ld - Using LD, the GNU linker
ld [options] objfile ...
ld combines a number of object and archive files, relocates their data
and ties up symbol references. Usually the last step in compiling a
program is to run ld.
ld accepts Linker Command Language files written in a superset of
AT&T's Link Editor Command Language syntax, to provide explicit and
total control over the linking process.
This man page does not describe the command language; see the ld entry
in "info", or the manual ld: the GNU linker, for full details on the
command language and on other aspects of the GNU linker.
This version of ld uses the general purpose BFD libraries to operate on
object files. This allows ld to read, combine, and write object files
in many different formats---for example, COFF or "a.out". Different
formats may be linked together to produce any available kind of object
file.
Aside from its flexibility, the GNU linker is more helpful than other
linkers in providing diagnostic information. Many linkers abandon execution
immediately upon encountering an error; whenever possible, ld
continues executing, allowing you to identify other errors (or, in some
cases, to get an output file in spite of the error).
The GNU linker ld is meant to cover a broad range of situations, and to
be as compatible as possible with other linkers. As a result, you have
many choices to control its behavior.
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 I<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 can not 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. Note that specifying a
script in this way merely augments the main linker script; use the -T
option to replace the default linker script entirely.
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 preceeded 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
(eg 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,--startgroup foo.o bar.o -Wl,--endgroup
This is important, because otherwise the compiler driver program may
silently drop the linker options, resulting in a bad link.
Here is a table of the generic command line switches accepted by the
GNU linker:
-akeyword
This 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.
-Aarchitecture
--architecture=architecture
In 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 archivelibrary
search path.
Future releases of ld may support similar functionality for other
architecture families.
-b input-format
--format=input-format
ld may be configured to support more than one kind of object file.
If your ld is configured this way, you can use the -b option to
specify the binary format for input object files that follow this
option on the command line. Even when ld is configured to support
alternative object formats, you don't usually need to specify this,
as ld should be configured to expect as a default input format the
most usual format on each machine. input-format is a text string,
the name of a particular format supported by the BFD libraries.
(You can list the available binary formats with objdump -i.)
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".
You can also define the input format from a script, using the command
"TARGET";
-c MRI-commandfile
--mri-script=MRI-commandfile
For compatibility with linkers produced by MRI, ld accepts script
files written in an alternate, restricted command language,
described in the MRI Compatible Script Files section of GNU ld documentation.
Introduce MRI script files with the option -c; use the
-T option to run linker scripts written in the general-purpose ld
scripting language. If MRI-cmdfile does not exist, ld looks for it
in the directories specified by any -L options.
-d
-dc
-dp These 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.
-e entry
--entry=entry
Use 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).
-E
--export-dynamic
When creating a dynamically linked executable, 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 this option, 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 version script to control what symbols should
be added to the dynamic symbol table if the output format supports
it. See the description of --version-script in @ref{VERSION}.
-EB Link big-endian objects. This affects the default output format.
-EL Link little-endian objects. This affects the default output format.
-f
--auxiliary name
When 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 name
When 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 name
When 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.
-g Ignored. Provided for compatibility with other tools.
-Gvalue
--gpsize=value
Set 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 ECOFF which supports putting large and small objects into
different sections. This is ignored for other object file formats.
-hname
-soname=name
When 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.
-i Perform an incremental link (same as option -r).
-init name
When 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.
-larchive
--library=archive
Add archive file archive to the list of files to link. This option
may be used any number of times. ld will search its path-list for
occurrences of "libarchive.a" for every archive specified.
On systems which support shared libraries, ld may also search for
libraries with extensions other than ".a". Specifically, on ELF
and SunOS systems, ld will search a directory for a library with an
extension of ".so" before searching for one with an extension of
".a". By convention, a ".so" extension indicates a shared library.
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.
-Lsearchdir
--library-path=searchdir
Add 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.
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.
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.
-memulation
Emulate 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-map
Print a link map to the standard output. A link map provides
information about the link, including the following:
o Where object files and symbols are mapped into memory.
o How common symbols are allocated.
o All archive members included in the link, with a mention of the
symbol which caused the archive member to be brought in.
-n
--nmagic
Turn off page alignment of sections, and mark the output as
"NMAGIC" if possible.
