how do I always include symbols from a static library?

Question

Suppose I have a static library libx.a. How to I make some symbols (not all) from this library to be always present in any binary I link with my library? Reason is that I need these symbols to be available via dlopen+dlsym. I'm aware of --whole-archive linker switch, but it forces all object files from library archive to linked into resulting binary, and that is not what I want...

Observations so far (CentOS 5.4, 32bit) (upd: this paragraph is wrong; I could not reproduce this behaviour)

ld main.o libx.a

will happily strip all non-referenced symbols, while

ld main.o -L. -lx

will link whole library in. I guess this depends on version of binutils used, however, and newer linkers will be able to cherry-pick individual objects from a static library.

Another question is how can I achieve the same effect under Windows?

Thanks in advance. Any hints will be greatly appreciated.

Answer 1

I would start with splitting off those symbols you always need into a seperate library, retaining only the optional ones in libx.a.

Answer 2

Take an address of the symbol you need to include.

If gcc's optimiser anyway eliminates it, do something with this address - should be enough.

Answer 3

First things first: ld main.o libx.a does not build a valid executable. In general, you should never use ld to link anything directly; always use proper compiler driver (gcc in this case) instead.

Also, "ld main.o libx.a" and "ld main.o -L. -lx" should be exactly equivalent. I am very doubtful you actually got different results from these two commands.

Now to answer your question: if you want foo, bar and baz to be exported from your a.out, do this:

gcc -Wl,-u,foo,-u,bar,-u,baz main.o -L. -lx -rdynamic

Update:
your statement: "symbols I want to include are used by library internally only" doesn't make much sense: if the symbols are internal to the library, why do you want to export them? And if something else uses them (via dlsym), then they are not internal to the library -- they are part of the library public API.

You should clarify your question and explain what you really are trying to achieve. Providing sample code will not hurt either.