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sum = (sum>>16) + (sum&0xffff);
可能又产生了近位。
我看到另一份实现。
unsigned short cksum(struct ip *ip, int len){
long sum = 0; /* assume 32 bit long, 16 bit short */
while(len > 1){
sum += *((unsigned short*) ip)++;
if(sum & 0x80000000) /* if high order bit set, fold */
sum = (sum & 0xFFFF) + (sum >> 16);
len -= 2;
}
if(len) /* take care of left over byte */
sum += (unsigned short) *(unsigned char *)ip;
while(sum>>16)
sum = (sum & 0xFFFF) + (sum >> 16);
return ~sum;
}