icmp_cksum.c

来自「压缩包中包含LINUX下多个命令的源码」· C语言 代码 · 共 49 行

/* Copyright (C) 1998, 2001 Free Software Foundation, Inc.

   This file is part of GNU Inetutils.

   GNU Inetutils is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

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

   You should have received a copy of the GNU General Public License
   along with GNU Inetutils; see the file COPYING.  If not, write to
   the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA. */

#ifdef HAVE_CONFIG_H
# include <config.h>
#endif

#include <sys/types.h>
#include <unistd.h>

u_short
icmp_cksum (u_char *addr, int len)
{
  register int sum = 0;
  u_short answer = 0;
  u_short *wp;

  for (wp = (u_short*)addr; len > 1; wp++, len -= 2)
    sum += *wp;

  /* Take in an odd byte if present */
  if (len == 1)
    {
      *(u_char *)&answer = *(u_char*)wp;
      sum += answer;
    }

  sum = (sum >> 16) + (sum & 0xffff);	/* add high 16 to low 16 */
  sum += (sum >> 16);			/* add carry */
  answer = ~sum;			/* truncate to 16 bits */
  return answer;
}