loop-6.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

main()
{
  char c;
  char d;
  int nbits;
  c = -1;
  for (nbits = 1 ; nbits < 100; nbits++) {
    d = (1 << nbits) - 1;
    if (d == c)
      break;
  }
  if (nbits == 100)
    abort();
  exit(0);
}