Sounds silly, but I found the following construct handy:

  int margin_t, margin_r, margin_b, margin_l;
  switch (sscanf (optarg, "%d %d %d %d",
                  &margin_t, &margin_r,
                  &margin_b, &margin_l))
  {
    case 0: err;
    case 1: margin_r = margin_t;
    case 2: margin_b = margin_t;
    case 3: margin_l = margin_r;
  }