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34 static char sccsid[] = "@(#)cksum.c 8.1 (Berkeley) 6/6/93";
36 static const char rcsid[] =
40 #include <sys/types.h>
45 * Checksum routine for Internet Protocol family headers (C Version)
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.
50 * All timed packets are smaller than 32K shorts, so there is no need to
51 * worry about carries except at the end.
54 in_cksum(u_short *addr, int len)
56 register int nleft = len;
57 register u_short *w = addr;
58 register u_short answer;
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
72 /* mop up an odd byte, if necessary */
74 sum += (*(u_char *)w) << 8;
77 * add back carry outs from top 16 bits to low 16 bits
79 sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
80 sum += (sum >> 16); /* add carry */
81 answer = ~sum; /* truncate to 16 bits */