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);
}