-N
--omagic
Set the text and data sections to be readable and writable. Also,
do not page-align the data segment. If the output format supports
Unix style magic numbers, mark the output as "OMAGIC".
-o output
--output=output
Use 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 level
If 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.
-q
--emit-relocs
Leave relocation sections and contents in fully linked exececutables.
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.
-r
--relocateable
Generate 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=filename
Read 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-all
Omit all symbol information from the output file.
-S
--strip-debug
Omit debugger symbol information (but not all symbols) from the
output file.
-t
--trace
Print the names of the input files as ld processes them.
-T scriptfile
--script=scriptfile
Use 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.
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.
-u symbol
--undefined=symbol
Force 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.
-Ur For 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.
--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
-V Display the version number for ld. The -V option also lists the
supported emulations.
-x
--discard-all
Delete all local symbols.
-X
--discard-locals
Delete all temporary local symbols. For most targets, this is all
local symbols whose names begin with L.
-y symbol
--trace-symbol=symbol
Print 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 path
Add path to the default library search path. This option exists
for Solaris compatibility.
-z keyword
The recognized keywords are "initfirst", "interpose", "loadfltr",
"nodefaultlib", "nodelete", "nodlopen", "nodump", "now", "origin",
"combreloc", "nocombreloc" and "nocopyreloc". The other keywords
are ignored for Solaris compatibility. "initfirst" marks the object
to be initialized first at runtime before any other objects.
"interpose" marks the object that its symbol table interposes
before all symbols but the primary executable. "loadfltr" marks the
object that its filtees be processed immediately at runtime.
"nodefaultlib" marks the object that the search for dependencies of
this object will ignore any default library search paths.
"nodelete" marks the object shouldn't be unloaded at runtime.
"nodlopen" marks the object not available to "dlopen". "nodump"
marks the object can not be dumped by "dldump". "now" marks the
object with the non-lazy runtime binding. "origin" marks the
object may contain $ORIGIN. "defs" disallows undefined symbols.
"combreloc" combines multiple reloc sections and sorts them to make
dynamic symbol lookup caching possible. "nocombreloc" disables
multiple reloc sections combining. "nocopyreloc" disables production
of copy relocs.
-( archives -)
--start-group archives --end-group
The 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.
-assert keyword
This option is ignored for SunOS compatibility.
-Bdynamic
-dy
-call_shared
Link 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.
-Bgroup
Set 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.
--no-undefined is implied. This option is only meaningful on ELF
platforms which support shared libraries.
-Bstatic
-dn
-non_shared
-static
Do 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.
-Bsymbolic
When 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 is only
meaningful on ELF platforms which support shared libraries.
--check-sections
--no-check-sections
Asks the linker not to check section addresses after they have been
assigned to see if there 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.
--cref
Output 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. The remaining files contain references
to the symbol.
--no-define-common
This option inhibits the assignment of addresses to common symbols.
The script command "INHIBIT_COMMON_ALLOCATION" has the same effect.
The --no-define-common option allows decoupling the decision to
assign addresses to Common symbols from the choice of the output
file type; otherwise a non-Relocatable output type forces assigning
addresses to Common symbols. Using --no-define-common allows Common
symbols that are referenced from a shared library to be
assigned addresses only in the main program. This eliminates the
unused duplicate space in the shared library, and also prevents any
possible confusion over resolving to the wrong duplicate when there
are many dynamic modules with specialized search paths for runtime
symbol resolution.
--defsym symbol=expression
Create 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. Note: there
should be no white space between symbol, the equals sign (``=''),
and expression.
--demangle[=style]
--no-demangle
These 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.
--dynamic-linker file
Set 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.
--embedded-relocs
This option is only meaningful when linking MIPS embedded PIC code,
generated by the -membedded-pic option to the GNU compiler and
assembler. It causes the linker to create a table which may be
used at runtime to relocate any data which was statically initialized
to pointer values. See the code in testsuite/ld-empic for
details.
--fatal-warnings
Treat all warnings as errors.
--force-exe-suffix
Make 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.
--no-gc-sections
--gc-sections
Enable garbage collection of unused input sections. It is ignored
on targets that do not support this option. This option is not
compatible with -r, nor should it be used with dynamic linking.
The default behaviour (of not performing this garbage collection)
can be restored by specifying --no-gc-sections on the command line.
