ping的实现（原始套接字系列三）

　// 填充data域
　const unsigned long int deadmeat = 0xDEADBEEF;
　char *datapart = (char*)icmp_hdr + sizeof(ICMPHeader);
　int bytes_left = packet_size - sizeof(ICMPHeader);
　while (bytes_left > 0)
　{
　　memcpy(datapart, &deadmeat, min(int(sizeof(deadmeat)), bytes_left));
　　bytes_left -= sizeof(deadmeat);
　　datapart += sizeof(deadmeat);
　}

　// 计算校验和
　icmp_hdr->checksum = ip_checksum((unsigned short*)icmp_hdr, packet_size);
}

　// 将所有的16数相加
　while (size > 1)
　{
　　cksum += *buffer++;
　　size -= sizeof(unsigned short);
　}
　if (size) //加上最后一个BYTE
　{
　　cksum += *(unsigned char*)buffer;
　}

　//和的前16位和后16位相加
　cksum = (cksum >> 16) + (cksum &0xffff);
　cksum += (cksum >> 16);

　return (unsigned short)(~cksum);
}

　if (setsockopt(sd, IPPROTO_IP, IP_TTL, (const char*) &ttl, sizeof(ttl)) ==SOCKET_ERROR)
　{
　　cerr << "TTL setsockopt failed: " << WSAGetLastError() << endl;
　　return - 1;
　}

　// 初始化目标主机信息块
　memset(&dest, 0, sizeof(dest));

　// 将第1个参数转换为目标IP地址
　unsigned int addr = inet_addr(host);
　if (addr != INADDR_NONE)
　{
　　// 为IP地址
　　dest.sin_addr.s_addr = addr;
　　dest.sin_family = AF_INET;
　}
　else
　{
　　// 非IP地址，进行主机名和IP地址的转换
　　hostent *hp = gethostbyname(host);
　　if (hp != 0)
　　{
　　　// 查找主机名对应的IP地址
　　　memcpy(&(dest.sin_addr), hp->h_addr, hp->h_length);
　　　dest.sin_family = hp->h_addrtype;
　　}
　　else
　　{
　　　// 不能识别的主机名
　　　cerr << "Failed to resolve " << host << endl;
　　　return - 1;
　　}
　}
　return 0;
}

　// 报文太短
　if (bytes < header_len + ICMP_MIN)
　{
　　cerr << "too few bytes from " << inet_ntoa(from->sin_addr) << endl;
　　return - 1;
　}
　// 解析回复报文类型
　else if (icmphdr->type != ICMP_ECHO_REPLY)
　{
　　//非正常回复
　　if (icmphdr->type != ICMP_TTL_EXPIRE)
　　{
　　　//ttl减为零
　　　if (icmphdr->type == ICMP_DEST_UNREACH)
　　　{
　　　　//主机不可达
　　　　cerr << "Destination unreachable" << endl;
　　　}
　　　else
　　　{
　　　　//非法的ICMP包类型
　　　　cerr << "Unknown ICMP packet type " << int(icmphdr->type) <<" received" << endl;
　　　}
　　　return - 1;
　　}
　}
　else if (icmphdr->id != (unsigned short)GetCurrentProcessId())
　{
　　//不是本进程发的包, 可能是同机的其它ping进程发的
　　return - 2;
　}

　// 指出往返时间TTL
　int nHops = int(256-reply->ttl);
　if (nHops == 192)
　{
　　// TTL came back 64, so ping was probably to a host on the
　　// LAN -- call it a single hop.
　　nHops = 1;
　}
　else if (nHops == 128)
　{
　　// Probably localhost
　　nHops = 0;
　}

　// 输出信息
　cout << endl << bytes << " bytes from " << inet_ntoa(from->sin_addr) <<", icmp_seq " << icmphdr->seq << ", ";
　if (icmphdr->type == ICMP_TTL_EXPIRE)
　{
　　cout << "TTL expired." << endl;
　}
　else
　{
　　cout << nHops << " hop" << (nHops == 1 ? "" : "s");
　　cout << ", time: " << (GetTickCount() - icmphdr->timestamp) << " ms." <<endl;
　}
　return 0;
}