The nm(1) command can report the list of symbols in a given library. It works on both static and shared libraries. For a given library nm(1) can list the symbol names defined, each symbol's value, and the symbol's type. It can also identify where the symbol was defined in the source code (by filename and line number), if that information is available in the library (see the -l option).
The symbol type requires a little more explanation. The type is displayed as a letter; lowercase means that the symbol is local, while uppercase means that the symbol is global (external). Typical symbol types include T (a normal definition in the code section), D (initialized data section), B (uninitialized data section), U (undefined; the symbol is used by the library but not defined by the library), and W (weak; if another library also defines this symbol, that definition overrides this one).
If you know the name of a function, but you truly can't remember what library it was defined in, you can use nm's ``-o'' option (which prefixes the filename in each line) along with grep to find the library name. From a Bourne shell, you can search all the libraries in /lib, /usr/lib, direct subdirectories of /usr/lib, and /usr/local/lib for ``cos'' as follows:
nm -o /lib/* /usr/lib/* /usr/lib/*/* \
/usr/local/lib/* 2> /dev/null | grep 'cos$' |
Much more information about nm can be found in the nm ``info'' documentation locally installed at info:binutils#nm.
It's worth noting that the GNU loader permits shared libraries to be text files using a specialized scripting language instead of the usual library format. This is useful for indirectly combining other libraries. For example, here's the listing of /usr/lib/libc.so on one of my systems:
/* GNU ld script Use the shared library, but some functions are only in the static library, so try that secondarily. */ GROUP ( /lib/libc.so.6 /usr/lib/libc_nonshared.a ) |
For more information about this, see the texinfo documentation on ld linker scripts (ld command language). General information is at info:ld#Options and info:ld#Commands, with likely commands discussed in info:ld#Option Commands.
If you're building an application that should port to many systems, you might consider using GNU libtool to build and install libraries. GNU libtool is a generic library support script. Libtool hides the complexity of using shared libraries behind a consistent, portable interface. Libtool provides portable interfaces to create object files, link libraries (static and shared), link executables, debug executables, install libraries, install executables. It also includes libltdl, a portability wrapper for dynamically loading programs. For more information, see its documentation at http://www.gnu.org/software/libtool/manual.html