--help
Print a summary of the command-line options on the standard output
and exit.
--target-help
Print a summary of all target specific options on the standard output
and exit.
-Map mapfile
Print a link map to the file mapfile. See the description of the
-M option, above.
--no-keep-memory
ld normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells ld to
instead optimize for memory usage, by rereading the symbol tables
as necessary. This may be required if ld runs out of memory space
while linking a large executable.
--no-undefined
-z defs
Normally when creating a non-symbolic shared library, undefined
symbols are allowed and left to be resolved by the runtime loader.
These options disallows such undefined symbols.
--allow-shlib-undefined
Allow undefined symbols in shared objects even when --no-undefined
is set. The net result will be that undefined symbols in regular
objects will still trigger an error, but undefined symbols in
shared objects will be ignored. The implementation of no_undefined
makes the assumption that the runtime linker will choke on undefined
symbols. However there is at least one system (BeOS) where
undefined symbols in shared libraries is normal since the kernel
patches them at load time to select which function is most appropriate
for the current architecture. I.E. dynamically select an
appropriate memset function. Apparently it is also normal for HPPA
shared libraries to have undefined symbols.
--no-warn-mismatch
Normally 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-whole-archive
Turn off the effect of the --whole-archive option for subsequent
archive files.
--noinhibit-exec
Retain 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.
-nostdlib
Only 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-format
ld may be configured to support more than one kind of object file.
If your ld is configured this way, you can use the --oformat option
to specify the binary format for the output object file. Even when
ld is configured to support alternative object formats, you don't
usually need to specify this, as ld should be configured to produce
as a default output format the most usual format on each machine.
output-format is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with objdump -i.) The script command "OUTPUT_FORMAT" can
also specify the output format, but this option overrides it.
-qmagic
This option is ignored for Linux compatibility.
-Qy This option is ignored for SVR4 compatibility.
--relax
An option with machine dependent effects. This option is only supported
on a few targets.
On some platforms, the --relax option performs global optimizations
that become possible when the linker resolves addressing in the
program, such as relaxing address modes and synthesizing new
instructions in the output object file.
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.
--retain-symbols-file filename
Retain 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 dir
Add 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 patch out of all the -L
options it is given. If a -rpath option is used, the runtime
search path will be formed exclusively using the -rpath options,
ignoring the -L options. This can be useful when using gcc, which
adds many -L options which may be on NFS mounted filesystems.
For compatibility with other ELF linkers, if the -R option is followed
by a directory name, rather than a file name, it is treated
as the -rpath option.
-rpath-link DIR
When 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.
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.
1. Any directories specified by -rpath-link options.
2. Any directories specified by -rpath options. The difference
between -rpath and -rpath-link is that directories specified by
-rpath options are included in the executable and used at runtime,
whereas the -rpath-link option is only effective at link
time. It is for the native linker only.
3. On an ELF system, if the -rpath and "rpath-link" options were
not used, search the contents of the environment variable
"LD_RUN_PATH". It is for the native linker only.
4. On SunOS, if the -rpath option was not used, search any directories
specified using -L options.
5. For a native linker, the contents of the environment variable
"LD_LIBRARY_PATH".
6. For a native ELF linker, the directories in "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.
7. The default directories, normally /lib and /usr/lib.
8. For a native linker on an ELF system, if the file
/etc/ld.so.conf exists, the list of directories found in that
file.
If the required shared library is not found, the linker will issue
a warning and continue with the link.
-shared
-Bshareable
Create 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
This option tells ld to sort the common symbols by size when it
places them in the appropriate output sections. First come all the
one byte symbols, then all the two byte, then all the four byte,
and then everything else. This is to prevent gaps between symbols
due to alignment constraints.
--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.
--stats
Compute and display statistics about the operation of the linker,
such as execution time and memory usage.
--traditional-format
For 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=org
Locate 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 org
Use org as the starting address for---respectively---the "bss",
"data", or the "text" segment of the output file. org must be a
single hexadecimal integer; for compatibility with other linkers,
you may omit the leading 0x usually associated with hexadecimal
values.
--dll-verbose
--verbose
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.
--version-script=version-scriptfile
Specify the name of a version script to the linker. This is typically
used when creating shared libraries to specify additional
information about the version heirarchy for the library being created.
