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      1 /*-
      2  * Copyright (c) 1985, 1993 The Regents of the University of California.
      3  * All rights reserved.
      4  *
      5  * Redistribution and use in source and binary forms, with or without
      6  * modification, are permitted provided that the following conditions
      7  * are met:
      8  * 1. Redistributions of source code must retain the above copyright
      9  *    notice, this list of conditions and the following disclaimer.
     10  * 2. Redistributions in binary form must reproduce the above copyright
     11  *    notice, this list of conditions and the following disclaimer in the
     12  *    documentation and/or other materials provided with the distribution.
     13  * 3. Neither the name of the University nor the names of its contributors
     14  *    may be used to endorse or promote products derived from this software
     15  *    without specific prior written permission.
     16  *
     17  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     27  * SUCH DAMAGE.
     28  */
     29 
     30 #include <sys/cdefs.h>
     31 #ifndef lint
     32 #if 0
     33 static char sccsid[] = "@(#)cksum.c	5.2 (Berkeley) 5/11/93";
     34 #else
     35 __RCSID("$NetBSD: cksum.c,v 1.7 2007/05/17 00:36:31 christos Exp $");
     36 #endif
     37 #endif /* not lint */
     38 
     39 #include <sys/types.h>
     40 #include "globals.h"
     41 
     42 /*
     43  *			I N _ C K S U M
     44  *
     45  * Checksum routine for Internet Protocol family headers (C Version)
     46  *
     47  * There is no profit in a specialized version of the checksum
     48  * function for any machine where int's are 32 bits and shorts are 16.
     49  *
     50  * All timed packets are smaller than 32K shorts, so there is no need to
     51  * worry about carries except at the end.
     52  */
     53 int
     54 in_cksum(const void *data, int len)
     55 {
     56 	const u_char *addr = data;
     57 	int nleft = len;
     58 	uint16_t answer;
     59 	int sum = 0;
     60 
     61 	/*
     62 	 *  Our algorithm is simple, using a 32 bit accumulator (sum),
     63 	 *  we add sequential 16 bit words to it, and at the end, fold
     64 	 *  back all the carry bits from the top 16 bits into the lower
     65 	 *  16 bits.
     66 	 */
     67 	while (nleft > 1) {
     68 		uint16_t w;
     69 
     70 		memcpy(&w, addr, sizeof(w));
     71 		sum += w;
     72 		addr += 2;
     73 		nleft -= 2;
     74 	}
     75 
     76 	/* mop up an odd byte, if necessary */
     77 	if (nleft == 1)
     78 		sum += *addr << 8;
     79 
     80 	/*
     81 	 * add back carry outs from top 16 bits to low 16 bits
     82 	 */
     83 	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
     84 	sum += (sum >> 16);			/* add carry */
     85 	answer = ~sum;				/* truncate to 16 bits */
     86 	return (answer);
     87 }
     88