LD_PRELOAD linux function hook example

ld_preload_hook.c

# helloworld.c
# gcc helloworld.c -o helloworld

#include <stdio.h>
#include <unistd.h>

int main() {
   puts("Hello world!\n");
   return 0;
}

# libexample.c
# gcc libexample.c -o libexample.so -fPIC -shared -ldl -D_GNU_SOURCE

#include <stdio.h>
#include <unistd.h>
#include <dlfcn.h>
#include <string.h>
 
int puts(const char *message) {
   int (*new_puts)(const char *message);
   int result;
   new_puts = dlsym(RTLD_NEXT, "puts");
   if (strcmp(message, "Hello world!\n") == 0) {
       result = new_puts("Goodbye FUCK!\n");
   } else {
       result = new_puts(message);
   }   
   return result;
}