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:
@fileOptions in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.
-a keyword--audit AUDITLIBDT_AUDIT entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the DT_SONAME
specified in the library. If specified multiple times DT_AUDIT
will contain a colon separated list of audit interfaces to use. If the linker
finds an object with an audit entry while searching for shared libraries,
it will add a corresponding DT_DEPAUDIT entry in the output file.
This option is only meaningful on ELF platforms supporting the rtld-audit
interface.
-b input-format--format=input-formatYou 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-commandfile-d-dc-dpFORCE_COMMON_ALLOCATION has the same effect.
See Miscellaneous Commands.
--depaudit AUDITLIB-P AUDITLIBDT_DEPAUDIT entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the DT_SONAME
specified in the library. If specified multiple times DT_DEPAUDIT
will contain a colon separated list of audit interfaces to use. This
option is only meaningful on ELF platforms supporting the rtld-audit interface.
The -P option is provided for Solaris compatibility.
-e entry--entry=entry--exclude-libs lib,lib,...--exclude-libs ALL excludes symbols in all archive libraries from
automatic export. This option is available only for the i386 PE targeted
port of the linker and for ELF targeted ports. For i386 PE, symbols
explicitly listed in a .def file are still exported, regardless of this
option. For ELF targeted ports, symbols affected by this option will
be treated as hidden.
--exclude-modules-for-implib module,module,...-E--export-dynamic--no-export-dynamicIf you do not use either of these options (or use the --no-export-dynamic option to restore the default behavior), the dynamic symbol table will normally contain only those symbols which are referenced by some dynamic object mentioned in the link.
If you use dlopen to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.
You can also use the dynamic list to control what symbols should be added to the dynamic symbol table if the output format supports it. See the description of ‘--dynamic-list’.
Note that this option is specific to ELF targeted ports. PE targets support a similar function to export all symbols from a DLL or EXE; see the description of ‘--export-all-symbols’ below.
-EB-EL-f name--auxiliary=nameIf 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=nameIf you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_FILTER field. The dynamic linker will resolve symbols according to the symbol table of the filter object as usual, but it will actually link to the definitions found in the shared object name. Thus the filter object can be used to select a subset of the symbols provided by the object name.
Some older linkers used the -F option throughout a compilation
toolchain for specifying object-file format for both input and output
object files.
The gnu linker uses other mechanisms for this purpose: the
-b, --format, --oformat options, the
TARGET command in linker scripts, and the GNUTARGET
environment variable.
The gnu linker will ignore the -F option when not
creating an ELF shared object.
-fini=name_fini as
the function to call.
-g-G value--gpsize=value-h name-soname=name-i-init=name_init as the
function to call.
-l namespec--library=namespecOn 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=searchdirIf 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 emulationIf 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-mapNote - 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.
When linker merges input .note.gnu.property sections into one output .note.gnu.property section, some properties are removed or updated, which are reported in the link map as
Removed property 0xc0000002 to merge foo.o (0x1) and bar.o (not found)
It indicates that property 0xc0000002 is removed from output when merging properties in foo.o, whose property 0xc0000002 value is 0x1, and bar.o, which doesn't have property 0xc0000002.
Updated property 0xc0000002 (0x1) to merge foo.o (0x1) and bar.o (0x1)
It indicates that property 0xc0010001 value is updated to 0x1 in output when merging properties in foo.o, whose 0xc0010001 property value is 0x1, and bar.o, whose 0xc0010001 property value is 0x1.
-n--nmagicNMAGIC.
-N--omagicOMAGIC. Note: Although a writable text section
is allowed for PE-COFF targets, it does not conform to the format
specification published by Microsoft.
--no-omagic-o output--output=outputOUTPUT can also specify the output file name.
-O level-plugin nameNote 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 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-state-q--emit-relocsThis option is currently only supported on ELF platforms.
--force-dynamic-r--relocatableOMAGIC.
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=filenameFor compatibility with other ELF linkers, if the -R option is followed by a directory name, rather than a file name, it is treated as the -rpath option.
-s--strip-all-S--strip-debug--strip-discarded--no-strip-discarded-t--trace-T scriptfile--script=scriptfileld looks for it in the directories
specified by any preceding ‘-L’ options. Multiple ‘-T’
options accumulate.
-dT scriptfile--default-script=scriptfileThis 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=symbolEXTERN 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=symbolEXTERN, ASSERT and DEFINED together. This option
can be used multiple times to require additional symbols.
-Ur--orphan-handling=MODEMODE can have any of the following values:
placediscardwarnplace and also
issue a warning.
errorThe default if ‘--orphan-handling’ is not given is place.
--unique[=SECTION]-v--version-V-x--discard-all-X--discard-locals-y symbol--trace-symbol=symbolThis option is useful when you have an undefined symbol in your link but don't know where the reference is coming from.
-Y path-z keywordNOP 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.
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.
dlopen.
dldump.
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’.
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.
PT_GNU_STACK segment.
Specifying zero will override any default non-zero sized
PT_GNU_STACK segment creation.
Other keywords are ignored for Solaris compatibility.
