Future releases of ld may support similar functionality for other architecture families.

You may want to use this option if you are linking files with an unusual binary format. You can also use -b to switch formats explicitly (when linking object files of different formats), by including -b input-format before each group of object files in a particular format.

The default format is taken from the environment variable ""GNUTARGET"".

You can also define the input format from a script, using the command ""TARGET"";

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 @ref{VERSION}.

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.

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.

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.

If searchdir begins with ""="", then the ""="" will be replaced by the sysroot prefix, a path specified when the linker is configured.

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.

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.

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.

This option is currently only supported on ELF platforms.

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.

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.

This option is useful when you have an undefined symbol in your link but don’t know where the reference is coming from.

Other keywords are ignored for Solaris compatibility.

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.

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.

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.

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.

The reason that --allow-shlib-undefined is the default is that the shared library being specified at link time may not be the same as the one that is available at load time, so the symbols might actually be resolvable at load time. Plus there are some systems, (eg BeOS) where undefined symbols in shared libraries is normal. (The kernel patches them at load time to select which function is most appropriate for the current architecture. This is used for example to dynamically select an appropriate memset function). Apparently it is also normal for HPPA shared libraries to have undefined symbols.

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 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.

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.

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.

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

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.

The behaviour for shared libraries on their own can also be controlled by the --[no-]allow-shlib-undefined option.

Normally the linker will generate an error message for each reported unresolved symbol but the option --warn-unresolved-symbols can change this to a warning.

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.

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.

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 (size_t c)
{
printf ("malloc called with %zu\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"".

Using ’auto-import’ generally will ’just work’ — but sometimes you may see this message:

"variable ’<var>’ can’t be auto-imported. Please read the documentation for ld’s ""--enable-auto-import"" for details."

This message occurs when some (sub)expression accesses an address ultimately given by the sum of two constants (Win32 import tables only allow one). Instances where this may occur include accesses to member fields of struct variables imported from a DLL, as well as using a constant index into an array variable imported from a DLL. Any multiword variable (arrays, structs, long long, etc) may trigger this error condition. However, regardless of the exact data type of the offending exported variable, ld will always detect it, issue the warning, and exit.

There are several ways to address this difficulty, regardless of the data type of the exported variable:

One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task of adjusting references in your client code for runtime environment, so this method works only when runtime environment supports this feature.

A second solution is to force one of the ’constants’ to be a variable — that is, unknown and un-optimizable at compile time. For arrays, there are two possibilities: a) make the indexee (the array’s address) a variable, or b) make the ’constant’ index a variable. Thus:

extern type extern_array[];
extern_array[1] -->
{ volatile type *t=extern_array; t[1] }

or

extern type extern_array[];
extern_array[1] -->
{ volatile int t=1; extern_array[t] }

For structs (and most other multiword data types) the only option is to make the struct itself (or the long long, or the ...) variable:

extern struct s extern_struct;
extern_struct.field -->
{ volatile struct s *t=&extern_struct; t->field }

or

extern long long extern_ll;
extern_ll -->
{ volatile long long * local_ll=&extern_ll; *local_ll }

A third method of dealing with this difficulty is to abandon ’auto-import’ for the offending symbol and mark it with ""__declspec(dllimport)"". However, in practise that requires using compile-time #defines to indicate whether you are building a DLL, building client code that will link to the DLL, or merely building/linking to a static library. In making the choice between the various methods of resolving the ’direct address with constant offset’ problem, you should consider typical real-world usage:

Original:

--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}

Solution 1:

--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
/* This workaround is for win32 and cygwin; do not "optimize" */
volatile int *parr = arr;
printf("%d\n",parr[1]);
}

Solution 2:

--foo.h
/* Note: auto-export is assumed (no __declspec(dllexport)) */
#if (defined(_WIN32) || defined(__CYGWIN__)) && \
!(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
#define FOO_IMPORT __declspec(dllimport)
#else
#define FOO_IMPORT
#endif
extern FOO_IMPORT int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}

A fourth way to avoid this problem is to re-code your library to use a functional interface rather than a data interface for the offending variables (e.g. set_foo() and get_foo() accessor functions). [This option is specific to the i386 PE targeted port of the linker]