is not usually checked for relocatable links. You can force checking in that case by using the --check-sections option.
--copy-dt-needed-entries--no-copy-dt-needed-entriesThis option affects the treatment of dynamic libraries referred to by DT_NEEDED tags inside ELF dynamic libraries mentioned on the command line. Normally the linker won’t add a DT_NEEDED tag to the output binary for each library mentioned in a DT_NEEDED tag in an input dynamic library. With --copy-dt-needed-entries specified on the command line however any dynamic libraries that follow it will have their DT_NEEDED entries added. The default behaviour can be restored with --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.
--crefOutput a cross reference table. If a linker map file is being generated, the cross reference table is printed to the map file. Otherwise, it is printed on the standard output.
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. If the symbol is defined as a common value then any files where this happens appear next. Finally any files that reference the symbol are listed.
--no-define-commonThis option inhibits the assignment of addresses to common symbols.
The script command INHIBIT_COMMON_ALLOCATION has the same effect.
See Miscellaneous 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.
--force-group-allocationThis option causes the linker to place section group members like
normal input sections, and to delete the section groups. This is the
default behaviour for a final link but this option can be used to
change the behaviour of a relocatable link (‘-r’). The script
command FORCE_GROUP_ALLOCATION has the same
effect. See Miscellaneous Commands.
--defsym=symbol=expressionCreate a global symbol in the output file, containing the absolute
address given by expression. You may use this option as many
times as necessary to define multiple symbols in the command line. A
limited form of arithmetic is supported for the expression in this
context: you may give a hexadecimal constant or the name of an existing
symbol, or use + 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 Assignments).
Note: there should be no white space between symbol, the
equals sign (“=”), and expression.
--demangle[=style]--no-demangleThese options control whether to demangle symbol names in error messages and other output. When the linker is told to demangle, it tries to present symbol names in a readable fashion: it strips leading underscores if they are used by the object file format, and converts C++ mangled symbol names into user readable names. Different compilers have different mangling styles. The optional demangling style argument can be used to choose an appropriate demangling style for your compiler. The linker will demangle by default unless the environment variable ‘COLLECT_NO_DEMANGLE’ is set. These options may be used to override the default.
-Ifile--dynamic-linker=fileSet the name of the dynamic linker. This is only meaningful when generating dynamically linked ELF executables. The default dynamic linker is normally correct; don’t use this unless you know what you are doing.
--no-dynamic-linkerWhen producing an executable file, omit the request for a dynamic linker to be used at load-time. This is only meaningful for ELF executables that contain dynamic relocations, and usually requires entry point code that is capable of processing these relocations.
--embedded-relocsThis option is similar to the --emit-relocs option except that the relocs are stored in a target specific section. This option is only supported by the ‘BFIN’, ‘CR16’ and M68K targets.
--fatal-warnings--no-fatal-warningsTreat all warnings as errors. The default behaviour can be restored with the option --no-fatal-warnings.
--force-exe-suffixMake sure that an output file has a .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-sectionsEnable garbage collection of unused input sections. It is ignored on targets that do not support this option. The default behaviour (of not performing this garbage collection) can be restored by specifying ‘--no-gc-sections’ on the command line. Note that garbage collection for COFF and PE format targets is supported, but the implementation is currently considered to be experimental.
‘--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-sectionsList all sections removed by garbage collection. The listing is printed on stderr. This option is only effective if garbage collection has been enabled via the ‘--gc-sections’) option. The default behaviour (of not listing the sections that are removed) can be restored by specifying ‘--no-print-gc-sections’ on the command line.
--gc-keep-exportedWhen ‘--gc-sections’ is enabled, this option prevents garbage collection of unused input sections that contain global symbols having default or protected visibility. This option is intended to be used for executables where unreferenced sections would otherwise be garbage collected regardless of the external visibility of contained symbols. Note that this option has no effect when linking shared objects since it is already the default behaviour. This option is only supported for ELF format targets.
--print-output-formatPrint the name of the default output format (perhaps influenced by
other command-line options). This is the string that would appear
in an OUTPUT_FORMAT linker script command (see File Commands).