This option is only meaningful on ELF platforms which support
shared libraries.
--warn-common
Warn 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:
int i = 1;
A definition, which goes in the initialized data section of the
output file.
extern int i;
An undefined reference, which does not allocate space. There
must be either a definition or a common symbol for the variable
somewhere.
int i;
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.
1. Turning a common symbol into a reference, because there is
already a definition for the symbol.
I<file>(I<section>): warning: common of `I<symbol>'
overridden by definition
I<file>(I<section>): warning: defined here
2. Turning a common symbol into a reference, because a later definition
for the symbol is encountered. This is the same as the
previous case, except that the symbols are encountered in a
different order.
I<file>(I<section>): warning: definition of `I<symbol>'
overriding common
I<file>(I<section>): warning: common is here
3. Merging a common symbol with a previous same-sized common symbol.
I<file>(I<section>): warning: multiple common
of `I<symbol>'
I<file>(I<section>): warning: previous common is here
4. Merging a common symbol with a previous larger common symbol.
I<file>(I<section>): warning: common of `I<symbol>'
overridden by larger common
I<file>(I<section>): warning: larger common is here
5. Merging a common symbol with a previous smaller common symbol.
This is the same as the previous case, except that the symbols
are encountered in a different order.
I<file>(I<section>): warning: common of `I<symbol>'
overriding smaller common
I<file>(I<section>): warning: smaller common is here
--warn-constructors
Warn 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-gp
Warn 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-once
Only warn once for each undefined symbol, rather than once per module
which refers to it.
--warn-section-align
Warn 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.
--whole-archive
For 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 symbol
Use a wrapper function for symbol. Any undefined reference to sym-
bol 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 (int c)
{
printf ("malloc called with %ld\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".
--enable-new-dtags
--disable-new-dtags
This 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 dynamic tags will be created as needed. 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.
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
If given, symbols with a stdcall suffix (@nn) will be exported asis
and also with the suffix stripped.
--base-file file
Use file as the name of a file in which to save the base addresses
of all the relocations needed for generating DLLs with dlltool.
--dll
Create a DLL instead of a regular executable. You may also use
-shared or specify a "LIBRARY" in a given ".def" file.
--enable-stdcall-fixup
--disable-stdcall-fixup
If 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 --dis-
able-stdcall-fixup, this feature is disabled and such mismatches
are considered to be errors.
--export-all-symbols
If 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".
--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.
--file-alignment
Specify the file alignment. Sections in the file will always begin
at file offsets which are multiples of this number. This defaults
to 512.
--heap reserve
--heap reserve,commit
Specify the amount of memory to reserve (and optionally commit) to
be used as heap for this program. The default is 1Mb reserved, 4K
committed.
--image-base value
Use 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.
--kill-at
If given, the stdcall suffixes (@nn) will be stripped from symbols
before they are exported.
--major-image-version value
Sets the major number of the "image version". Defaults to 1.
--major-os-version value
Sets the major number of the "os version". Defaults to 4.
--major-subsystem-version value
Sets the major number of the "subsystem version". Defaults to 4.
--minor-image-version value
Sets the minor number of the "image version". Defaults to 0.
--minor-os-version value
Sets the minor number of the "os version". Defaults to 0.
--minor-subsystem-version value
Sets the minor number of the "subsystem version". Defaults to 0.
--output-def file
The 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.
--out-implib file
The linker will create the file file which will contain an import
lib corresponding to the DLL the linker is generating. This import
lib (which should be called "*.dll.a" or "*.a" may be used to link
clients against the generated DLL; this behavior makes it possible
to skip a separate "dlltool" import library creation step.
--enable-auto-image-base
Automatically choose the image base for DLLs, 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.
--disable-auto-image-base
Do not automatically generate a unique image base. If there is no
user-specified image base ("--image-base") then use the platform
default.
--dll-search-prefix string
When linking dynamically to a dll without an import library, i
search for "<string><basename>.dll" in preference to "lib<basename>.dll".
This behavior 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".
--enable-auto-import
Do sophisticated linking of "_symbol" to "__imp__symbol" for DATA
imports from DLLs, and create the necessary thunking symbols when
building the DLLs with those DATA exports. This 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 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
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