Command Line Options

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Command line options for `ld`

The following example shows the options you can use on the ld command line. Following the example is a discussion of the options.

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 ]
[ -e entry ] [ -F ] [ -F format ]
[ -format input-format ] [ -g ] [ -G size ]
[ -help ] [ -i ] [ -l archive ] [-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 ]

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, use the following example.

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 search directories with the -L option .)

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 specific option.

The exceptionswhich may meaningfully be used more than onceare the following: -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 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, --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.

-A architecture

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 identifies the particular architecture in the 960 family, enabling some safeguards and modifying the archive-library search path. See 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 dont usually need to specify this, as ld should be configured to expect as a default input format the most usual for-mat 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 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 Environmental variables. You can also define the input format from a script, using the command, TARGET; see Commands dealing with object file formats.

-B static

Do not link against shared libraries. This is only meaningful on platforms for which shared libraries are supported.

-B dynamic

Link against dynamic libraries. This is only meaningful on platforms for which shared libraries are supported. This option is normally the default on such platforms.

-B symbolic

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 option is only meaningful on ELF platforms which support shared libraries.

-c MRI-commandfile

For compatibility with linkers produced by MRI, 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

These three options are equivalent; multiple forms are sup-ported for compatibility with other linkers. They assign space to common symbols even if a relocatable output file is specified (with -r). The script command, FORCE_COMMON_ ALLOCATION, has the same effect.

-defsym symbol=expression

Create a global symbol in the output file, containing 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 expression in this context: you may give a hexadecimal constant or the name of an existing symbol, or use + 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 Simple linker script example).

Note:
There should be no white space between symbol, the equals sign (=), and expression.

-dynamic-linker file

Set the name of the dynamic linker. This is only meaningful when generating dynamically linked ELF executables. The default dynamic linker is normally correct; dont use this unless you know what you are doing.

-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 causes the linker to create a table which may be used at runtime to relocate any data which was statically initialized to pointer values. See the code in testsuite/ld-empic for details.

-e entry

Use entry as the explicit symbol for beginning execution of your program, rather than the default entry point. See Setting the entry point for a discussion of defaults and other ways of specifying the entry point.

-F
-F format

Ignored. Some older linkers used this option throughout a compilation toolchain for specifying object-file format for both input and output object files.

The mechanisms 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

Synonym for -b input-format.

-g

Ignored. Provided for compatibility with other tools.

-G value
-G value

Set the maximum size of objects to be optimized using the GP register to size under MIPS ECOFF. Ignored for other object file formats.

-help

Print a summary of the command-line options on the standard output and exit.

-I

Perform an incremental link (same as option -r).

-l ar

Add archive file archive 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 archive specified.

-L searchdir
-L searchdir

Add path, searchdir, to the list of paths that 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 Environmental 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

Print (to the standard output) a link mapdiagnostic information about where symbols are mapped by ld, and information on global common storage allocation.

-Map mapfile

Print to the file mapfile a link mapdiagnostic information about where symbols are mapped by ld, and information on global common storage allocation.

-m emulation
-m emulation

Emulate the emulation linker. You can list the available emulations with the --verbose or -V options. The default depends on lds configuration.

-N

Set the text and data sections to be readable and writable. Also, do not page-align the data segment. If the output format supports Unix style magic numbers, mark the output as OMAGIC.

-n

Set the text segment to be read only, and mark the output as NMAGIC if possible.

-noinhibit-exec

Retain the executable output file whenever it is still usable. Normally, the linker will not produce an output file if it encounters errors during the link process; it exits without writing an output file when it issues any error whatsoever.

-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

Use output as the 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 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 dont 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 BFD.

-R filename

Read symbol names and their addresses from filename, but do not relocate it or include it in the output. This allows your output file to refer symbolically to absolute locations of memory defined in other programs.

-relax

An option with machine dependent effects. Currently this option is only supported on the H8/300 and the Intel 960. See ld and the H8/300 and 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

Retain only the symbols listed in the file, filename, discarding all others. filename is simply a flat file, with one symbol name per line. This option is especially useful in environments (such as VxWorks) where a large global symbol table is accumulated gradually, to conserve runtime memory.

-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

Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All -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.