--print-memory-usagePrint used size, total size and used size of memory regions created with the MEMORY command. This is useful on embedded targets to have a quick view of amount of free memory. The format of the output has one headline and one line per region. It is both human readable and easily parsable by tools. Here is an example of an output:
Memory region Used Size Region Size %age Used
ROM: 256 KB 1 MB 25.00%
RAM: 32 B 2 GB 0.00%
--helpPrint a summary of the command-line options on the standard output and exit.
--target-helpPrint a summary of all target specific options on the standard output and exit.
-Map=mapfilePrint a link map to the file mapfile. See the description of the -M option, above.
--no-keep-memoryld 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 defsReport unresolved symbol references from regular object files. This is done even if the linker is creating a non-symbolic shared library. The switch --[no-]allow-shlib-undefined controls the behaviour for reporting unresolved references found in shared libraries being linked in.
The effects of this option can be reverted by using -z undefs.
--allow-multiple-definition-z muldefsNormally when a symbol is defined multiple times, the linker will report a fatal error. These options allow multiple definitions and the first definition will be used.
--allow-shlib-undefined--no-allow-shlib-undefinedAllows or disallows undefined symbols in shared libraries. This switch is similar to --no-undefined except that it determines the behaviour when the undefined symbols are in a shared library rather than a regular object file. It does not affect how undefined symbols in regular object files are handled.
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-versionNormally when a symbol has an undefined version, the linker will ignore it. This option disallows symbols with undefined version and a fatal error will be issued instead.
--default-symverCreate and use a default symbol version (the soname) for unversioned exported symbols.
--default-imported-symverCreate and use a default symbol version (the soname) for unversioned imported symbols.
--no-warn-mismatchNormally 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-mismatchNormally ld will give a warning if it finds an incompatible
library during a library search. This option silences the warning.
--no-whole-archiveTurn off the effect of the --whole-archive option for subsequent archive files.
--noinhibit-execRetain the executable output file whenever it is still usable. Normally, the linker will not produce an output file if it encounters errors during the link process; it exits without writing an output file when it issues any error whatsoever.
-nostdlibOnly search library directories explicitly specified on the command line. Library directories specified in linker scripts (including linker scripts specified on the command line) are ignored.
--oformat=output-formatld 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 BFD.
--out-implib fileCreate an import library in file corresponding to the executable
the linker is generating (eg. a DLL or ELF program). This import
library (which should be called *.dll.a or *.a for DLLs)
may be used to link clients against the generated executable; this
behaviour makes it possible to skip a separate import library creation
step (eg. dlltool for DLLs). This option is only available for
the i386 PE and ELF targetted ports of the linker.
-pie--pic-executableCreate a position independent executable. This is currently only supported on ELF platforms. Position independent executables are similar to shared libraries in that they are relocated by the dynamic linker to the virtual address the OS chooses for them (which can vary between invocations). Like normal dynamically linked executables they can be executed and symbols defined in the executable cannot be overridden by shared libraries.
-qmagicThis option is ignored for Linux compatibility.
-QyThis option is ignored for SVR4 compatibility.
--relax--no-relaxAn option with machine dependent effects.
This option is only supported on a few targets.
See ld and the H8/300.
See ld and the Intel 960 family.
See ld and Xtensa Processors.
See ld and the 68HC11 and 68HC12.
See ld and the Altera Nios II.
See ld and PowerPC 32-bit ELF Support.
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 combining 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=filenameRetain only the symbols listed in the file filename, discarding all others. filename is simply a flat file, with one symbol name per line. This option is especially useful in environments (such as VxWorks) where a large global symbol table is accumulated gradually, to conserve run-time memory.
‘--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=dirAdd a directory to the runtime library search path. This is used when
linking an ELF executable with shared objects. All -rpath
arguments are concatenated and passed to the runtime linker, which uses
them to locate shared objects at runtime. The -rpath option is
also used when locating shared objects which are needed by shared
objects explicitly included in the link; see the description of the
-rpath-link option. If -rpath is not used when linking an
ELF executable, the contents of the environment variable
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 path 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=dirWhen using ELF or SunOS, one shared library may require another. This
happens when an 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.
