Function mocking (for testing) in C?

Question

I would like to write tests for a C library, in C. I'd like to mock out some functions for the test.

Suppose my library is compiled from the following source:

/* foo.h */
int myfunction(int x, int y);

/* foo.c */
#include "foo.h"

static int square(int x) { return x * x; }

int myfunction(int x, int y) {
    return square(x) + square(y);
}

I want to write a test like this:

/* foo_test.c */
#include "foo.h"

static int square(int x) { return x + 1; }

int main(void) {
    assert(myfunction(0, 0) == 2);
    return 0;
}

Is there any way I can compile so that myfunction will use the definition of square in foo_test.c, instead of the one in foo.c, only when linking the executable foo_test? That is, I want to compile foo.c into a library (let's call it libfoo.so), and then compile foo_test.c with libfoo.so and some magic so that I'll get an executable foo_test which uses the different implementation of square.

It would be helpful to hear solutions for when square is not declared static, but solving the above case would be even better.

EDIT: It seems hopeless, but here's an idea: Suppose I compile with -O0 -g so it's unlikely that square will get inlined and I should have symbols showing where the call was resolved. Is there a way to sneak into the object file and swap out the resolved reference?

Answer 1

It looks like you are using GCC, so you can use the weak attribute:

The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.

http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html