Computer Science
ld(1) GNU Development Tools ld(1)
NAME
ld - the GNU linker
SYNOPSIS
ld [-o output] objfile...
[-Aarchitecture] [-b input-format] [-Bstatic]
[-Bdynamic] [-Bsymbolic] [-c commandfile] [--cref]
[-d|-dc|-dp]
[-defsym symbol = expression] [-e entry]
[-embedded-relocs] [-E] [-export-dynamic] [-f name]
[--auxiliary name] [-F name] [--filter name]
[-format input-format] [-g] [-G size] [-h name]
[-soname name] [--help] [-i] [-lar] [-Lsearchdir]
[-M] [-Map mapfile] [-m emulation] [-n|-N]
[-noinhibit-exec] [-no-keep-memory]
[-no-warn-mismatch] [-oformat output-format]
[-R filename] [-relax] [-r|-Ur] [-rpath directory]
[-rpath-link directory] [-S] [-s] [-shared]
[-sort-common] [-split-by-reloc count]
[-split-by-file] [-T commandfile] [-Ttext textorg]
[-Tdata dataorg] [-Tbss bssorg] [-t] [-u sym] [-V]
[-v] [--verbose] [--version] [-warn-common]
[-warn-constructors] [-warn-multiple-gp]
[-warn-once] [-warn-section-align]
[--whole-archive] [--no-whole-archive]
[--wrap symbol] [-X] [-x]
DESCRIPTION
ld combines a number of object and archive files, relo-
cates their data and ties up symbol references. Often the
last step in building a new compiled program to run is a
call to ld.
ld accepts Linker Command Language files to provide ex-
plicit 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 as-
pects of the GNU linker.
This version of ld uses the general purpose BFD libraries
to operate on object files. This allows ld to read, com-
bine, 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. You can use `objdump -i' to get a list of formats
supported on various architectures; see objdump(1).
Aside from its flexibility, the GNU linker is more helpful
than other linkers in providing diagnostic information.
Many linkers abandon execution immediately upon encounter-
ing 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 situ-
ations, and to be as compatible as possible with other
linkers. As a result, you have many choices to control
its behavior through the command line, and through envi-
ronment variables.
OPTIONS
The plethora of command-line options may seem intimidat-
ing, 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, support-
ed 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 re-
sult of linking the file /lib/crt0.o with hello.o and the
library libc.a which will come from the standard search
directories.
The command-line options to ld may be specified in any or-
der, and may be repeated at will. For the most part, re-
peating an option with a different argument will either
have no further effect, or override prior occurrences
(those further to the left on the command line) of an op-
tion.
The exceptions--which may meaningfully be used more than
once--are -A, -b (or its synonym -format), -defsym, -L,
-l, -R, and -u.
The list of object files to be linked together, shown as
objfile, may follow, precede, or be mixed in with command-
line options; save that an objfile argument may not be
placed between an option flag and its argument.
Usually the linker is invoked with at least one object
file, but other forms of binary input files can also be
specified with -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'.
Option arguments must either follow the option letter
without intervening whitespace, or be given as separate
arguments immediately following the option that requires
them.
-Aarchitecture
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 is
one of the two-letter names identifying members of
the 960 family; the option specifies the desired
output target, and warns of any incompatible in-
structions in the input files. It also modifies
the linker's search strategy for archive libraries,
to support the use of libraries specific to each
particular architecture, by including in the search
loop names suffixed with the string identifying the
architecture.
For example, if your ld command line included
`-ACA' as well as `-ltry', the linker would look
(in its built-in search paths, and in any paths you
specify with -L) for a library with the names
try
libtry.a
tryca
libtryca.a
The first two possibilities would be considered in
any event; the last two are due to the use of
`-ACA'.
Future releases of ld may support similar function-
ality for other architecture families.
You can meaningfully use -A more than once on a
command line, if an architecture family allows com-
bination of target architectures; each use will add
another pair of name variants to search for when -l
specifies a library.
-b input-format
Specify the binary format for input object files
that follow this option on the command line. You
don't usually need to specify this, as ld is con-
figured 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 sup-
ported by the BFD libraries. -format input-format
has the same effect, as does the script command
TARGET.
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.
-Bstatic
Do not link against shared libraries. This is only
meaningful on platforms for which shared libraries
are supported.
-Bdynamic
Link against dynamic libraries. This is only mean-
ingful on platforms for which shared libraries are
supported. This option is normally the default on
such platforms.
