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.
Two special functions aid in initializing and finalizing a module: _init and _fini. If a function ``_init'' is exported in a library, then it is called when the library is first opened (via dlopen() or simply as a shared library). In a C program, this just means that you defined some function named _init. There is a corresponding function called _fini, which is called whenever a client finishes using the library (via a call dlclose() that brings its reference count to zero, or on normal exit of the program). The C prototypes for these functions are:
void _init(void); void _fini(void); |
When compiling the file into a ``.o'' file in gcc, be sure to add the gcc option ``-nostartfiles''. This keeps the C compiler from linking the system startup libraries against the .so file. Otherwise, you'll get a ``multiple-definition'' error. My thanks to Jim Mischel and Tim Gentry for their suggestion to add this discussion of _init and _fini, as well as help in creating it.
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
All the symbols included in generated files are useful for debugging, but take up space. If you need space, you can eliminate some of it.
The best approach is to first generate the object files normally, and do all your debugging and testing first (debugging and testing is much easier with them). Afterwards, once you've tested the program thoroughly, use strip(1) to remove the symbols. The strip(1) command gives you a good deal of control over what symbols to eliminate; see its documentation for details.
Another approach is to use the GNU ld options ``-S'' and ``-s''; ``-S'' omits debugger symbol information (but not all symbols) from the output file, while ``-s'' omits all symbol information from the output file. You can invoke these options through gcc as ``-Wl,-S'' and ``-Wl,-s''. If you always strip the symbols and these options are sufficient, feel free, but this is a less flexible approach.
You might find the paper Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux useful. It describes how to make a truly tiny program executable. Frankly, you shouldn't use most of these tricks under normal circumstances, but they're quite instructive in showing how ELF really works.