LD_PRELOAD example

/*
 * this define is important
 * do not delete it or gcc will complain that RTLD_NEXT is undefined
 * http://stackoverflow.com/questions/1777397/rtld-next-undeclared
 */
#define _GNU_SOURCE

#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
#include <stdarg.h>

int printf(const char *format, ...)
{
    va_list list;
    char *parg;
    /* function pointer for original printf */
    typeof(printf) *old_printf;

    /* get the parameters */
    va_start(list, format);
    vasprintf(&parg, format, list);
    va_end(list);

    /* what the log function should be */
    puts("hooked printf success\n");

    /* call the original printf */
    old_printf = dlsym(RTLD_NEXT, "printf");
    (*old_printf)("%s", parg);
}