The tokens $ORIGIN and $LIB can appear in these search directories. They will be replaced by the full path to the directory containing the program or shared object in the case of $ORIGIN and either ‘lib’ - for 32-bit binaries - or ‘lib64’ - for 64-bit binaries - in the case of $LIB.
The alternative form of these tokens - ${ORIGIN} and ${LIB} can also be used. The token $PLATFORM is not supported.
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-BshareableCreate a shared library. This is currently only supported on ELF, XCOFF and SunOS platforms. On SunOS, the linker will automatically create a shared library if the -e option is not used and there are undefined symbols in the link.
--sort-common--sort-common=ascending--sort-common=descendingThis option tells 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=nameThis option will apply SORT_BY_NAME to all wildcard section
patterns in the linker script.
--sort-section=alignmentThis option will apply SORT_BY_ALIGNMENT to all wildcard section
patterns in the linker script.
--spare-dynamic-tags=countThis option specifies the number of empty slots to leave in the .dynamic section of ELF shared objects. Empty slots may be needed by post processing tools, such as the prelinker. The default is 5.
--split-by-file[=size]Similar to --split-by-reloc but creates a new output section for each input file when size is reached. size defaults to a size of 1 if not given.
--split-by-reloc[=count]Tries to creates extra sections in the output file so that no single output section in the file contains more than count relocations. This is useful when generating huge relocatable files for downloading into certain real time kernels with the COFF object file format; since COFF cannot represent more than 65535 relocations in a single section. Note that this will fail to work with object file formats which do not support arbitrary sections. The linker will not split up individual input sections for redistribution, so if a single input section contains more than count relocations one output section will contain that many relocations. count defaults to a value of 32768.
--statsCompute and display statistics about the operation of the linker, such as execution time and memory usage.
--sysroot=directoryUse directory as the location of the sysroot, overriding the configure-time default. This option is only supported by linkers that were configured using --with-sysroot.
--task-linkThis is used by COFF/PE based targets to create a task-linked object file where all of the global symbols have been converted to statics.
--traditional-formatFor some targets, the output of 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=orgLocate a section in the output file at the absolute address given by org. You may use this option as many times as necessary to locate multiple sections in the command line. org must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading ‘0x’ usually associated with hexadecimal values. Note: there should be no white space between sectionname, the equals sign (“=”), and org.
-Tbss=org-Tdata=org-Ttext=orgSame as --section-start, with .bss, .data or
.text as the sectionname.
-Ttext-segment=orgWhen creating an ELF executable, it will set the address of the first byte of the text segment.
-Trodata-segment=orgWhen creating an ELF executable or shared object for a target where the read-only data is in its own segment separate from the executable text, it will set the address of the first byte of the read-only data segment.
-Tldata-segment=orgWhen creating an ELF executable or shared object for x86-64 medium memory model, it will set the address of the first byte of the ldata segment.
--unresolved-symbols=methodDetermine how to handle unresolved symbols. There are four possible values for ‘method’:
Do not report any unresolved symbols.
Report all unresolved symbols. This is the default.
Report unresolved symbols that are contained in shared libraries, but ignore them if they come from regular object files.
Report unresolved symbols that come from regular object files, but ignore them if they come from shared libraries. This can be useful when creating a dynamic binary and it is known that all the shared libraries that it should be referencing are included on the linker’s command line.
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]Display the version number for 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-scriptfileSpecify the name of a version script to the linker. This is typically used when creating shared libraries to specify additional information about the version hierarchy for the library being created. This option is only fully supported on ELF platforms which support shared libraries; see VERSION. It is partially supported on PE platforms, which can use version scripts to filter symbol visibility in auto-export mode: any symbols marked ‘local’ in the version script will not be exported. See WIN32.
--warn-commonWarn when a common symbol is combined with another common symbol or with a symbol definition. Unix linkers allow this somewhat sloppy practice, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C examples:
A definition, which goes in the initialized data section of the output file.
An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere.
A common symbol. If there are only (one or more) common symbols for a variable, it goes in the uninitialized data area of the output file. The linker merges multiple common symbols for the same variable into a single symbol. If they are of different sizes, it picks the largest size. The linker turns a common symbol into a declaration, if there is a definition of the same variable.
