Override a C function defined in a static library

Question

I have a static library of C files, compiled with g++ on Cygwin. I wish to unit test one function that is defined in the library. That function calls another function defined in that library and I wish to override the dependency to replace it with my own version of that function. I can't modify what's in the static library, so this solution [ Override a function call in C ] doesn't apply.

Usually, I can write a .cpp file and include the .c file containing the function I want to unit test, which essentially extends that file with the code I add. It's a dirty trick I'd never use for production code but it's handy for unit testing C files, because it gives my test code access to static things in that C file. Then, I can write in my fake version of the dependency, and my unit test function that calls the function I'm testing. I compile my.cpp to get my.o, then link it with the static library. In theory, since the linker has found a definition for the dependency already (the one I provide) it won't look in the library and there will be no conflict. Usually this works, but now I'm getting a "multiple definition" error where the linker first finds my fake and then finds the real one. I don't know what might cause this and don't know what to look for. I also can't boil this down to a simple example because my simple examples don't have this problem.

Ideas please?

Answer 1

One possibility (admittedly, and ugly one, but...) is to extract the individual object files from the static library. If the function you're calling and the function it's calling are in separate object files, you can link against the object file containing the function you need to call, but not against the one containing the function it calls.

This only gives you granularity on the level of complete object files though, so if the two functions involved are both in the same object file, it won't work. If you really need to get things to work, and don't mind making a really minor modification to the object file in question, you may be able to use a binary editor to mark the second function as a weak external, which means it'll be used in the absence of any other external with the same name, but if another is provided, that will be used instead.

Whether that latter qualifies as "modifying the library" or not depends a bit on your viewpoint. It's not modifying the code in the library, but is modifying a bit of the object file wrapper around that code. My guess is that you'd rather not do it, but it may still be the cleanest way out of an otherwise untenable situation.