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(const void *ptr, int nleft)
  10{
  11        const uint16_t *addr = ptr;
  12
  13        /*
  14         * Our algorithm is simple, using a 32 bit accumulator,
  15         * we add sequential 16 bit words to it, and at the end, fold
  16         * back all the carry bits from the top 16 bits into the lower
  17         * 16 bits.
  18         */
  19        unsigned sum = 0;
  20        while (nleft > 1) {
  21                sum += *addr++;
  22                nleft -= 2;
  23        }
  24
  25        /* Mop up an odd byte, if necessary */
  26        if (nleft == 1) {
  27                if (BB_LITTLE_ENDIAN)
  28                        sum += *(uint8_t*)addr;
  29                else
  30                        sum += *(uint8_t*)addr << 8;
  31        }
  32
  33        /* Add back carry outs from top 16 bits to low 16 bits */
  34        sum = (sum >> 16) + (sum & 0xffff);     /* add hi 16 to low 16 */
  35        sum += (sum >> 16);                     /* add carry */
  36
  37        return (uint16_t)~sum;
  38}
  39