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-constructorsWarn if any global constructors are used. This is only useful for a few object file formats. For formats like COFF or ELF, the linker can not detect the use of global constructors.
--warn-multiple-gpWarn if multiple global pointer values are required in the output file. This is only meaningful for certain processors, such as the Alpha. Specifically, some processors put large-valued constants in a special section. A special register (the global pointer) points into the middle of this section, so that constants can be loaded efficiently via a base-register relative addressing mode. Since the offset in base-register relative mode is fixed and relatively small (e.g., 16 bits), this limits the maximum size of the constant pool. Thus, in large programs, it is often necessary to use multiple global pointer values in order to be able to address all possible constants. This option causes a warning to be issued whenever this case occurs.
--warn-onceOnly warn once for each undefined symbol, rather than once per module which refers to it.
--warn-section-alignWarn if the address of an output section is changed because of
alignment. Typically, the alignment will be set by an input section.
The address will only be changed if it not explicitly specified; that
is, if the SECTIONS command does not specify a start address for
the section (see SECTIONS).
--warn-shared-textrelWarn if the linker adds a DT_TEXTREL to a shared object.
--warn-alternate-emWarn if an object has alternate ELF machine code.
--warn-unresolved-symbolsIf the linker is going to report an unresolved symbol (see the option --unresolved-symbols) it will normally generate an error. This option makes it generate a warning instead.
--error-unresolved-symbolsThis restores the linker’s default behaviour of generating errors when it is reporting unresolved symbols.
--whole-archiveFor each archive mentioned on the command line after the --whole-archive option, include every object file in the archive in the link, rather than searching the archive for the required object files. This is normally used to turn an archive file into a shared library, forcing every object to be included in the resulting shared library. This option may be used more than once.
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=symbolUse a wrapper function for symbol. Any undefined reference to
symbol will be resolved to __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--no-eh-frame-hdrRequest (--eh-frame-hdr) or suppress
(--no-eh-frame-hdr) the creation of .eh_frame_hdr
section and ELF PT_GNU_EH_FRAME segment header.
--no-ld-generated-unwind-infoRequest creation of .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-dtagsThis linker can create the new dynamic tags in ELF. But the older ELF systems may not understand them. If you specify --enable-new-dtags, the new dynamic tags will be created as needed and older dynamic tags will be omitted. If you specify --disable-new-dtags, no new dynamic tags will be created. By default, the new dynamic tags are not created. Note that those options are only available for ELF systems.
--hash-size=numberSet the default size of the linker’s hash tables to a prime number close to number. Increasing this value can reduce the length of time it takes the linker to perform its tasks, at the expense of increasing the linker’s memory requirements. Similarly reducing this value can reduce the memory requirements at the expense of speed.
--hash-style=styleSet the type of linker’s hash table(s). style can be either
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.
--compress-debug-sections=none--compress-debug-sections=zlib--compress-debug-sections=zlib-gnu--compress-debug-sections=zlib-gabiOn ELF platforms, these options control how DWARF debug sections are compressed using zlib.
--compress-debug-sections=none doesn’t compress DWARF debug sections. --compress-debug-sections=zlib-gnu compresses DWARF debug sections and renames them to begin with ‘.zdebug’ instead of ‘.debug’. --compress-debug-sections=zlib-gabi also compresses DWARF debug sections, but rather than renaming them it sets the SHF_COMPRESSED flag in the sections’ headers.
The --compress-debug-sections=zlib option is an alias for --compress-debug-sections=zlib-gabi.
Note that this option overrides any compression in input debug sections, so if a binary is linked with --compress-debug-sections=none for example, then any compressed debug sections in input files will be uncompressed before they are copied into the output binary.
The default compression behaviour varies depending upon the target involved and the configure options used to build the toolchain. The default can be determined by examining the output from the linker’s --help option.
--reduce-memory-overheadsThis option reduces memory requirements at ld runtime, at the expense of linking speed. This was introduced to select the old O(n^2) algorithm for link map file generation, rather than the new O(n) algorithm which uses about 40% more memory for symbol storage.
