Using LD, the GNU linker - 2. Invocation

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# 2. Invocation

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.

## 2.1 Command Line Options

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 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, ``--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`
  This option is supported for HP/UX compatibility. The keyword argument must be one of the strings ``archive'`, ``shared'`, or ``default'`. ``-aarchive'` is functionally equivalent to ``-Bstatic'`, and the other two keywords are functionally equivalent to ``-Bdynamic'`. This option may be used any number of times.
`-Aarchitecture`
`--architecture=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 section 4.2 `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 section 3.4.3 Commands dealing with object file formats.
`-c MRI-commandfile`
`--mri-script=MRI-commandfile`
  For compatibility with linkers produced by MRI, `ld` accepts script files written in an alternate, restricted command language, described in section A. 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 supported 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. See section 3.4.4 Other linker script commands.
`-e entry`
`--entry=entry`
  Use entry as the explicit symbol for beginning execution of your program, rather than the default entry point. If there is no symbol named entry, the linker will try to parse entry as a number, and use that as the entry address (the number will be interpreted in base 10; you may use a leading ``0x'` for base 16, or a leading ``0'` for base 8). See section 3.4.1 Setting the entry point, for a discussion of defaults and other ways of specifying the entry point.
`-E`
`--export-dynamic`
  When creating a dynamically linked executable, add all symbols to the dynamic symbol table. The dynamic symbol table is the set of symbols which are visible from dynamic objects at run time. 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.
`-EB`
  Link big-endian objects. This affects the default output format.
`-EL`
  Link little-endian objects. This affects the default output format.
`-f`
`--auxiliary name`
  When creating an ELF shared object, set the internal 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. 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`
  When creating an ELF shared object, set the internal DT_FILTER field to the specified name. This tells the dynamic linker that the symbol table of the shared object which is being created should be used as a filter on the symbol table of the shared object 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.
`-g`
  Ignored. Provided for compatibility with other tools.
`-Gvalue`
`--gpsize=value`
  Set the maximum size of objects to be optimized using the GP register to size. This is only meaningful for object file formats such as MIPS ECOFF which supports putting large and small objects into different sections. This is ignored for other object file formats.
`-hname`
`-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.
`-i`
  Perform an incremental link (same as option ``-r'`).
`-larchive`
`--library=archive`
  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 `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`
  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 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`
  Emulate the emulation linker. You can list the available emulations with the ``--verbose'` or ``-V'` options. 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`
  Print a link map to the standard output. A link map provides information about the link, including the following:
  - Where object files and symbols are mapped into memory.
  - How common symbols are allocated.
  - All archive members included in the link, with a mention of the symbol which caused the archive member to be brought in.
`-n`
`--nmagic`
  Set the text segment to be read only, and mark the output as `NMAGIC` if possible.
`-N`
`--omagic`
  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`.
`-o output`
`--output=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.
`-O level`
  If level is a numeric values greater than zero `ld` optimizes the output. This might take significantly longer and therefore probably should only be enabled for the final binary.
`-r`
`--relocateable`
  Generate 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; to do that, use ``-Ur'`. This option does the same thing as ``-i'`.
`-R filename`
`--just-symbols=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. You may use this option more than once. 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`
  Omit all symbol information from the output file.
`-S`
`--strip-debug`
  Omit debugger symbol information (but not all symbols) from the output file.
`-t`
`--trace`
  Print the names of the input files as `ld` processes them.
`-T scriptfile`
`--script=scriptfile`
  Use scriptfile as the linker script. This script replaces `ld`'s default linker script (rather than adding to it), so commandfile must specify everything necessary to describe the output file. 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 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`
  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. This option is equivalent to the `EXTERN` linker script command.
`-Ur`
  For anything other than C++ programs, this option is equivalent to ``-r'`: it generates relocatable output--i.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.
`-v`
`--version`
`-V`
  Display the version number for `ld`. The `-V` option also lists the supported emulations.
`-x`
`--discard-all`
