Is it possible to exchange a C function implementation at run time?

Question

I have implemented a facade pattern that uses C functions underneath and I would like to test it properly.

I do not really have control over these C functions. They are implemented in a header. Right now I #ifdef to use the real headers in production and my mock headers in tests. Is there a way in C to exchange the C functions at runtime by overwriting the C function address or something? I would like to get rid of the #ifdef in my code.

Answer 1

To expand on Bart's answer, consider the following trivial example.

#include <stdio.h>
#include <stdlib.h>

int (*functionPtr)(const char *format, ...);

int myPrintf(const char *fmt, ...)
{
    char *tmpFmt = strdup(fmt);
    int i;
    for (i=0; i<strlen(tmpFmt); i++)
        tmpFmt[i] = toupper(tmpFmt[i]);

// notice - we only print an upper case version of the format
// we totally disregard all but the first parameter to the function
    printf(tmpFmt);

    free(tmpFmt);
}

int main()
{

    functionPtr = printf;
    functionPtr("Hello world! - %d\n", 2013);

    functionPtr = myPrintf;
    functionPtr("Hello world! - %d\n", 2013);

    return 0;
}

Output

Hello World! - 2013
HELLO WORLD! - %D