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=styleRequest the creation of a .note.gnu.build-id ELF note section
or a .buildid COFF 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.
The i386 PE linker supports the -shared option, which causes
the output to be a dynamically linked library (DLL) instead of a
normal executable. You should name the output *.dll when you
use this option. In addition, the linker fully supports the standard
*.def files, which may be specified on the linker command line
like an object file (in fact, it should precede archives it exports
symbols from, to ensure that they get linked in, just like a normal
object file).
In addition to the options common to all targets, the i386 PE linker support additional command line options that are specific to the i386 PE target. Options that take values may be separated from their values by either a space or an equals sign.
--add-stdcall-aliasIf given, symbols with a stdcall suffix (@nn) will be exported as-is and also with the suffix stripped. [This option is specific to the i386 PE targeted port of the linker]
--base-file fileUse file as the name of a file in which to save the base addresses of all the relocations needed for generating DLLs with dlltool. [This is an i386 PE specific option]
--dllCreate a DLL instead of a regular executable. You may also use
-shared or specify a LIBRARY in a given .def
file.
[This option is specific to the i386 PE targeted port of the linker]
--enable-long-section-names--disable-long-section-namesThe PE variants of the COFF object format add an extension that permits
the use of section names longer than eight characters, the normal limit
for COFF. By default, these names are only allowed in object files, as
fully-linked executable images do not carry the COFF string table required
to support the longer names. As a GNU extension, it is possible to
allow their use in executable images as well, or to (probably pointlessly!)
disallow it in object files, by using these two options. Executable images
generated with these long section names are slightly non-standard, carrying
as they do a string table, and may generate confusing output when examined
with non-GNU PE-aware tools, such as file viewers and dumpers. However,
GDB relies on the use of PE long section names to find Dwarf-2 debug
information sections in an executable image at runtime, and so if neither
option is specified on the command-line, ld will enable long
section names, overriding the default and technically correct behaviour,
when it finds the presence of debug information while linking an executable
image and not stripping symbols.
[This option is valid for all PE targeted ports of the linker]
--enable-stdcall-fixup--disable-stdcall-fixupIf the link finds a symbol that it cannot resolve, it will attempt to
do “fuzzy linking” by looking for another defined symbol that differs
only in the format of the symbol name (cdecl vs stdcall) and will
resolve that symbol by linking to the match. For example, the
undefined symbol _foo might be linked to the function
_foo@12, or the undefined symbol _bar@16 might be linked
to the function _bar. When the linker does this, it prints a
warning, since it normally should have failed to link, but sometimes
import libraries generated from third-party dlls may need this feature
to be usable. If you specify --enable-stdcall-fixup, this
feature is fully enabled and warnings are not printed. If you specify
--disable-stdcall-fixup, this feature is disabled and such
mismatches are considered to be errors.
[This option is specific to the i386 PE targeted port of the linker]
--leading-underscore--no-leading-underscoreFor most targets default symbol-prefix is an underscore and is defined in target’s description. By this option it is possible to disable/enable the default underscore symbol-prefix.
--export-all-symbolsIf given, all global symbols in the objects used to build a DLL will
be exported by the DLL. Note that this is the default if there
otherwise wouldn’t be any exported symbols. When symbols are
explicitly exported via DEF files or implicitly exported via function
attributes, the default is to not export anything else unless this
option is given. Note that the symbols DllMain@12,
DllEntryPoint@0, DllMainCRTStartup@12, and
impure_ptr will not be automatically
exported. Also, symbols imported from other DLLs will not be
re-exported, nor will symbols specifying the DLL’s internal layout
such as those beginning with _head_ or ending with
_iname. In addition, no symbols from libgcc,
libstd++, libmingw32, or crtX.o will be exported.
Symbols whose names begin with __rtti_ or __builtin_ will
not be exported, to help with C++ DLLs. Finally, there is an
extensive list of cygwin-private symbols that are not exported
(obviously, this applies on when building DLLs for cygwin targets).
These cygwin-excludes are: _cygwin_dll_entry@12,
_cygwin_crt0_common@8, _cygwin_noncygwin_dll_entry@12,
_fmode, _impure_ptr, cygwin_attach_dll,
cygwin_premain0, cygwin_premain1, cygwin_premain2,
cygwin_premain3, and environ.
