Override a function call in C

Question

I want to override certain function calls to various APIs for the sake of logging the calls, but I also might want to manipulate data before it is sent to the actual function.

For example, say I use a function called getObjectName thousands of times in my source code. I want to temporarily override this function sometimes because I want to change the behaviour of this function to see the different result.

I create a new source file like this:

#include <apiheader.h>    

const char *getObjectName (object *anObject)
{
    if (anObject == NULL)
        return "(null)";
    else
        return "name should be here";
}

I compile all my other source as I normally would, but I link it against this function first before linking with the API's library. This works fine except I can obviously not call the real function inside my overriding function.

Is there an easier way to "override" a function without getting linking/compiling errors/warnings? Ideally I want to be able to override the function by just compiling and linking an extra file or two rather than fiddle around with linking options or altering the actual source code of my program.

Answer 1

If it's only for your source that you want to capture/modify the calls, the simplest solution is to put together a header file (intercept.h) with:

#ifdef INTERCEPT
    #define getObjectName(x) myGetObjectName(x)
#endif

Then you implement the function as follows (in intercept.c which doesn't include intercept.h):

const char *myGetObjectName (object *anObject) {
    if (anObject == NULL) return "(null)";
    return getObjectName(anObject);

Then make sure each source file where you want to intercept the call has the following at the top:

#include "intercept.h"

When you compile with "-DINTERCEPT", all files will call your function rather than the real one, whereas your function will still call the real one.

Compiling without the "-DINTERCEPT" will prevent interception from occurring.

It's a bit trickier if you want to intercept all calls (not just those from your source) - this can generally be done with dynamic loading and resolution of the real function (with dlload- and dlsym-type calls) but I don't think it's necessary in your case.