Can gdb make a function pointer point to another location?

Question

I\'ll explain:

Let\'s say I\'m interested in replacing the rand() function used by a certain application.

So I attach gdb to this process and ma

Answer 1

This question intrigued me, so I did a little research. What you are looking for is a 'dll injection'. You write a function to replace some library function, put it in a .so, and tell ld to preload your dll. I just tried it out and it worked great! I realize this doesn't really answer your question in relation to gdb, but I think it offers a viable workaround.

For a gdb-only solution, see my other solution.

---

// -*- compile-command: "gcc -Wall -ggdb -o test test.c"; -*-
// test.c

#include "stdio.h"
#include "stdlib.h"

int main(int argc, char** argv)
{
    //should print a fairly random number...
    printf("Super random number: %d\n", rand());

    return 0;
}

---

/ -*- compile-command: "gcc -Wall -fPIC -shared my_rand.c -o my_rand.so"; -*-
//my_rand.c

int rand(void)
{
    return 42;
}

---

compile both files, then run: LD_PRELOAD="./my_rand.so" ./test

Super random number: 42