ping中用到的校验和算法

in_cksum(addr, len)

u_short *addr;

int len;

{

        register int nleft = len;

        register u_short *w = addr;

        register u_short answer;

        register int sum = 0;



        /*

         *  Our algorithm is simple, using a 32 bit accumulator (sum),

         *  we add sequential 16 bit words to it, and at the end, fold

         *  back all the carry bits from the top 16 bits into the lower

         *  16 bits.

         */

        while( nleft > 1 )  {

                sum += *w++;

                nleft -= 2;

        }



        /* mop up an odd byte, if necessary */

        if( nleft == 1 ) {

                u_short u = 0;



                *(u_char *)(&u) = *(u_char *)w ;

                sum += u;

        }



      /*

         * add back carry outs from top 16 bits to low 16 bits

         */

       sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */ （将高位的数值加到低位）

       sum += (sum >> 16); /* add carry */ （如果超过16位的最大的情况也加起来） 

       answer = ~sum; /* truncate to 16 bits */ （取反）

        return (answer);

}