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

				f (b, c)				     unsigned char *b;				     int c;				{				  unsigned long v = 0;				  switch (c)				    {				    case 'd':				      v = ((unsigned long)b[0] 				      v >>= 9;				      break;								    case 'k':				      v = b[3] >> 4;				      break;								    default:				      abort ();				    }								  return v;				}				main ()				{				  char buf[4];				  buf[0] = 170; buf[1] = 5;				  if (f (buf, 'd') != 85)				    abort ();				  exit (0);				}