Returning a shared library symbol table

Question

For instance:

void* sdl_library = dlopen("libSDL.so", RTLD_LAZY);
void* initializer = dlsym(sdl_library,"SDL_Init");

Assuming no errors, initializer will point to the function SD_Init in the shared library libSDK.so.

However this requires knowing the symbol "SDL_Init" exists.

Is it possibly to query a library for all its symbols? Eg, in this case it would return SDL_Init, the function pointer, and any other symbols exported by libSDL.so.

Answer 1

There is no libc function to do that. However, you can write one yourself (though the code is somewhat involved).

On Linux, dlopen() in fact returns the address of a link_map structure, which has a member named l_addr that points to the base address of the loaded shared object (assuming your system doesn't randomize shared library placement, and that your library has not been prelinked).

On Linux, a sure way to find the base address (the address of Elf*_Ehdr) is to use dl_iterate_phdr() after dlopen()ing the library.

Having the ELF header, you should be able to iterate over a list of exported symbols (the dynamic symbol table), by first locating the Elf*_Phdr of type PT_DYNAMIC, and then locating DT_SYMTAB, DT_STRTAB entries, and iterating over all symbols in the dynamic symbol table. Use /usr/include/elf.h to guide you.

Additionally, you could use libelf, but I'm unable to guide you since I don't have previous experience with it.

Finally note that the exercise is somewhat futile: you'll get a list of defined functions, but you'll have no idea how to call them (what parameters they expect), so what's the point?

Answer 2

I don't think there is a published API for this. You can either use the nm tool from binutils or examine its source code: http://sourceware.org/cgi-bin/cvsweb.cgi/src/binutils/?cvsroot=src

http://sourceware.org/cgi-bin/cvsweb.cgi/src/binutils/nm.c?rev=1.63&content-type=text/x-cvsweb-markup&cvsroot=src

(obviously assuming elf)