  Delete all local symbols.
`-X`
`--discard-locals`
  Delete all temporary local symbols. For most targets, this is all local symbols whose names begin with ``L'`.
`-y symbol`
`--trace-symbol=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 don't know where the reference is coming from.
`-Y path`
  Add path to the default library search path. This option exists for Solaris compatibility.
`-z keyword`
  This option is ignored for Solaris compatibility.
`-( 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. It is best to use it only when there are unavoidable circular references between two or more archives.
`-assert keyword`
  This option is ignored for SunOS compatibility.
`-Bdynamic`
`-dy`
`-call_shared`
  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. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for `-l` options which follow it.
`-Bstatic`
`-dn`
`-non_shared`
`-static`
  Do not link against shared libraries. This is only meaningful on platforms for which shared libraries are supported. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for `-l` options which follow it.
`-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 option is only meaningful on ELF platforms which support shared libraries.
`--check-sections`
`--no-check-sections`
  Asks the linker not to check section addresses after they have been assigned to see if there any overlaps. Normally the linker will perform this check, and if it finds any overlaps it will produce suitable error messages. The linker does know about, and does make allowances for sections in overlays. The default behaviour can be restored by using the command line switch ``--check-sections'`.
`--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. 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.
`--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 section 3.5 Assigning Values to Symbols). 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; don't 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.
`--force-exe-suffix`
  Make sure that an output file has a .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`
  Enable garbage collection of unused input sections. It is ignored on targets that do not support this option. This option is not compatible with ``-r'`, nor should it be used with dynamic linking. The default behaviour (of not performing this garbage collection) can be restored by specifying ``--no-gc-sections'` on the command line.
`--help`
  Print a summary of the command-line options on the standard output and exit.
`-Map mapfile`
  Print a link map to the file mapfile. See the description of the ``-M'` option, above.
`--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`
  Normally `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`
  Turn off the effect of the `--whole-archive` option for subsequent archive files.
`--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.
`--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`
  This option is ignored for Linux compatibility.
`-Qy`
  This option is ignored for SVR4 compatibility.
`--relax`
  An option with machine dependent effects. This option is only supported on a few targets. See section 4.1 `ld` and the H8/300. See section 4.2 `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 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 run-time 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. 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`
  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-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. The linker uses the following search paths to locate required shared libraries.
  1. Any directories specified by `-rpath-link` options.
  2. Any directories specified by `-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.
  3. On an ELF system, if the `-rpath` and `rpath-link` options were not used, search the contents of the environment variable `LD_RUN_PATH`.
  4. On SunOS, if the `-rpath` option was not used, search any directories specified using `-L` options.
  5. For a native linker, the contents of the environment variable `LD_LIBRARY_PATH`.
  6. The default directories, normally ``/lib'` and ``/usr/lib'`.
  7. For a native linker on an ELF system, if the file ``/etc/ld.so.conf'` exists, the list of directories found in that file.
  If the required shared library is not found, the linker will issue a warning and continue with the link.
`-shared`
`-Bshareable`
  Create a shared library. This is currently only supported on ELF, XCOFF 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`
  This option tells `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`
  Similar to `--split-by-reloc` but creates a new output section for each input file.
`--split-by-reloc count`
  Trys 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.
`--stats`
  Compute and display statistics about the operation of the linker, such as execution time and memory usage.
`--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.
`-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. 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`
  Display the version number for `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`
  Specify the name of a version script to the linker. This is typically used when creating shared libraries to specify additional information about the version heirarchy for the library being created. This option is only meaningful on ELF platforms which support shared libraries. See section 3.9 VERSION Command.
`--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 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.
  1. 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

