ping的c实现

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

}
/*剥去ICMP报头*/
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报头长度,即ip报头的长度标志乘4*/
       icmp=(struct icmp *)(buf+iphdrlen);  /*越过ip报头,指向ICMP报头*/
       len-=iphdrlen;            /*ICMP报头及ICMP数据报的总长度*/
       if( len<8)                /*小于ICMP报头长度则不合理*/
       {       printf("ICMP packets\ s length is less than 8\n");
               return -1;
       }

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