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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.)
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.
See section 3. Linker Scripts.
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
-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 5. 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 2.2 Environment Variables.
You can also define the input format from a script, using the command
TARGET
;
see 3.4.3 Commands dealing with object file formats.
-c MRI-commandfile
--mri-script=MRI-commandfile
ld
accepts script
files written in an alternate, restricted command language, described in
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 3.4.4 Other linker script commands.
-e entry
--entry=entry
-E
--export-dynamic
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 3.9 VERSION Command.
-EB
-EL
-f
--auxiliary 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
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
_fini
as
the function to call.
-g
-Gvalue
--gpsize=value
-hname
-soname=name
-i
-init name
_init
as the
function to call.
-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 2.2 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
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
-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.
-O level
ld
optimizes
the output. This might take significantly longer and therefore probably
should only be enabled for the final binary.
-q
--emit-relocs
This option is currently only supported on ELF platforms.
-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'.
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
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
-S
--strip-debug
-t
--trace
ld
processes them.
-T scriptfile
--script=scriptfile
ld
's default linker script (rather than adding to it), so
commandfile must specify everything necessary to describe the
output file. See section 3. Linker Scripts. 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
EXTERN
linker script command.
-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.
--unique[=SECTION]
-v
--version
-V
ld
. The `-V' option also
lists the supported emulations.
-x
--discard-all
-X
--discard-locals
-y symbol
--trace-symbol=symbol
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
-z keyword
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.
muldefs
allows multiple definitions.
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 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
-Bdynamic
-dy
-call_shared
-Bgroup
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
-Bsymbolic
--check-sections
--no-check-sections
--cref
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
INHIBIT_COMMON_ALLOCATION
has the same effect.
See section 3.4.4 Other linker script commands.
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
+
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: Symbol Definitions). Note: there should be no white
space between symbol, the equals sign ("="), and
expression.
--demangle[=style]
--no-demangle
--dynamic-linker file
--embedded-relocs
--fatal-warnings
--force-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
--help
--target-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-undefined
-z defs
--allow-multiple-definition
-z muldefs
--allow-shlib-undefined
--no-undefined-version
--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
--noinhibit-exec
-nostdlib
--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 5. 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 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-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
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-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.
rpath-link
options
were not used, search the contents of the environment variable
LD_RUN_PATH
. It is for the native linker only.
LD_LIBRARY_PATH
.
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.
If the required shared library is not found, the linker will issue a warning and continue with the link.
-shared
-Bshareable
--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 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]
--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.
--section-start sectionname=org
-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.
--dll-verbose
--verbose
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
--warn-common
There are three kinds of global symbols, illustrated here by C examples:
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.
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 3.6 SECTIONS command).
--whole-archive
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
__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
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