-rpath-link DIR

When using ELF or SunOS, one shared library may require 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 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.

- Any directories specified by -rpath-link options.
- Any directories specified by -rpath options. The difference between -rpath and -rpath-link is that directories specified by
- -rpath are included in the executable to use at runtime, whereas the -rpath-link is only effective at link time.
- On an ELF system, if the -rpath and rpath-link options were not used, search the contents of the environment variable LD_RUN_PATH.
- On SunOS, if the -rpath option was not used, search any directories specified using -L options.
- For a native linker, the contents of the environment variable LD_LIBRARY_PATH.
- The default directories, normally /lib and /usr/lib.

If the required shared library is not found, the linker will issue a warning and continue with the link.

-r

Generate relocatable outputi.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 files 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

Omit debugger symbol information (but not all symbols) from the output file.

-s

Omit all symbol information from the output file.

-soname name

When creating an ELF shared object, set the internal 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.

-shared

Create a shared library. This is currently only supported on ELF and SunOS platforms. On SunOS, the linker will automatically create a shared library if the -e option is not used and there are undefined symbols in the link.

-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 between symbols due to alignment constraints. This option disables that sorting.

-split-by-reloc count

Tries to creates extra sections in the output file so that no single output section in the file contains 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 linker will not split up individual input sections for redistribution, so if a single input section contains more than count relocations one output section will contain that many relocations.

-split-by-file

Similar to -split-by-reloc but creates a new output section for each input file.

-stats

Compute and display statistics about the operation of the linker, such as execution time and memory usage.

-Tbss org
-Tdata org
-Ttext org

Use org as the starting address forrespectivelythe 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.

-Tcommandfile
-T commandfile

Read link commands from the file, commandfile. These commands replace lds default link script (rather than adding to it), so commandfile must specify everything necessary to describe the target format. See Linker scripts. If commandfiledoes not exist, ld looks for it in the directories specified by any preceding -L options. Multiple -T options accumulate.

-t

Print the names of the input files as ld processes them.

-traditional-format

For some targets, the output of 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

Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. -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 outputi.e., an output file that can in turn serve as input to 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

Display the version number for ld and list the linker emulations supported. Display which input files can and cannot 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. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs.

There are three kinds of global symbols, illustrated here by C examples:

- int i = 1;
A definition, which goes in the initialized data section of the output file.
- extern int i;
An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere.
- int i;
A common symbol. If there are only (one or more) common symbols for a variable, it goes in the uninitialized data area of the output file. The linker merges multiple common symbols for the same variable into a single symbol. If they are of different sizes, it picks the largest size. The linker turns a common symbol into a declaration, if there is a definition of the same variable.

The -warn-common option can produce the following 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.

- Turning a common symbol into a reference, because there is already a definition for the symbol.

file( section): warning: common of 'symbol'
overridden by definition
file( section): warning: defined here

- Turning a common symbol into a reference, because a later definition for the symbol is encountered. This is the same as the previous case, except that the symbols are encountered in a different order.

file( section): warning: definition of symbol
overriding common
file( section): warning: common is here

- Merging a common symbol with a previous same-sized common symbol.

file( section): warning: multiple common
of symbol
file( section): warning: previous common is here

- Merging a common symbol with a previous larger common symbol.

file( section): warning: common of symbol
overridden by larger common
file( section): warning: larger common is here

- Merging a common symbol with a previous smaller common symbol. The following is the same as the previous case, except that the symbols are encountered in a different order.

file( section): warning: common of symbol
overriding smaller common
file( section): warning: smaller common is here

-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-once

Only warn once for each undefined symbol, rather than once per module which refers to it.

For each archive mentioned on the command line, 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.

-X

Delete all temporary local symbols. For most targets, this is all local symbols whose names begin with L.

-x

Delete all local symbols.

-y symbol

Print the name of each linked file in which symbol appears. This option may be given any number of times. On many systems it is necessary to prepend an underscore. This option is useful when you have an undefined symbol in your link but dont know where the reference is coming from.

-(archives-)
--start-group archives--end-group

The archives should be a list of archive files. They may be either explicit file names, or -l options.

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. Use it only when there are unavoidable circular references between two or more archives.

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