c - ICMP 校验和不正确

Question

我在 x64 机器上。以下是我计算 ICMP 校验和的方法:

unsigned short in_checksum(unsigned short *ptr, int n_bytes)
{
  register long sum;
  u_short odd_byte;
  register u_short ret_checksum;

  while (n_bytes > 1)
  {
    sum += *ptr++;
    n_bytes -= 2;
  }

  if (n_bytes == 1)
  {
    odd_byte = 0;
    *((u_char *) & odd_byte) = * (u_char *) ptr;
    sum += odd_byte;
  }

  sum = (sum >> 16) + (sum & 0xffff);
  sum += (sum >> 16);
  ret_checksum = ~sum;

  return ret_checksum;
}

当我通过 wireshark 嗅探发送的数据包时，我总是说每个 icmp 数据包的校验和不正确。这是怎么回事？

Answer 1

你忘了初始化

  register long sum;

到 0。将选项 -W 传递给 gcc 会告诉您。

...: In function 'in_checksum':...: warning: 'sum' may be used uninitialized in this function