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LD_PRELOAD not working

Hi,

I'm trying to create my own version of the printf function. I'm using the following code:

Code:

#ifndef _GNU_SOURCE

#define _GNU_SOURCE

#endif



#include <dlfcn.h>

#include <stdio.h>



int printf(const char *format, ...){



    static int (*real_printf) (const char *format, ...) = NULL;



    if (!real_printf){

            real_printf = dlsym(RTLD_NEXT, "printf");

    }



    return real_printf("my printf\n");

}

I compiled it with:

Code:

$ gcc -shared -o libprinthack.so printhack.o -ldl

I'm trying it out with:

Code:



#include <stdio.h>



int main(void){

  printf("hello world\n");

  return 0;

}

I'm trying to run it with:

Code:

$ LD_PRELOAD=$PWD/libprinthack.so ./test

However, the program still calls the original version. Anyone know what the problem could be? Thanks!

Set LD_DEBUG to 'all' and see the output.

Yeah, gcc is being [un]helpful and changing your printf() to a puts().

You'll need to overload puts as well.

Or you could build the hosting app with -fno-builtin-printf, but that kind of defeats most of the point of LD_PRELOAD..