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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