-( archives -)--start-group archives --end-groupThe 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 will all be searched repeatedly until all possible references are resolved.
Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives.
--accept-unknown-input-arch--no-accept-unknown-input-arch--as-needed--no-as-needed--add-needed--no-add-needed-assert keyword-Bdynamic-dy-call_shared-BgroupDF_1_GROUP flag in the DT_FLAGS_1 entry in the dynamic
section. This causes the runtime linker to handle lookups in this
object and its dependencies to be performed only inside the group.
--unresolved-symbols=report-all is implied. This option is
only meaningful on ELF platforms which support shared libraries.
-Bstatic-dn-non_shared-static-Bsymbolic-Bsymbolic-functions--dynamic-list=dynamic-list-fileThe format of the dynamic list is the same as the version node without scope and node name. See VERSION for more information.
--dynamic-list-data--dynamic-list-cpp-new--dynamic-list-cpp-typeinfo--check-sections--no-check-sections--copy-dt-needed-entries--no-copy-dt-needed-entriesThis 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.
--crefThe 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-commonINHIBIT_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-allocationFORCE_GROUP_ALLOCATION has the same
effect. See Miscellaneous Commands.
--defsym=symbol=expression+ and - to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (see Assignments).
Note: there should be no white space between symbol, the
equals sign (“<=>”), and expression.
--demangle[=style]--no-demangle-Ifile--dynamic-linker=file--no-dynamic-linker--embedded-relocs--disable-multiple-abs-defs--fatal-warnings--no-fatal-warnings--force-exe-suffixIf a successfully built fully linked output file does not have a
.exe or .dll suffix, this option forces the linker to copy
the output file to one of the same name with a .exe suffix. This
option is useful when using unmodified Unix makefiles on a Microsoft
Windows host, since some versions of Windows won't run an image unless
it ends in a .exe suffix.
--gc-sections--no-gc-sections‘--gc-sections’ decides which input sections are used by examining symbols and relocations. The section containing the entry symbol and all sections containing symbols undefined on the command-line will be kept, as will sections containing symbols referenced by dynamic objects. Note that when building shared libraries, the linker must assume that any visible symbol is referenced. Once this initial set of sections has been determined, the linker recursively marks as used any section referenced by their relocations. See ‘--entry’, ‘--undefined’, and ‘--gc-keep-exported’.
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 one of the options ‘--entry’,
‘--undefined’, or ‘--gc-keep-exported’ or by a ENTRY
command in the linker script.
--print-gc-sections--no-print-gc-sections--gc-keep-exported--print-output-formatOUTPUT_FORMAT linker script command (see File Commands).
--print-memory-usage Memory region Used Size Region Size %age Used
ROM: 256 KB 1 MB 25.00%
RAM: 32 B 2 GB 0.00%
--help--target-help-Map=mapfile--no-keep-memory--no-undefined-z defsThe effects of this option can be reverted by using -z undefs.
--allow-multiple-definition-z muldefs--allow-shlib-undefined--no-allow-shlib-undefinedThe default behaviour is to report errors for any undefined symbols referenced in shared libraries if the linker is being used to create an executable, but to allow them if the linker is being used to create a shared library.
The reasons for allowing undefined symbol references in shared libraries specified at link time are that:
The BeOS kernel for example patches shared libraries at load time to select whichever function is most appropriate for the current architecture. This is used, for example, to dynamically select an appropriate memset function.
--no-undefined-version--default-symver--default-imported-symver--no-warn-mismatch--no-warn-search-mismatch--no-whole-archive--noinhibit-exec-nostdlib--oformat=output-formatOUTPUT_FORMAT can also specify the output format, but
this option overrides it. See BFD.
--out-implib file*.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-executable-qmagic-Qy--relax--no-relaxOn 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=filename‘--retain-symbols-file’ does not discard undefined symbols, or symbols needed for relocations.
You may only specify ‘--retain-symbols-file’ once in the command
line. It overrides ‘-s’ and ‘-S’.
-rpath=dirLD_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=dirld -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-Bshareable--sort-common--sort-common=ascending--sort-common=descending--sort-section=nameSORT_BY_NAME to all wildcard section
patterns in the linker script.
--sort-section=alignmentSORT_BY_ALIGNMENT to all wildcard section
patterns in the linker script.
--spare-dynamic-tags=count--split-by-file[=size]--split-by-reloc[=count]--stats--sysroot=directory--task-link--traditional-formatFor example, on SunOS, ld combines duplicate entries in the
symbol string table. This can reduce the size of an output file with
full debugging information by over 30 percent. Unfortunately, the SunOS
dbx program can not read the resulting program (gdb has no
trouble). The ‘--traditional-format’ switch tells ld to not
combine duplicate entries.
--section-start=sectionname=org-Tbss=org-Tdata=org-Ttext=org.bss, .data or
.text as the sectionname.
