ping的c實現

                       (nsend-nreceived)/nsend*100);
       close(sockfd);
       exit(1);
}
/*校驗和算法*/
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報頭二進制數據以2字節為單位累加起來*/
       while(nleft>1)
       {       sum+=*w++;
               nleft-=2;
       }
/*若ICMP報頭為奇數個字節，會剩下最后一字節。把最后一個字節視為一個2字節數據的高字節，
這個2字節數據的低字節為0，繼續累加*/
       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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