ping的c實現

               if( (n=recvfrom(sockfd,recvpacket,sizeof(recvpacket),0,
                               (struct sockaddr *)&from,&fromlen)) <0)
               {       if(errno==EINTR)continue;
                       perror("recvfrom error");
                       continue;
               }
               gettimeofday(&tvrecv,NULL);  /*記錄接收時間*/
               if(unpack(recvpacket,n)==-1)continue;
               nreceived++;
       }

}
/*剝去ICMP報頭*/
int unpack(char *buf,int len)
{       int i,iphdrlen;
       struct ip *ip;
       struct icmp *icmp;
       struct timeval *tvsend;
       double rtt;

       ip=(struct ip *)buf;
       iphdrlen=ip->ip_hl<<2;    /*求ip報頭長度,即ip報頭的長度標志乘4*/
       icmp=(struct icmp *)(buf+iphdrlen);  /*越過ip報頭,指向ICMP報頭*/
       len-=iphdrlen;            /*ICMP報頭及ICMP數據報的總長度*/
       if( len<8)                /*小于ICMP報頭長度則不合理*/
       {       printf("ICMP packets\ s length is less than 8\n");
               return -1;
       }

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