Link Options - Using the GNU Compiler Collection (GCC) (2024)

It makes a difference where in the command you write this option; thelinker searches and processes libraries and object files in the order theyare specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’after file foo.o but before bar.o. If bar.o refersto 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 linkerthen uses this file as if it had been specified precisely by name.

The directories searched include several standard system directoriesplus any that you specify with -L.

Normally the files found this way are library files—archive fileswhose members are object files. The linker handles an archive file byscanning through it for members which define symbols that have so farbeen referenced but not defined. But if the file that is found is anordinary object file, it is linked in the usual fashion. The onlydifference between using an -l option and specifying a file nameis that -l surrounds library with ‘lib’ and ‘.a’and searches several directories.

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

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

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

There are several situations in which an application should use theshared libgcc instead of the static version. The most commonof these is when the application wishes to throw and catch exceptionsacross different shared libraries. In that case, each of the librariesas 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 mainexecutable, because C++ and Java programs typically use exceptions, sothis is the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you mayfind that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linkeror a GNU linker that does not support option --eh-frame-hdr,it links the shared version of libgcc into shared librariesby default. Otherwise, it takes advantage of the linker and optimizesaway the linking with the shared version of libgcc, linking withthe static version of libgcc by default. This allows exceptions topropagate through such shared libraries, without incurring relocationcosts at library load time.

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

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 entirestring as a single argument, which is not what the linker expects.

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