in_cksum.c

早期freebsd实现

/*-
 * Copyright (c) 1990, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * from tahoe:	in_cksum.c	1.2	86/01/05
 *	@(#)in_cksum.c	8.1 (Berkeley) 6/11/93
 */

#include <sys/param.h>
#include <sys/mbuf.h>

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

/*
 * Checksum routine for Internet Protocol family headers.
 *
 * This routine is very heavily used in the network
 * code and should be modified for each CPU to be as fast as possible.
 * 
 * This implementation is 386 version.
 */

#undef	ADDCARRY
#define ADDCARRY(sum)  {				\
			if (sum & 0xffff0000) {		\
				sum &= 0xffff;		\
				sum++;			\
			}				\
		}
in_cksum(m, len)
	register struct mbuf *m;
	register int len;
{
	union word {
		char	c[2];
		u_short	s;
	} u;
	register u_short *w;
	register int sum = 0;
	register int mlen = 0;

	for (;m && len; m = m->m_next) {
		if (m->m_len == 0)
			continue;
		w = mtod(m, u_short *);
		if (mlen == -1) {
			/*
			 * The first byte of this mbuf is the continuation
			 * of a word spanning between this mbuf and the
			 * last mbuf.
			 */

			/* u.c[0] is already saved when scanning previous 
			 * mbuf.
			 */
			u.c[1] = *(u_char *)w;
			sum += u.s;
			ADDCARRY(sum);
			w = (u_short *)((char *)w + 1);
			mlen = m->m_len - 1;
			len--;
		} else
			mlen = m->m_len;

		if (len < mlen)
			mlen = len;
		len -= mlen;

		/*
		 * add by words.
		 */
		while ((mlen -= 2) >= 0) {
			if ((int)w & 0x1) {
				/* word is not aligned */
				u.c[0] = *(char *)w;
				u.c[1] = *((char *)w+1);
				sum += u.s;
				w++;
			} else
				sum += *w++;
			ADDCARRY(sum);
		}
		if (mlen == -1)
			/*
			 * This mbuf has odd number of bytes. 
			 * There could be a word split betwen
			 * this mbuf and the next mbuf.
			 * Save the last byte (to prepend to next mbuf).
			 */
			u.c[0] = *(u_char *)w;
	}
	if (len)
		printf("cksum: out of data\n");
	if (mlen == -1) {
		/* The last mbuf has odd # of bytes. Follow the
		   standard (the odd byte is shifted left by 8 bits) */
		u.c[1] = 0;
		sum += u.s;
		ADDCARRY(sum);
	}
	return (~sum & 0xffff);
}