[This option is specific to the i386 PE targeted port of the linker]
--exclude-symbols symbol,symbol,...Specifies a list of symbols which should not be automatically exported. The symbol names may be delimited by commas or colons. [This option is specific to the i386 PE targeted port of the linker]
--exclude-all-symbolsSpecifies no symbols should be automatically exported. [This option is specific to the i386 PE targeted port of the linker]
--file-alignmentSpecify the file alignment. Sections in the file will always begin at file offsets which are multiples of this number. This defaults to 512. [This option is specific to the i386 PE targeted port of the linker]
--heap reserve--heap reserve,commitSpecify the number of bytes of memory to reserve (and optionally commit) to be used as heap for this program. The default is 1MB reserved, 4K committed. [This option is specific to the i386 PE targeted port of the linker]
--image-base valueUse value as the base address of your program or dll. This is the lowest memory location that will be used when your program or dll is loaded. To reduce the need to relocate and improve performance of your dlls, each should have a unique base address and not overlap any other dlls. The default is 0x400000 for executables, and 0x10000000 for dlls. [This option is specific to the i386 PE targeted port of the linker]
--kill-atIf given, the stdcall suffixes (@nn) will be stripped from symbols before they are exported. [This option is specific to the i386 PE targeted port of the linker]
--large-address-awareIf given, the appropriate bit in the “Characteristics” field of the COFF header is set to indicate that this executable supports virtual addresses greater than 2 gigabytes. This should be used in conjunction with the /3GB or /USERVA=value megabytes switch in the “[operating systems]” section of the BOOT.INI. Otherwise, this bit has no effect. [This option is specific to PE targeted ports of the linker]
--disable-large-address-awareReverts the effect of a previous ‘--large-address-aware’ option. This is useful if ‘--large-address-aware’ is always set by the compiler driver (e.g. Cygwin gcc) and the executable does not support virtual addresses greater than 2 gigabytes. [This option is specific to PE targeted ports of the linker]
--major-image-version valueSets the major number of the “image version”. Defaults to 1. [This option is specific to the i386 PE targeted port of the linker]
--major-os-version valueSets the major number of the “os version”. Defaults to 4. [This option is specific to the i386 PE targeted port of the linker]
--major-subsystem-version valueSets the major number of the “subsystem version”. Defaults to 4. [This option is specific to the i386 PE targeted port of the linker]
--minor-image-version valueSets the minor number of the “image version”. Defaults to 0. [This option is specific to the i386 PE targeted port of the linker]
--minor-os-version valueSets the minor number of the “os version”. Defaults to 0. [This option is specific to the i386 PE targeted port of the linker]
--minor-subsystem-version valueSets the minor number of the “subsystem version”. Defaults to 0. [This option is specific to the i386 PE targeted port of the linker]
--output-def fileThe linker will create the file file which will contain a DEF
file corresponding to the DLL the linker is generating. This DEF file
(which should be called *.def) may be used to create an import
library with dlltool or may be used as a reference to
automatically or implicitly exported symbols.
[This option is specific to the i386 PE targeted port of the linker]
--enable-auto-image-base--enable-auto-image-base=valueAutomatically choose the image base for DLLs, optionally starting with base
value, unless one is specified using the --image-base argument.
By using a hash generated from the dllname to create unique image bases
for each DLL, in-memory collisions and relocations which can delay program
execution are avoided.
[This option is specific to the i386 PE targeted port of the linker]
--disable-auto-image-baseDo not automatically generate a unique image base. If there is no
user-specified image base (--image-base) then use the platform
default.
[This option is specific to the i386 PE targeted port of the linker]
--dll-search-prefix stringWhen linking dynamically to a dll without an import library,
search for <string><basename>.dll in preference to
lib<basename>.dll. This behaviour allows easy distinction
between DLLs built for the various "subplatforms": native, cygwin,
uwin, pw, etc. For instance, cygwin DLLs typically use
--dll-search-prefix=cyg.
