Setgid example

Compile a program like the following

 main() {
  int fd;

  /* get access to privileged resource */
  fd = open( "/etc/shadow", O_RDONLY );
  
  /* become invoking user again */
  if ( setgid( getgid() ) < 0 ) {
      printf(stderr, "Can't get back to original gid - bailing out!\n");
      exit(1);
  }

  /* do something with fd now */
 }