Override a function call in C

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.

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.

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.

You can override a function using LD_PRELOAD trick - see man ld.so. You compile shared lib with your function and start the binary (you even don't need to modify the binary!) like LD_PRELOAD=mylib.so myprog.

In the body of your function (in shared lib) you write like this:

const char *getObjectName (object *anObject) {
  static char * (*func)();

  if(!func)
    func = (char *(*)()) dlsym(RTLD_NEXT, "getObjectName");
  printf("Overridden!\n");     
  return(func(anObject));    // call original function
}

You can override any function from shared library, even from stdlib, without modifying/recompiling the program, so you could do the trick on programs you don't have a source for. Isn't it nice?

If you use GCC, you can make your function weak. Those can be overridden by non-weak functions:

test.c:

#include <stdio.h>

__attribute__((weak)) void test(void) { 
    printf("not overridden!\n"); 
}

int main() {
    test();
}

What does it do?

$ gcc test.c
$ ./a.out
not overridden!

test1.c:

#include <stdio.h>

void test(void) {
    printf("overridden!\n");
}

What does it do?

$ gcc test1.c test.c
$ ./a.out
overridden!

Sadly, that won't work for other compilers. But you can have the weak declarations that contain overridable functions in their own file, placing just an include into the API implementation files if you are compiling using GCC:

weakdecls.h:

__attribute__((weak)) void test(void);
... other weak function declarations ...

functions.c:

/* for GCC, these will become weak definitions */
#ifdef __GNUC__
#include "weakdecls.h"
#endif

void test(void) { 
    ...
}

... other functions ...

Downside of this is that it does not work entirely without doing something to the api files (needing those three lines and the weakdecls). But once you did that change, functions can be overridden easily by writing a global definition in one file and linking that in.

Override a function call in C

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