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<hr><p align="left"><small>发信人: digger (欧阳疯), 信区: Socket <br>
标 题: RAW sockets example code: in_cksum.c <br>
发信站: 华南网木棉站 (Wed Aug 5 19:57:33 1998), 转信 <br>
<br>
/* <br>
* Copyright (c) 1989, 1993 <br>
* The Regents of the University of California. All rights reserved. <br>
* <br>
* This code is derived from software contributed to Berkeley by <br>
* Mike Muuss. <br>
* <br>
* Redistribution and use in source and binary forms, with or without <br>
* modification, are permitted provided that the following conditions <br>
* are met: <br>
* 1. Redistributions of source code must retain the above copyright <br>
* notice, this list of conditions and the following disclaimer. <br>
* 2. Redistributions in binary form must reproduce the above copyright <br>
* notice, this list of conditions and the following disclaimer in the <br>
* documentation and/or other materials provided with the distribution. <br>
* 3. All advertising materials mentioning features or use of this software <br>
* must display the following acknowledgement: <br>
* This product includes software developed by the University of <br>
* California, Berkeley and its contributors. <br>
* 4. Neither the name of the University nor the names of its contributors <br>
* may be used to endorse or promote products derived from this software <br>
* without specific prior written permission. <br>
* <br>
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND <br>
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE <br>
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE <br>
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE <br>
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL <br>
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS <br>
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) <br>
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT <br>
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY <br>
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF <br>
* SUCH DAMAGE. <br>
*/ <br>
<br>
#include <stdlib.h> <br>
#include <stdio.h> <br>
#include <sys/types.h> <br>
<br>
/* <br>
/* <br>
* in_cksum -- <br>
* Checksum routine for Internet Protocol family headers (C Version) <br>
*/ <br>
unsigned short in_cksum(unsigned short *addr,int len) <br>
{ <br>
register int sum = 0; <br>
u_short answer = 0; <br>
register u_short *w = addr; <br>
register int nleft = len; <br>
<br>
/* <br>
* Our algorithm is simple, using a 32 bit accumulator (sum), we add <br>
* sequential 16 bit words to it, and at the end, fold back all the <br>
* carry bits from the top 16 bits into the lower 16 bits. <br>
*/ <br>
while (nleft > 1) { <br>
sum += *w++; <br>
nleft -= 2; <br>
} <br>
<br>
/* mop up an odd byte, if necessary */ <br>
if (nleft == 1) { <br>
*(u_char *)(&answer) = *(u_char *)w ; <br>
sum += answer; <br>
} <br>
<br>
/* add back carry outs from top 16 bits to low 16 bits */ <br>
sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */ <br>
sum += (sum >> 16); /* add carry */ <br>
answer = ~sum; /* truncate to 16 bits */ <br>
return(answer); <br>
} <br>
<br>
-- <br>
#################### <br>
# 遥 眺 淑 女 # <br>
####################################### <br>
# 君 子 好 球 # <br>
#################### <br>
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