[This option is specific to the i386 PE targeted port of the linker]
--enable-auto-importDo sophisticated linking of _symbol to __imp__symbol for
DATA imports from DLLs, and create the necessary thunking symbols when
building the import libraries with those DATA exports. Note: Use of the
’auto-import’ extension will cause the text section of the image file
to be made writable. This does not conform to the PE-COFF format
specification published by Microsoft.
Note - use of the ’auto-import’ extension will also cause read only data which would normally be placed into the .rdata section to be placed into the .data section instead. This is in order to work around a problem with consts that is described here: http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
Using ’auto-import’ generally will ’just work’ – but sometimes you may see this message:
"variable ’<var>’ can’t be auto-imported. Please read the
documentation for ld’s --enable-auto-import for details."
This message occurs when some (sub)expression accesses an address ultimately given by the sum of two constants (Win32 import tables only allow one). Instances where this may occur include accesses to member fields of struct variables imported from a DLL, as well as using a constant index into an array variable imported from a DLL. Any multiword variable (arrays, structs, long long, etc) may trigger this error condition. However, regardless of the exact data type of the offending exported variable, ld will always detect it, issue the warning, and exit.
There are several ways to address this difficulty, regardless of the data type of the exported variable:
One way is to use –enable-runtime-pseudo-reloc switch. This leaves the task of adjusting references in your client code for runtime environment, so this method works only when runtime environment supports this feature.
A second solution is to force one of the ’constants’ to be a variable – that is, unknown and un-optimizable at compile time. For arrays, there are two possibilities: a) make the indexee (the array’s address) a variable, or b) make the ’constant’ index a variable. Thus:
extern type extern_array[];
extern_array[1] -->
{ volatile type *t=extern_array; t[1] }
or
extern type extern_array[];
extern_array[1] -->
{ volatile int t=1; extern_array[t] }
For structs (and most other multiword data types) the only option is to make the struct itself (or the long long, or the ...) variable:
extern struct s extern_struct;
extern_struct.field -->
{ volatile struct s *t=&extern_struct; t->field }
or
extern long long extern_ll;
extern_ll -->
{ volatile long long * local_ll=&extern_ll; *local_ll }
A third method of dealing with this difficulty is to abandon
’auto-import’ for the offending symbol and mark it with
__declspec(dllimport). However, in practice that
requires using compile-time #defines to indicate whether you are
building a DLL, building client code that will link to the DLL, or
merely building/linking to a static library. In making the choice
between the various methods of resolving the ’direct address with
constant offset’ problem, you should consider typical real-world usage:
Original:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
Solution 1:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
/* This workaround is for win32 and cygwin; do not "optimize" */
volatile int *parr = arr;
printf("%d\n",parr[1]);
}
Solution 2:
--foo.h
/* Note: auto-export is assumed (no __declspec(dllexport)) */
#if (defined(_WIN32) || defined(__CYGWIN__)) && \
!(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
#define FOO_IMPORT __declspec(dllimport)
#else
#define FOO_IMPORT
#endif
extern FOO_IMPORT int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
A fourth way to avoid this problem is to re-code your library to use a functional interface rather than a data interface for the offending variables (e.g. set_foo() and get_foo() accessor functions). [This option is specific to the i386 PE targeted port of the linker]
--disable-auto-importDo not attempt to do sophisticated linking of _symbol to
__imp__symbol for DATA imports from DLLs.
[This option is specific to the i386 PE targeted port of the linker]
--enable-runtime-pseudo-relocIf your code contains expressions described in –enable-auto-import section, that is, DATA imports from DLL with non-zero offset, this switch will create a vector of ’runtime pseudo relocations’ which can be used by runtime environment to adjust references to such data in your client code. [This option is specific to the i386 PE targeted port of the linker]
--disable-runtime-pseudo-relocDo not create pseudo relocations for non-zero offset DATA imports from DLLs. [This option is specific to the i386 PE targeted port of the linker]
--enable-extra-pe-debugShow additional debug info related to auto-import symbol thunking. [This option is specific to the i386 PE targeted port of the linker]
--section-alignmentSets the section alignment. Sections in memory will always begin at addresses which are a multiple of this number. Defaults to 0x1000. [This option is specific to the i386 PE targeted port of the linker]
--stack reserve--stack reserve,commitSpecify the number of bytes of memory to reserve (and optionally commit) to be used as stack for this program. The default is 2MB reserved, 4K committed. [This option is specific to the i386 PE targeted port of the linker]
--subsystem which--subsystem which:major--subsystem which:major.minorSpecifies the subsystem under which your program will execute. The
legal values for which are native, windows,
console, posix, and xbox. You may optionally set
the subsystem version also. Numeric values are also accepted for
which.