-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 op-
tion is only meaningful on ELF platforms which sup-
port shared libraries.
-c commandfile
Directs ld to read link commands from the file com-
mandfile. These commands will completely override
ld's default link format (rather than adding to
it); commandfile must specify everything necessary
to describe the target format.
You may also include a script of link commands di-
rectly in the command line by bracketing it between
`{' and `}' characters.
--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.
-d
-dc
-dp These three options are equivalent; multiple forms
are supported for compatibility with other linkers.
Use any of them to make ld assign space to common
symbols even if a relocatable output file is speci-
fied (-r). The script command FORCE_COMMON_ALLOCA-
TION has the same effect.
-defsym symbol = expression
Create a global symbol in the output file, contain-
ing 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 ex-
pression in this context: you may give a hexadeci-
mal constant or the name of an existing symbol, or
use + and - to add or subtract hexadecimal con-
stants or symbols. If you need more elaborate ex-
pressions, consider using the linker command lan-
guage from a script.
-e entry
Use entry as the explicit symbol for beginning ex-
ecution of your program, rather than the default
entry point. for a discussion of defaults and oth-
er ways of specifying the entry point.
-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 caus-
es the linker to create a table which may be used
at runtime to relocate any data which was statical-
ly initialized to pointer values. See the code in
testsuite/ld-empic for details.
-E
-export-dynamic
When creating an ELF file, add all symbols to the
dynamic symbol table. Normally, the dynamic symbol
table contains only symbols which are used by a dy-
namic object. This option is needed for some uses
of dlopen.
-f name
--auxiliary name
When creating an ELF shared object, set the inter-
nal 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.
-F name
--filter name
When creating an ELF shared object, set the inter-
nal DT_FILTER field to the specified name. This
tells the dynamic linker that the symbol table of
the shared object should be used as a filter on the
symbol table of the shared object name.
-format input-format
Synonym for -b input-format.
-g Accepted, but ignored; provided for compatibility
with other tools.
-G size
Set the maximum size of objects to be optimized us-
ing the GP register to size under MIPS ECOFF. Ig-
nored for other object file formats.
-h name
-soname name
When creating an ELF shared object, set the inter-
nal 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.
--help Print a summary of the command-line options on the
standard output and exit. This option and --ver-
sion begin with two dashes instead of one for com-
patibility with other GNU programs. The other op-
tions start with only one dash for compatibility
with other linkers.
-i Perform an incremental link (same as option -r).
-lar Add an archive file ar 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
libar.a for every ar specified.
-Lsearchdir
This command adds path searchdir to the list of
paths that ld will search for archive libraries.
You may use this option any number of times.
The default set of paths searched (without being
specified with -L) depends on what 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.
-M Print (to the standard output file) a link map--di-
agnostic information about where symbols are mapped
by ld, and information on global common storage al-
location.
-Map mapfile
Print to the file mapfile a link map--diagnostic
information about where symbols are mapped by ld,
and information on global common storage alloca-
tion.
-m emulation
Emulate the emulation linker. You can list the
available emulations with the --verbose or -V op-
tions. This option overrides the compiled-in de-
fault, which is the system for which you configured
ld.
-N specifies readable and writable text and data sec-
tions. If the output format supports Unix style
magic numbers, the output is marked as OMAGIC.
When you use the `-N' option, the linker does not
page-align the data segment.
-n sets the text segment to be read only, and NMAGIC
is written if possible.
-noinhibit-exec
Normally, the linker will not produce an output
file if it encounters errors during the link pro-
cess. With this flag, you can specify that you
wish the output file retained even after non-fatal
errors.
-no-keep-memory
The linker normally optimizes for speed over memory
usage by caching the symbol tables of input files
in memory. This option tells the linker to instead
optimize for memory usage, by rereading the symbol
tables as necessary. This may be required if the
linker runs out of memory space while linking a
large executable.
-no-warn-mismatch
Normally the linker 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 the linker 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.
-o output
output is a 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 al-
so specify the output file name.
-oformat output-format
Specify the binary format for the output object
file. You don't usually need to specify this, as
ld is configured to produce as a default output
format the most usual format on each machine. out-
put-format is a text string, the name of a particu-
lar format supported by the BFD libraries. The
script command OUTPUT_FORMAT can also specify the
output format, but this option overrides it.
-R filename
Read symbol names and their addresses from file-
name, but do not relocate it or include it in the
output. This allows your output file to refer sym-
bolically to absolute locations of memory defined
in other programs.
