IP组播的简单例子

发送端代码，基本步骤：

1，创建一个udpsocket

sock = socket(PF_INET, SOCK_DGRAM, 0);

2，绑定到一个发送数据的网卡，或者绑定到ADDR_ANY, 

   saddr.sin_family = AF_INET;
   saddr.sin_port = htons(0); // 发送端任意使用个端口，因为要调用bind，所以用0，让系统随便算个可用的
   saddr.sin_addr.s_addr = htonl(INADDR_ANY); // 绑定到任意地址，如果要在某个物理网卡上组播，设置那个地址。这个地址设置和路由有关，如果设置ANADDR_ANY,但是路由表中localhost的组播地址优先的话，可能导致不走局域网网卡来组播。
   int status = bind(sock, (struct sockaddr *)&saddr, sizeof(struct sockaddr_in));//绑定

3，连接到组播地址，或者在发送的时候用sendto指定。

4，设置ttl

   // Set multicast packet TTL to 3; default TTL is 1
   setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl,
              sizeof(unsigned char));

5，设置回环组播，这样自己可以在本机localhost上测试

   // send multicast traffic to myself too
   status = setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP,
                       &one, sizeof(unsigned char));

6，发送数据，和普通的udp数据发送是一样的

   // set destination multicast address
   saddr.sin_family = PF_INET;
   saddr.sin_addr.s_addr = inet_addr("226.0.0.1");
   saddr.sin_port = htons(4096);

   // put some data in buffer
   strcpy(buffer, "Hello world\n");

   socklen = sizeof(struct sockaddr_in);
   // receive packet from socket
   status = sendto(sock, buffer, strlen(buffer), 0,
                     (struct sockaddr *)&saddr, socklen);

接收端代码，基本步骤：

1，创建一个udpsocket

   // open a UDP socket
   sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);

2，绑定到一个希望用于接收的网卡，或者绑定到ADDR_ANY

   saddr.sin_family = PF_INET;
   saddr.sin_port = htons(4096); // listen on port 4096
   saddr.sin_addr.s_addr = htonl(INADDR_ANY); // 解释参考上面
   status = bind(sock, (struct sockaddr *)&saddr, sizeof(struct sockaddr_in));

3，加入组播组

   imreq.imr_multiaddr.s_addr = inet_addr("226.0.0.1");
   imreq.imr_interface.s_addr = INADDR_ANY; // use DEFAULT (route configuration?) interface, e.g. "127.0.0.1" for loop test, "192.168.0.100" for LAN test.

   // JOIN multicast group on default interface
   status = setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, 
              (const void *)&imreq, sizeof(struct ip_mreq));

4，开始接受数据

   // receive packet from socket
   status = recvfrom(sock, buffer, MAXBUFSIZE, 0, 
                     (struct sockaddr *)&saddr, &socklen);