深入理解Linux多播编程

Linux多播编程是一种具有多接收者功能的协议，它提供了一种有效的利用程序发送消息的方式。不管接收者在何处，小包就能够到达所有的系统，从而节省了巨大的带宽消耗。多播是内部网络（如局域网）中一种极具吸引力的编程技术，它允许多个利用程序发送和接收包数据。本文将深入探讨Linux的多播编程。

Multi-cast programming under Linux is based on the BSD sockets API and make use of the special IP protocol of the multi-cast protocol. To start using multi-cast programming, a program uses a call to the socket system call, passing the family PF_INET and the associated type. This creates a UDP datagram socket:

// 创建UDP socket

int sockfd = socket(PF_INET, SOCK_DGRAM, 0);

Once the socket has been created, the client can specify a multi-cast address using the bind system call. This designates the local IP address associated with the socket and allows the system to join a given multi-cast group. For example, to join the 224.0.1.5 multi-cast group, one could use the following system call:

// 假定有机器的IP为192.168.1.100，加入 224.0.1.5多播组

struct sockaddr_in group_addr;

group_addr.sin_family = AF_INET;

group_addr.sin_addr.s_addr = inet_addr(“224.0.1.5”);

group_addr.sin_port = htons(5000);

// 启动加入多播组

bind(sockfd, (struct sockaddr *)&group_addr, sizeof(group_addr));

Once the socket has been bound, the client can begin to send data packets to the multi-cast group using the sendto system call. Since multi-cast packets are sent over the network one time only, they are often referred to as one-time transmissions. In order to achieve this one-time transmission, the program must use the IP_MULTICAST_LOOP option, which is sent along with the IP_ADD_MEMBERSHIP socket option when the socket is created. Finally, since multi-cast transmissions occur over UDP, when sending data to the group one must specify the port on which the data will be sent.

// 给 224.0.1.5多播组发送UDP数据

char buf[100]

struct sockaddr_in group_addr;

group_addr.sin_family = AF_INET;

group_addr.sin_addr.s_addr = inet_addr(“224.0.1.5”);

group_addr.sin_port = htons(5000);

// 启动设置发送多播组

setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_LOOP, &val, sizeof(val));

// 启动发送，指定机器192.168.1.100发送

sendto(sockfd, buf, sizeof(buf), 0, (struct sockaddr *)&group_addr, sizeof(group_addr));

Receiving data from multi-cast groups is much the same as sending data. The main difference, of course, is that the client must use the recvfrom system call rather than the sendto system call. After the socket has been bound to the multi-cast group, the client can begin receiving data in a continuous loop using the recvfrom system call:

// 开启一个接收循环

while(1)

{

recvfrom(sockfd, buf, sizeof(buf), 0, NULL, NULL);

}

When a multi-cast client has finished receiving data, it must clean up after itself before exiting by calling the leave_group system call, which instructs the socket to leave the multi-cast group.

// 要求离开多播组

setsockopt(sockfd, IPPROTO_IP, IP_DROP_MEMBERSHIP, &group_addr, sizeof(group_addr));

本文重点介绍了Linux多播编程，其中介绍了怎样创建UDP socket，如何加入多播组，怎么发送和接收数据和如何离开多播组。多播编程使网络编程更加方便，可以高效的发送数据，值得学习和研究。