-relax An option with machine dependent effects. Current-
ly this option is only supported on the H8/300.
On some platforms, use this option to perform glob-
al optimizations that become possible when the
linker resolves addressing in your program, such as
relaxing address modes and synthesizing new in-
structions in the output object file.
On platforms where this is not supported, `-relax'
is accepted, but has no effect.
-r Generates 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; -Ur is an alternative.
This option does the same as -i.
-rpath directory
Add a directory to the runtime library search path.
This is used when linking an ELF executable with
shared objects. All -rpath arguments are concate-
nated and passed to the runtime linker, which uses
them to locate shared objects at runtime. The
-rpath option is also used when locating shared ob-
jects 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 en-
vironment 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 op-
tions, ignoring the -L options. This can be useful
when using gcc, which adds many -L options which
may be on NFS mounted filesystems.
-rpath-link directory
When using ELF or SunOS, one shared library may re-
quire 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-relocateable link, it will
automatically try to locate the required shared li-
brary and include it in the link, if it is not in-
cluded explicitly. In such a case, the -rpath-link
option specifies the first set of directories to
search. The -rpath-link option may specify a se-
quence of directory names either by specifying a
list of names separated by colons, or by appearing
multiple times.
If the required shared library is not found, the
linker will issue a warning and continue with the
link.
-S Omits debugger symbol information (but not all sym-
bols) from the output file.
-s Omits all symbol information from the output file.
-shared
Create a shared library. This is currently only
supported on ELF and SunOS platforms (on SunOS it
is not required, as the linker will automatically
create a shared library when there are undefined
symbols and the -e option is not used).
-sort-common
Normally, when 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 be-
tween symbols due to alignment constraints. This
option disables that sorting.
-split-by-reloc count
Trys to creates extra sections in the output file
so that no single output section in the file con-
tains more than count relocations. This is useful
when generating huge relocatable 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 link-
er will not split up individual input sections for
redistribution, so if a single input section con-
tains more than count relocations one output sec-
tion will contain that many relocations.
-split-by-file
Similar to -split-by-reloc but creates a new output
section for each input file.
-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. textorg must be a hexadecimal integer.
-T commandfile
Equivalent to -c commandfile; supported for compat-
ibility with other tools.
-t Prints names of input files as ld processes them.
-u sym Forces sym to be entered in the output file as an
undefined symbol. This may, for example, trigger
linking of additional modules from standard li-
braries. -u may be repeated with different option
arguments to enter additional undefined symbols.
-Ur For anything other than C++ programs, this option
is equivalent to -r: it generates relocatable out-
put--i.e., an output file that can in turn serve as
input to ld. When linking C++ programs, -Ur will
resolve references to constructors, unlike -r.
--verbose
Display the version number for ld and list the sup-
ported emulations. Display which input files can
and can not be opened.
-v, -V Display the version number for ld. The -V option
also lists the supported emulations.
--version
Display the version number for ld and exit.
-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.
-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 the output file requires multiple global-
pointer values. This option is only meaningful for
certain processors, such as the Alpha.
-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 af-
ter the --whole-archive option, include every ob-
ject 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.
--no-whole-archive
Turn off the effect of the --whole-archive option
for archives which appear later on the command
line.
--wrap symbol
Use a wrapper function for symbol. Any undefined
reference to symbol will be resolved to __wrap_sym-
bol. Any undefined reference to __real_symbol will
be resolved to symbol.
-X Delete all temporary local symbols. For most tar-
gets, this is all local symbols whose names begin
with `L'.
-x Delete all local symbols.
ENVIRONMENT
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. If there is
no GNUTARGET in the environment, ld uses the natural for-
mat of the host. 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 po-
tential ambiguities, since there is no method of ensuring
that the magic number used to flag 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.
SEE ALSO
objdump(1)
`ld' and `binutils' entries in info
ld: the GNU linker, Steve Chamberlain and Roland Pesch;
The GNU Binary Utilities, Roland H. Pesch.
COPYING
Copyright (c) 1991, 1992 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim
copies of this manual provided the copyright notice and
this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified ver-
sions of this manual under the conditions for verbatim
copying, provided that the entire resulting derived work
is distributed under the terms of a permission notice
identical to this one.
Permission is granted to copy and distribute translations
of this manual into another language, under the above con-
ditions for modified versions, except that this permission
notice may be included in translations approved by the
Free Software Foundation instead of in the original En-
glish.
cygnus support 17 August 1992 1
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