IP效验和计算的代码

unsigned short check_sum(unsigned short *addr,int len)
{
    register int nleft=len;
    register int sum=0;
    register short *w=addr;
    short answer=0;
    while(nleft>1)
    {
        sum+=*w++;
        nleft-=2;
    }
    if(nleft==1)
    {
        *(unsigned char *)(&answer)=*(unsigned char *)w;
        sum+=answer;
    }
    sum=(sum>>16)+(sum&0xffff);
    sum+=(sum>>16);
    answer=~sum;
    return(answer);
}