1  int
       2  ffs (x)
       3       int x;
       4  {
       5    int bit, mask;
       6  
       7    if (x == 0)
       8      return 0;
       9  
      10    for (bit = 1, mask = 1; !(x & mask); bit++, mask <<= 1)
      11      ;
      12  
      13    return bit;
      14  }
      15  
      16  f (x)
      17       int x;
      18  {
      19    int y;
      20    y = ffs (x) - 1;
      21    if (y < 0) 
      22      abort ();
      23  }
      24  
      25  main ()
      26  {
      27    f (1);
      28    exit (0);
      29  }