c - Restricting symbols in a Linux static library

Question

I'm looking for ways to restrict the number of C symbols exported to a Linux static library (archive). I'd like to limit these to only those symbols that are part of the official API for the library. I already use 'static' to declare most functions as static, but this restricts them to file scope. I'm looking for a way to restrict to scope to the library.

I can do this for shared libraries using the techniques in Ulrich Drepper's How to Write Shared Libraries, but I can't apply these techniques to static archives. In his earlier Good Practices in Library Design paper, he writes:

The only possibility is to combine all object files which need certain internal resources into one using 'ld -r' and then restrict the symbols which are exported by this combined object file. The GNU linker has options to do just this.

Could anyone help me discover what these options might be? I've had some success with 'strip -w -K prefix_*', but this feels brutish. Ideally, I'd like a solution that will work with both GCC 3 and 4.

Thanks!

Answer 1

I don't believe GNU ld has any such options; Ulrich must have meant objcopy, which has many such options: --localize-hidden, --localize-symbol=symbolname, --localize-symbols=filename.

The --localize-hidden in particular allows one to have a very fine control over which symbols are exposed. Consider:

int foo() { return 42; }
int __attribute__((visibility("hidden"))) bar() { return 24; }

gcc -c foo.c
nm foo.o
000000000000000b T bar
0000000000000000 T foo

objcopy --localize-hidden foo.o bar.o
nm bar.o
000000000000000b t bar
0000000000000000 T foo

So bar() is no longer exported from the object (even though it is still present and usable for debugging). You could also remove bar() all together with objcopy --strip-unneeded.