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 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.)
The command-line options to ld may be specified in any order, and
may be repeated at will. Repeating most 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 objects files 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 should only be used to augment the main
linker script; if you want to use some command that logically can only
appear once, such as the SECTIONS or MEMORY command, you
must replace the default linker script using the `-T' option.
See section Command Language.
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, `--oformat' and `--oformat' are equivalent. 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, `--oformat srec' and `--oformat=srec' are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
-akeyword
-Aarchitecture
--architecture=architecture
ld, this option is useful only for the
Intel 960 family of architectures. In that ld configuration, the
architecture argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. See section ld and the Intel 960 family, for details.
Future releases of ld may support similar functionality for
other architecture families.
-b input-format
--format=input-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`-b' option to specify the binary format for input object files
that follow this option on the command line. Even when ld is
configured to support alternative object formats, you don't usually need
to specify this, as ld should be configured to expect as a
default input format the most usual format on each machine.
input-format is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with `objdump -i'.)
See section BFD.
You may want to use this option if you are linking files with an unusual
binary format. You can also use `-b' to switch formats explicitly (when
linking object files of different formats), by including
`-b input-format' before each group of object files in a
particular format.
The default format is taken from the environment variable
GNUTARGET.
See section Environment Variables.
You can also define the input
format from a script, using the command TARGET; see section Option Commands.
-c MRI-commandfile
--mri-script=MRI-commandfile
ld accepts script
files written in an alternate, restricted command language, described in
section MRI Compatible Script Files. Introduce MRI script files with
the option `-c'; use the `-T' option to run linker
scripts written in the general-purpose ld scripting language.
If MRI-cmdfile does not exist, ld looks for it in the directories
specified by any `-L' options.
-d
-dc
-dp
FORCE_COMMON_ALLOCATION has the same effect. See section Option Commands.
-e entry
--entry=entry
-E
--export-dynamic
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.
-f
--auxiliary name
-F name
--filter name
-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.
--force-exe-suffix
.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.
-g
-Gvalue
--gpsize=value
-hname
-soname=name
-i
-larchive
--library=archive
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
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. See section Environment Variables.
The paths can also be specified in a link script with the
SEARCH_DIR command. Directories specified this way are searched
at the point in which the linker script appears in the command line.
-memulation
LDEMULATION environment variable, if that is defined.
Otherwise, the default emulation depends upon how the linker was
configured.
-M
--print-map
-n
--nmagic
NMAGIC if possible.
-N
--omagic
OMAGIC.
-o output
--output=output
ld; if this
option is not specified, the name `a.out' is used by default. The
script command OUTPUT can also specify the output file name.
-r
--relocateable
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'.
This option does the same thing as `-i'.
-R filename
--just-symbols=filename
-R option is
followed by a directory name, rather than a file name, it is treated as
the -rpath option.
-s
--strip-all
-S
--strip-debug
-t
--trace
ld processes them.
-T commandfile
--script=commandfile
ld's default link script (rather than adding to it), so
commandfile must specify everything necessary to describe the
target format. You must use this option if you want to use a command
which can only appear once in a linker script, such as the
SECTIONS or MEMORY command. See section Command Language. If
commandfile does not exist, ld looks for it in the
directories specified by any preceding `-L' options. Multiple
`-T' options accumulate.
-u symbol
--undefined=symbol
-v
--version
-V
ld. The -V option also
lists the supported emulations.
-x
--discard-all
-X
--discard-locals
-y symbol
--trace-symbol=symbol
-Y path
-z keyword
-( archives -)
--start-group archives --end-group
-assert keyword
-Bdynamic
-dy
-call_shared
-l options which follow it.
-Bstatic
-dn
-non_shared
-static
-l options which follow it.
-Bsymbolic
--cref
--defsym symbol=expression
+ and - to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (see section Assignment: Defining Symbols). Note: there should be no
white space between symbol, the equals sign ("="), and
expression.
--dynamic-linker file
-EB
-EL
--embedded-relocs
--help
-Map mapfile
--no-keep-memory
ld normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells ld to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if ld runs out of memory space
while linking a large executable.
