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3.13 Options for Linking

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.

object-file-name

A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.

-c
-S
-E

If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See section Options Controlling the Kind of Output.

-llibrary
-l library

Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file ‘foo.o’ but before ‘bar.o’. If ‘bar.o’ refers to functions in ‘z’, those functions may not be loaded.

The linker searches a standard list of directories for the library, which is actually a file named ‘liblibrary.a’. The linker then uses this file as if it had been specified precisely by name.

The directories searched include several standard system directories plus any that you specify with ‘-L’.

Normally the files found this way are library files—archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an ‘-l’ option and specifying a file name is that ‘-l’ surrounds library with ‘lib’ and ‘.a’ and searches several directories.

-lobjc

You need this special case of the ‘-l’ option in order to link an Objective-C or Objective-C++ program.

-nostartfiles

Do not use the standard system startup files when linking. The standard system libraries are used normally, unless ‘-nostdlib’ or ‘-nodefaultlibs’ is used.

-nodefaultlibs

Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless ‘-nostartfiles’ is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

-nostdlib

Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by ‘-nostdlib’ and ‘-nodefaultlibs’ is ‘libgcc.a’, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See Interfacing to GCC Output in GNU Compiler Collection (GCC) Internals, for more discussion of ‘libgcc.a’.) In most cases, you need ‘libgcc.a’ even when you want to avoid other standard libraries. In other words, when you specify ‘-nostdlib’ or ‘-nodefaultlibs’ you should usually specify ‘-lgcc’ as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is ‘__main’, used to ensure C++ constructors are called; see collect2 in GNU Compiler Collection (GCC) Internals.)

-pie

Produce a position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (‘-fpie’, ‘-fPIE’, or model suboptions) when you specify this linker option.

-rdynamic

Pass the flag ‘-export-dynamic’ to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program.

-s

Remove all symbol table and relocation information from the executable.

-static

On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.

-shared

Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (‘-fpic’, ‘-fPIC’, or model suboptions) when you specify this linker option.(1)

-shared-libgcc
-static-libgcc

On systems that provide ‘libgcc’ as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of ‘libgcc’ was built when the compiler was configured, these options have no effect.

There are several situations in which an application should use the shared ‘libgcc’ instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared ‘libgcc’.

Therefore, the G++ and GCJ drivers automatically add ‘-shared-libgcc’ whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared ‘libgcc’. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option ‘--eh-frame-hdr’, it links the shared version of ‘libgcc’ into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of ‘libgcc’, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option ‘-shared-libgcc’, such that it is linked with the shared ‘libgcc’.

-static-libasan

When the ‘-fsanitize=address’ option is used to link a program, the GCC driver automatically links against ‘libasan’. If ‘libasan’ is available as a shared library, and the ‘-static’ option is not used, then this links against the shared version of ‘libasan’. The ‘-static-libasan’ option directs the GCC driver to link ‘libasan’ statically, without necessarily linking other libraries statically.

-static-libtsan

When the ‘-fsanitize=thread’ option is used to link a program, the GCC driver automatically links against ‘libtsan’. If ‘libtsan’ is available as a shared library, and the ‘-static’ option is not used, then this links against the shared version of ‘libtsan’. The ‘-static-libtsan’ option directs the GCC driver to link ‘libtsan’ statically, without necessarily linking other libraries statically.

-static-libstdc++

When the g++ program is used to link a C++ program, it normally automatically links against ‘libstdc++’. If ‘libstdc++’ is available as a shared library, and the ‘-static’ option is not used, then this links against the shared version of ‘libstdc++’. That is normally fine. However, it is sometimes useful to freeze the version of ‘libstdc++’ used by the program without going all the way to a fully static link. The ‘-static-libstdc++’ option directs the g++ driver to link ‘libstdc++’ statically, without necessarily linking other libraries statically.

-symbolic

Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option ‘-Xlinker -z -Xlinker defs’). Only a few systems support this option.

-T script

Use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the ‘-T’ option may be required when linking to avoid references to undefined symbols.

-Xlinker option

Pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.

If you want to pass an option that takes a separate argument, you must use ‘-Xlinker’ twice, once for the option and once for the argument. For example, to pass ‘-assert definitions’, you must write ‘-Xlinker -assert -Xlinker definitions’. It does not work to write ‘-Xlinker "-assert definitions"’, because this passes the entire string as a single argument, which is not what the linker expects.

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the ‘option=value’ syntax than as separate arguments. For example, you can specify ‘-Xlinker -Map=output.map’ rather than ‘-Xlinker -Map -Xlinker output.map’. Other linkers may not support this syntax for command-line options.

-Wl,option

Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, ‘-Wl,-Map,output.map’ passes ‘-Map output.map’ to the linker. When using the GNU linker, you can also get the same effect with ‘-Wl,-Map=output.map’.

-u symbol

Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use ‘-u’ multiple times with different symbols to force loading of additional library modules.


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