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

With gcc, under Linux you can use the --wrap linker flag like this:

gcc program.c -Wl,-wrap,getObjectName -o program

and define your function as:

const char *__wrap_getObjectName (object *anObject)
{
    if (anObject == NULL)
        return "(null)";
    else
        return __real_getObjectName( anObject ); // call the real function
}

This will ensure that all calls to getObjectName() are rerouted to your wrapper function (at link time). This very useful flag is however absent in gcc under Mac OS X.

Remember to declare the wrapper function with extern "C" if you're compiling with g++ though.