cksumlib.c

vxworks源码源码解读是学习vxworks的最佳途径

/* cksumLib.c - checksum routine */

/* Copyright 1992 Wind River Systems, Inc. */

/*
modification history
--------------------
01b,04jul92,smb  added everything above sys/types.h
01a,23mar92,jmm  taken from net source  
*/

/*
DESCRIPTION

*/
#include "vxWorks.h"
#include "sys/types.h"

/*******************************************************************************
*
* checksum - portable internet checksum calculation routine
*
* This checksums the buffer <pAddr> of length <len>.  This routine differs
* from the standard checksum algorithm (in_cksum) in that it checksums a buffer
* while the other checksums an mbuf chain.  For of a description of the
* internet checksum algorithm, please refer to RFC 1071.
*
* RETURNS: checksum
*
* NOMANUAL
*/

u_short checksum
    (
    u_short *		pAddr,			/* pointer to buffer  */
    int			len 			/* length of buffer   */
    )

    {
    int 	nLeft	= len;
    int 	sum 	= 0;
    u_short *	w 	= pAddr;
    u_short 	answer;

    while (nLeft > 1)
	{
	sum 	+= *w++;
	nLeft	-= 2;
	}

    if (nLeft == 1)
	sum += 0 | ((*(u_char *) w) << 8);

    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    answer = sum;

    return (~answer & 0xffff);
    }