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.
Here is a summary of the options you can use on the ld
command
line:
ld [ -o output ] objfile... [ -Aarchitecture ] [ -b input-format ] [ -Bstatic ] [ -Bdynamic ] [ -Bsymbolic ] [ -c MRI-commandfile ] [ -d | -dc | -dp ] [ -defsym symbol=expression ] [ -dynamic-linker file ] [ -embedded-relocs ] [ -export-dynamic ] [ -e entry ] [ -F ] [ -F format ] [ -format input-format ] [ -g ] [ -G size ] [ -help ] [ -i ] [ -larchive ] [ -Lsearchdir ] [ -M ] [ -Map mapfile ] [ -m emulation ] [ -N | -n ] [ -noinhibit-exec ] [ -no-keep-memory ] [ -oformat output-format ] [ -R filename ] [ -relax ] [ -retain-symbols-file filename ] [ -r | -Ur ] [ -rpath dir ] [-rpath-link dir ] [ -S ] [ -s ] [ -soname name ] [ -shared ] [ -sort-common ] [ -stats ] [ -T commandfile ] [ -Ttext org ] [ -Tdata org ] [ -Tbss org ] [ -t ] [ -traditional-format ] [ -u symbol] [-V] [-v] [ -verbose] [ -version ] [ -warn-common ] [ -warn-constructors] [ -warn-once ] [ -y symbol ] [ -X ] [-x ] [ -( [ archives ] -) ] [ --start-group [ archives ] --end-group ] [ -split-by-reloc count ] [ -split-by-file ] [ --whole-archive ] [ --no-whole-archive ]
This plethora of command-line options may seem intimidating, 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.
The exceptions--which may meaningfully be used more than once--are `-A', `-b' (or its synonym `-format'), `-defsym', `-L', `-l', `-R', `-u', and `-(' (or its synonym `--start-group')..
The list of object files to be linked together, shown as objfile..., may follow, precede, or be mixed in with command-line options, except that an objfile 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. 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.
-Aarchitecture
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
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'.) `-format input-format'
has the same effect, as does the script command TARGET
.
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.
-Bstatic
-Bdynamic
-Bsymbolic
-c 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.
-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
-embedded-relocs
-e entry
-export-dynamic
dlopen
.
-F
-Fformat
ld
uses for this purpose (the
`-b' or `-format' options for input files, `-oformat'
option or the TARGET
command in linker scripts for output files,
the GNUTARGET
environment variable) are more flexible, but
ld
accepts the `-F' option for compatibility with scripts
written to call the old linker.
-format input-format
-g
-Gvalue
-G value
-help
-i
-lar
ld
will search its
path-list for occurrences of libar.a
for every archive
specified.
-Lsearchdir
-L 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.
-M
ld
, and information on global common
storage allocation.
-Map mapfile
ld
, and information on global
common storage allocation.
-memulation
-m emulation
ld
was configured.
-N
OMAGIC
.
-n
NMAGIC
if possible.
-noinhibit-exec
-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.
-o 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.
-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.
-R filename
-R
option is
followed by a directory name, rather than a file name, it is treated as
the -rpath
option.
-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
.
-r
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'.
-S
-s
-soname name
-shared
-e
option is not used and there are
undefined symbols in the link.
-sort-common
ld
places the global common symbols in the
appropriate output sections, it sorts them by size. 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. This option disables that sorting.
-split-by-reloc count
-split-by-file
-stats
-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.
-T commandfile
-Tcommandfile
ld
's default link script (rather than adding
to it), so commandfile must specify everything necessary to describe
the target format. 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.
-t
ld
processes them.
-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.
-u symbol
-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.
-v
-V
ld
. The -V
option also
lists the supported emulations.
-version
ld
and exit.
-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-once
--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.
Turn off the effect of the --whole-archive
option for archives
which appear later on the command line.
-X
-x
-y symbol
-( archives -)
--start-group archives --end-group
You can change the behavior of ld
with the environment
variable GNUTARGET
.
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.