  2. 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

  3. 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

  4. 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

  5. Merging a common symbol with a previous smaller common symbol. This 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-multiple-gp`
  Warn if multiple global pointer values are required in the output file. This is only meaningful for certain processors, such as the Alpha. Specifically, some processors put large-valued constants in a special section. A special register (the global pointer) points into the middle of this section, so that constants can be loaded efficiently via a base-register relative addressing mode. Since the offset in base-register relative mode is fixed and relatively small (e.g., 16 bits), this limits the maximum size of the constant pool. Thus, in large programs, it is often necessary to use multiple global pointer values in order to be able to address all possible constants. This option causes a warning to be issued whenever this case occurs.
`--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 (see section 3.6 SECTIONS command).
`--whole-archive`
  For each archive mentioned on the command line after the `--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`
  Use a wrapper function for symbol. Any undefined reference to symbol will be resolved to `__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`.

### 2.1.1 Options specific to i386 PE targets

The i386 PE linker supports the `-shared` option, which causes the output to be a dynamically linked library (DLL) instead of a normal executable. You should name the output `*.dll` when you use this option. In addition, the linker fully supports the standard `*.def` files, which may be specified on the linker command line like an object file (in fact, it should precede archives it exports symbols from, to ensure that they get linked in, just like a normal object file).

In addition to the options common to all targets, the i386 PE linker support additional command line options that are specific to the i386 PE target. Options that take values may be separated from their values by either a space or an equals sign.

`--add-stdcall-alias`
  If given, symbols with a stdcall suffix (@nn) will be exported as-is and also with the suffix stripped.
`--base-file file`
  Use file as the name of a file in which to save the base addresses of all the relocations needed for generating DLLs with ``dlltool'`.
`--dll`
  Create a DLL instead of a regular executable. You may also use `-shared` or specify a `LIBRARY` in a given `.def` file.
`--enable-stdcall-fixup`
`--disable-stdcall-fixup`
  If the link finds a symbol that it cannot resolve, it will attempt to do "fuzzy linking" by looking for another defined symbol that differs only in the format of the symbol name (cdecl vs stdcall) and will resolve that symbol by linking to the match. For example, the undefined symbol `_foo` might be linked to the function `_foo@12`, or the undefined symbol `_bar@16` might be linked to the function `_bar`. When the linker does this, it prints a warning, since it normally should have failed to link, but sometimes import libraries generated from third-party dlls may need this feature to be usable. If you specify `--enable-stdcall-fixup`, this feature is fully enabled and warnings are not printed. If you specify `--disable-stdcall-fixup`, this feature is disabled and such mismatches are considered to be errors.
`--export-all-symbols`
  If given, all global symbols in the objects used to build a DLL will be exported by the DLL. Note that this is the default if there otherwise wouldn't be any exported symbols. When symbols are explicitly exported via DEF files or implicitly exported via function attributes, the default is to not export anything else unless this option is given. Note that the symbols `DllMain@12`, `DllEntryPoint@0`, and `impure_ptr` will not be automatically exported.
`--exclude-symbols symbol,symbol,...`
  Specifies a list of symbols which should not be automatically exported. The symbol names may be delimited by commas or colons.
`--file-alignment`
  Specify the file alignment. Sections in the file will always begin at file offsets which are multiples of this number. This defaults to 512.
`--heap reserve`
`--heap reserve,commit`
  Specify the amount of memory to reserve (and optionally commit) to be used as heap for this program. The default is 1Mb reserved, 4K committed.
`--image-base value`
  Use value as the base address of your program or dll. This is the lowest memory location that will be used when your program or dll is loaded. To reduce the need to relocate and improve performance of your dlls, each should have a unique base address and not overlap any other dlls. The default is 0x400000 for executables, and 0x10000000 for dlls.
`--kill-at`
  If given, the stdcall suffixes (@nn) will be stripped from symbols before they are exported.
`--major-image-version value`
  Sets the major number of the "image version". Defaults to 1.
`--major-os-version value`
  Sets the major number of the "os version". Defaults to 4.
`--major-subsystem-version value`
  Sets the major number of the "subsystem version". Defaults to 4.
`--minor-image-version value`
  Sets the minor number of the "image version". Defaults to 0.
`--minor-os-version value`
  Sets the minor number of the "os version". Defaults to 0.
`--minor-subsystem-version value`
  Sets the minor number of the "subsystem version". Defaults to 0.
`--output-def file`
  The linker will create the file file which will contain a DEF file corresponding to the DLL the linker is generating. This DEF file (which should be called `*.def`) may be used to create an import library with `dlltool` or may be used as a reference to automatically or implicitly exported symbols.
`--section-alignment`
  Sets the section alignment. Sections in memory will always begin at addresses which are a multiple of this number. Defaults to 0x1000.
`--stack reserve`
`--stack reserve,commit`
  Specify the amount of memory to reserve (and optionally commit) to be used as stack for this program. The default is 32Mb reserved, 4K committed.
`--subsystem which`
`--subsystem which:major`
`--subsystem which:major.minor`
  Specifies the subsystem under which your program will execute. The legal values for which are `native`, `windows`, `console`, and `posix`. You may optionally set the subsystem version also.

## 2.2 Environment Variables

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 5. 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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