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36 static char sccsid[] = "@(#)cksum.c 8.1 (Berkeley) 6/6/93";
38 static const char rcsid[] =
42 #include <sys/types.h>
47 * Checksum routine for Internet Protocol family headers (C Version)
49 * There is no profit in a specialized version of the checksum
50 * function for any machine where int's are 32 bits and shorts are 16.
52 * All timed packets are smaller than 32K shorts, so there is no need to
53 * worry about carries except at the end.
60 register int nleft = len;
61 register u_short *w = addr;
62 register u_short answer;
66 * Our algorithm is simple, using a 32 bit accumulator (sum),
67 * we add sequential 16 bit words to it, and at the end, fold
68 * back all the carry bits from the top 16 bits into the lower
76 /* mop up an odd byte, if necessary */
78 sum += (*(u_char *)w) << 8;
81 * add back carry outs from top 16 bits to low 16 bits
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 */