--no-warn-mismatch
ld will give an error if you try to link together input
files that are mismatched for some reason, perhaps because they have
been compiled for different processors or for different endiannesses.
This option tells ld that it should silently permit such possible
errors. This option should only be used with care, in cases when you
have taken some special action that ensures that the linker errors are
inappropriate.
--no-whole-archive
--whole-archive option for subsequent
archive files.
--noinhibit-exec
--oformat output-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`--oformat' option to specify the binary format for the output
object file. Even when ld is configured to support alternative
object formats, you don't usually need to specify this, as ld
should be configured to produce as a default output format the most
usual format on each machine. output-format is a text string, the
name of a particular format supported by the BFD libraries. (You can
list the available binary formats with `objdump -i'.) The script
command OUTPUT_FORMAT can also specify the output format, but
this option overrides it. See section BFD.
-qmagic
-Qy
--relax
ld and the H8/300.
See section ld and the Intel 960 family.
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 platforms where this is not supported, `--relax' is accepted,
but ignored.
--retain-symbols-file filename
-rpath dir
-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
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-relocateable 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 linker uses the following search paths to locate required shared
libraries.
-rpath-link options.
-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.
-rpath and rpath-link options
were not used, search the contents of the environment variable
LD_RUN_PATH.
-rpath option was not used, search any
directories specified using -L options.
LD_LIBRARY_PATH.
-shared
-Bshareable
-e option is not used and there are
undefined symbols in the link.
--sort-common
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 bytes, then all the four bytes, and then
everything else. This is to prevent gaps between symbols due to
alignment constraints.
--split-by-file
--split-by-reloc but creates a new output section for
each input file.
--split-by-reloc count
--stats
--traditional-format
ld is different in some ways from
the output of some existing linker. This switch requests ld to
use the traditional format instead.
For example, on SunOS, ld combines duplicate entries in the
symbol string table. This can reduce the size of an output file with
full debugging information by over 30 percent. Unfortunately, the SunOS
dbx program can not read the resulting program (gdb has no
trouble). The `--traditional-format' switch tells ld to not
combine duplicate entries.
-Tbss org
-Tdata org
-Ttext org
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.
-Ur
ld. When linking C++ programs, `-Ur'
does resolve references to constructors, unlike `-r'.
It does not work to use `-Ur' on files that were themselves linked
with `-Ur'; once the constructor table has been built, it cannot
be added to. Use `-Ur' only for the last partial link, and
`-r' for the others.
--verbose
ld and list the linker emulations
supported. Display which input files can and cannot be opened. Display
the linker script if using a default builtin script.
--version-script=version-scriptfile
--warn-common
file(section): warning: common of `symbol' overridden by definition file(section): warning: defined here
file(section): warning: definition of `symbol' overriding common file(section): warning: common is here
file(section): warning: multiple common of `symbol' file(section): warning: previous common is here
file(section): warning: common of `symbol' overridden by larger common file(section): warning: larger common is here
file(section): warning: common of `symbol' overriding smaller common file(section): warning: smaller common is here
--warn-constructors
--warn-multiple-gp
--warn-once
--warn-section-align
SECTIONS command does not specify a start address for
the section (see section Specifying Output Sections).
--whole-archive
--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.
--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 (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.
You can change the behavior of ld with the environment variables
GNUTARGET and LDEMULATION.
GNUTARGET determines the input-file object format if you don't
use `-b' (or its synonym `--format'). Its value should be one
of the BFD names for an input format (see section BFD). If there is no
GNUTARGET in the environment, ld uses the natural format
of the target. If GNUTARGET is set to default then BFD
attempts to discover the input format by examining binary input files;
this method often succeeds, but there are potential ambiguities, since
there is no method of ensuring that the magic number used to specify
object-file formats is unique. However, the configuration procedure for
BFD on each system places the conventional format for that system first
in the search-list, so ambiguities are resolved in favor of convention.
LDEMULATION determines the default emulation if you don't use the
`-m' option. The emulation can affect various aspects of linker
behaviour, particularly the default linker script. You can list the
available emulations with the `--verbose' or `-V' options. If
the `-m' option is not used, and the LDEMULATION environment
variable is not defined, the default emulation depends upon how the
linker was configured.
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