[This option is specific to the i386 PE targeted port of the linker]
The following options set flags in the DllCharacteristics field
of the PE file header:
[These options are specific to PE targeted ports of the linker]
--high-entropy-vaImage is compatible with 64-bit address space layout randomization (ASLR).
--dynamicbaseThe image base address may be relocated using address space layout randomization (ASLR). This feature was introduced with MS Windows Vista for i386 PE targets.
--forceintegCode integrity checks are enforced.
--nxcompatThe image is compatible with the Data Execution Prevention. This feature was introduced with MS Windows XP SP2 for i386 PE targets.
--no-isolationAlthough the image understands isolation, do not isolate the image.
--no-sehThe image does not use SEH. No SE handler may be called from this image.
--no-bindDo not bind this image.
--wdmdriverThe driver uses the MS Windows Driver Model.
--tsawareThe image is Terminal Server aware.
--insert-timestamp--no-insert-timestampInsert a real timestamp into the image. This is the default behaviour as it matches legacy code and it means that the image will work with other, proprietary tools. The problem with this default is that it will result in slightly different images being produced each time the same sources are linked. The option --no-insert-timestamp can be used to insert a zero value for the timestamp, this ensuring that binaries produced from identical sources will compare identically.
The C6X uClinux target uses a binary format called DSBT to support shared libraries. Each shared library in the system needs to have a unique index; all executables use an index of 0.
--dsbt-size sizeThis option sets the number of entries in the DSBT of the current executable or shared library to size. The default is to create a table with 64 entries.
--dsbt-index indexThis option sets the DSBT index of the current executable or shared library
to index. The default is 0, which is appropriate for generating
executables. If a shared library is generated with a DSBT index of 0, the
R_C6000_DSBT_INDEX relocs are copied into the output file.
The ‘--no-merge-exidx-entries’ switch disables the merging of adjacent exidx entries in frame unwind info.
The 68HC11 and 68HC12 linkers support specific options to control the memory bank switching mapping and trampoline code generation.
--no-trampolineThis option disables the generation of trampoline. By default a trampoline
is generated for each far function which is called using a jsr
instruction (this happens when a pointer to a far function is taken).
--bank-window nameThis option indicates to the linker the name of the memory region in the ‘MEMORY’ specification that describes the memory bank window. The definition of such region is then used by the linker to compute paging and addresses within the memory window.
The following options are supported to control handling of GOT generation when linking for 68K targets.
--got=typeThis option tells the linker which GOT generation scheme to use. type should be one of ‘single’, ‘negative’, ‘multigot’ or ‘target’. For more information refer to the Info entry for ld.
The following options are supported to control microMIPS instruction generation and branch relocation checks for ISA mode transitions when linking for MIPS targets.
--insn32--no-insn32These options control the choice of microMIPS instructions used in code generated by the linker, such as that in the PLT or lazy binding stubs, or in relaxation. If ‘--insn32’ is used, then the linker only uses 32-bit instruction encodings. By default or if ‘--no-insn32’ is used, all instruction encodings are used, including 16-bit ones where possible.
--ignore-branch-isa--no-ignore-branch-isaThese options control branch relocation checks for invalid ISA mode
transitions. If ‘--ignore-branch-isa’ is used, then the linker
accepts any branch relocations and any ISA mode transition required
is lost in relocation calculation, except for some cases of BAL
instructions which meet relaxation conditions and are converted to
equivalent JALX instructions as the associated relocation is
calculated. By default or if ‘--no-ignore-branch-isa’ is used
a check is made causing the loss of an ISA mode transition to produce
an error.
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