LXR busybox/libbb/inet

   1/*
   2 * Checksum routine for Internet Protocol family headers (C Version)
   3 *
   4 * Licensed under GPLv2, see file LICENSE in this source tree.
   5 */
   6
   7#include "libbb.h"
   8
   9uint16_t FAST_FUNC inet_cksum(uint16_t *addr, int nleft)
  10{
  11        /*
  12         * Our algorithm is simple, using a 32 bit accumulator,
  13         * we add sequential 16 bit words to it, and at the end, fold
  14         * back all the carry bits from the top 16 bits into the lower
  15         * 16 bits.
  16         */
  17        unsigned sum = 0;
  18        while (nleft > 1) {
  19                sum += *addr++;
  20                nleft -= 2;
  21        }
  22
  23        /* Mop up an odd byte, if necessary */
  24        if (nleft == 1) {
  25                if (BB_LITTLE_ENDIAN)
  26                        sum += *(uint8_t*)addr;
  27                else
  28                        sum += *(uint8_t*)addr << 8;
  29        }
  30
  31        /* Add back carry outs from top 16 bits to low 16 bits */
  32        sum = (sum >> 16) + (sum & 0xffff);     /* add hi 16 to low 16 */
  33        sum += (sum >> 16);                     /* add carry */
  34
  35        return (uint16_t)~sum;
  36}
  37