How to wrap a library function with custom code?

Question

I have a library which implements malloc. I want to override this function with a custom malloc function that does something and then calls the malloc function of the library.

How can i redefine the symbol malloc without losing the function from the library?

Answer 1

The GNU linker provides the --wrap symbol flag to wrap a custom function around an existing function.

As you can read here, last flag: http://ieee.uwaterloo.ca/coldfire/gcc-doc/docs/ld_3.html#SEC3

--wrap symbol

Use a wrapper function for symbol. Any undefined reference to symbol will be resolved to __wrap_symbol. Any undefined reference to __real_symbol will be resolved to symbol. This can used to provide a wrapper for a system function. The wrapper function should be called __wrap_symbol. If it wishes to call the system function, it should call __real_symbol. Here is a trivial example:

void *
__wrap_malloc (int c) {
    printf ("malloc called with %ld\n", c);
    return __real_malloc (c);
}

If you link other code with this file using --wrap malloc, then all calls to malloc will call the function __wrap_malloc instead. The call to __real_malloc in __wrap_malloc will call the real malloc function. You may wish to provide a __real_malloc function as well, so that links without the --wrap option will succeed. If you do this, you should not put the definition of __real_malloc in the same file as __wrap_malloc; if you do, the assembler may resolve the call before the linker has a chance to wrap it to malloc.

Answer 2

it is not a good practice to create new functions with the names of library functions. If at all you want to create such a function, your new function will work fine but you can not use the library function. If you want to create your own version of printf(), you can do so but you can not use the original printf() function as you are overriding.

Answer 3

If both functions have the same prototype, you can use a function pointer

#include <stdlib.h>

void *my_malloc(size_t len) {
    return malloc(len);
}

int main(void) {
    void *(*malloc_pointer)(size_t);

    malloc_pointer = my_malloc;
    malloc_pointer(42); // uses my_malloc;
    malloc_pointer = malloc;
    malloc_pointer(42); // uses malloc;

    return 0;
}