Geekなページさんから ( http://www.geekpage.jp/programming/linux-network/book/12/12-5.php ) checksumのコードを調整したもの。よく忘れるので...

checksum.c

/*
 * 16bit Checksum
 * Endianness?
 */
uint16_t
checksum(uint16_t *buf, size_t bufsz) 
{ 
  uint_least32_t sum = 0; 
  while (bufsz > 1) { 
    sum += *buf; 
    buf++; 
    bufsz -= 2; 
  } 
  if (bufsz == 1) { 
    sum += *(uint8_t *)buf; 
  } 
  sum = (sum & 0xffff) + (sum >> 16); 
  sum = (sum & 0xffff) + (sum >> 16); 
  return ~(uint16_t)(sum & 0xffff); 
}