ping的c实现

                       (nsend-nreceived)/nsend*100);
       close(sockfd);
       exit(1);
}
/*校验和算法*/
unsigned short cal_chksum(unsigned short *addr,int len)
{       int nleft=len;
       int sum=0;
       unsigned short *w=addr;
       unsigned short answer=0;

/*把ICMP报头二进制数据以2字节为单位累加起来*/
       while(nleft>1)
       {       sum+=*w++;
               nleft-=2;
       }
/*若ICMP报头为奇数个字节，会剩下最后一字节。把最后一个字节视为一个2字节数据的高字节，
这个2字节数据的低字节为0，继续累加*/
       if( nleft==1)
       {       *(unsigned char *)(&answer)=*(unsigned char *)w;
               sum+=answer;
       }
       sum=(sum>>16)+(sum&0xffff);
       sum+=(sum>>16);
       answer=~sum;

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