-Ttext-segment=org-Trodata-segment=org-Tldata-segment=org--unresolved-symbols=methodThe 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]--version-script=version-scriptfile--warn-commonThere are three kinds of global symbols, illustrated here by C examples:
The ‘--warn-common’ option can produce five kinds of warnings. Each warning consists of a pair of lines: the first describes the symbol just encountered, and the second describes the previous symbol encountered with the same name. One or both of the two symbols will be a common symbol.
file(section): warning: common of `symbol'
overridden by definition
file(section): warning: defined here
file(section): warning: definition of `symbol'
overriding common
file(section): warning: common is here
file(section): warning: multiple common
of `symbol'
file(section): warning: previous common is here
file(section): warning: common of `symbol'
overridden by larger common
file(section): warning: larger common is here
file(section): warning: common of `symbol'
overriding smaller common
file(section): warning: smaller common is here
--warn-constructors--warn-multiple-gp--warn-once--warn-section-alignSECTIONS command does not specify a start address for
the section (see SECTIONS).
--warn-shared-textrel--warn-alternate-em--warn-unresolved-symbols--error-unresolved-symbols--whole-archiveTwo notes when using this option from gcc: First, gcc doesn't know about this option, so you have to use -Wl,-whole-archive. Second, don't forget to use -Wl,-no-whole-archive after your list of archives, because gcc will add its own list of archives to your link and you may not want this flag to affect those as well.
--wrap=symbol__wrap_symbol. Any
undefined reference to __real_symbol will be resolved to
symbol.
This can be used to provide a wrapper for a system function. The
wrapper function should be called __wrap_symbol. If it
wishes to call the system function, it should call
__real_symbol.
Here is a trivial example:
void *
__wrap_malloc (size_t c)
{
printf ("malloc called with %zu\n", c);
return __real_malloc (c);
}
If you link other code with this file using --wrap malloc, then
all calls to malloc will call the function __wrap_malloc
instead. The call to __real_malloc in __wrap_malloc will
call the real malloc function.
You may wish to provide a __real_malloc function as well, so that
links without the --wrap option will succeed. If you do this,
you should not put the definition of __real_malloc in the same
file as __wrap_malloc; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to malloc.
Only undefined references are replaced by the linker. So, translation unit
internal references to symbol are not resolved to
__wrap_symbol. In the next example, the call to f in
g is not resolved to __wrap_f.
int
f (void)
{
return 123;
}
int
g (void)
{
return f();
}
Please keep in mind that with link-time optimization (LTO) enabled, your whole program may be a translation unit.
--eh-frame-hdr--no-eh-frame-hdr.eh_frame_hdr
section and ELF PT_GNU_EH_FRAME segment header.
--no-ld-generated-unwind-info.eh_frame unwind info for linker
generated code sections like PLT. This option is on by default
if linker generated unwind info is supported.
--enable-new-dtags--disable-new-dtags--hash-size=number--hash-style=stylesysv 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 depends upon how the linker was configured,
but for most Linux based systems it will be both.
--compress-debug-sections=none--compress-debug-sections=zlib--compress-debug-sections=zlib-gnu--compress-debug-sections=zlib-gabi--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-overheadsAnother effect of the switch is to set the default hash table size to 1021, which again saves memory at the cost of lengthening the linker's run time. This is not done however if the --hash-size switch has been used.
The --reduce-memory-overheads switch may be also be used to enable other tradeoffs in future versions of the linker.
--build-id--build-id=style.note.gnu.build-id ELF note section
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-alias--base-file file--dllLIBRARY 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-names--enable-stdcall-fixup--disable-stdcall-fixup_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-underscore--export-all-symbolsDllMain@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,...--exclude-all-symbols--file-alignment--heap reserve--heap reserve,commit--image-base value--kill-at--large-address-aware--disable-large-address-aware--major-image-version value--major-os-version value--major-subsystem-version value--minor-image-version value--minor-os-version value--minor-subsystem-version value--output-def file*.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=value--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-base--image-base) then use the platform
default.
[This option is specific to the i386 PE targeted port of the linker]
--dll-search-prefix string<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-import_symbol to __imp__symbol for
DATA imports from DLLs, thus making it possible to bypass the dllimport
mechanism on the user side and to reference unmangled symbol names.
[This option is specific to the i386 PE targeted port of the linker]
The following remarks pertain to the original implementation of the feature and are obsolete nowadays for Cygwin and MinGW targets.
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).
--disable-auto-import_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-reloc--disable-runtime-pseudo-reloc--enable-extra-pe-debug--section-alignment--stack reserve--stack reserve,commit--subsystem which--subsystem which:major--subsystem which:major.minornative, 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-va--dynamicbase--forceinteg--nxcompat--no-isolation--no-seh--no-bind--wdmdriver--tsaware--insert-timestamp--no-insert-timestampThe 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 size--dsbt-index indexR_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.
--branch-stub--stub-group-size=NThe 68HC11 and 68HC12 linkers support specific options to control the memory bank switching mapping and trampoline code generation.
--no-trampolinejsr
instruction (this happens when a pointer to a far function is taken).
--bank-window nameThe following options are supported to control handling of GOT generation when linking for 68K targets.
--got=typeThe following options are supported to control microMIPS instruction generation and branch relocation checks for ISA mode transitions when linking for MIPS targets.
--insn32--no-insn32--ignore-branch-isa--no-ignore-branch-isaBAL
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.