c++ - LITTLE_ENDIAN 机器也将修复 BIG_ENDIAN 代码

Question

我有这段遗留代码，我需要在 BIG 和 LITTLE Endian 机器上运行。问题出在 hton() 上。

msg->Mac 是 char Mac[16+1]现有代码:(仅适用于 BIG)

if (sscanf(msg->Mac, "%4hx.%4hx.%4hx", (unsigned short *)&new_mac[0],
              (unsigned short *)&new_mac[2],
              (unsigned short *)&new_mac[4]) != 3) {
  return (ERROR_ADDRESS_TRANSLATION);
 }

*(unsigned short *)&new_mac[0] = hton(*(unsigned short *)&new_mac[0]);
*(unsigned short *)&new_mac[2] = hton(*(unsigned short *)&new_mac[2]);
*(unsigned short *)&new_mac[4] = hton(*(unsigned short *)&new_mac[4]);

sprintf((char *)newMac, "%04x.%04x.%04x", *(unsigned short *)&new_mac[0],
        *(unsigned short *)&new_mac[2], *(unsigned short *)&new_mac[4]);

 /* Get the MAC address */
if (sscanf((char *)newMac, "%4hx.%4hx.%4hx", (unsigned short *)&mac_addr[0],
              (unsigned short *)&mac_addr[2],
              (unsigned short *)&mac_addr[4]) != 3) {
      return (ERROR_ADDRESS_TRANSLATION);
 }


 /* Convert to network order */
 *(unsigned short *)&mac_addr[0] = hton(*(unsigned short *)&mac_addr[0]);
 *(unsigned short *)&mac_addr[2] = hton(*(unsigned short *)&mac_addr[2]);
 *(unsigned short *)&mac_addr[4] = hton(*(unsigned short *)&mac_addr[4]);

为了在 LITTLE Endian 机器上解决这个问题，我使用了一个 SWAP 宏，它可以在短时间内交换字节。这是正确的方法吗？

我在上面添加的代码:(使其也适用于 LITTLE)

#if __BYTE_ORDER != __BIG_ENDIAN

*(unsigned short *)&mac_addr[0] = SWAP(*(unsigned short *)&mac_addr[0]);
*(unsigned short *)&mac_addr[2] = SWAP(*(unsigned short *)&mac_addr[2]);
*(unsigned short *)&mac_addr[4] = SWAP(*(unsigned short *)&mac_addr[4]);

#endif

Answer 1

你在小端机器上测试过吗？由于 hton 已经考虑了拱字节性:

/* Copyright (C) 1993-2015 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <stdint.h>
#include <netinet/in.h>

#undef  htonl
#undef  ntohl

uint32_t
htonl (uint32_t x)
{
#if BYTE_ORDER == BIG_ENDIAN
  return x;
#elif BYTE_ORDER == LITTLE_ENDIAN
  return __bswap_32 (x);
#else
# error "What kind of system is this?"
#endif
}
weak_alias (htonl, ntohl)