多播的测试代码如下：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
int main(int argc, char*argv[])
{
int sockfd; // 套接字文件描述符
struct sockaddr_in local_addr; // 本地地址
int err = -1;
char group[16] = "224.0.0.88"; // 多播组 IP
sockfd = socket(AF_INET, SOCK_DGRAM, 0); //建立套接字
if (sockfd == -1)
{
perror("socket()");
return -1;
}
// 初始化地址
memset(&local_addr, 0, sizeof(local_addr));
local_addr.sin_family = AF_INET;
local_addr.sin_addr.s_addr = htonl(INADDR_ANY);
local_addr.sin_port = htons(8080);
// 绑定socket
err = bind(sockfd,(struct sockaddr*)&local_addr, sizeof(local_addr));
if(err < 0)
{
perror("bind()");
return -2;
}
struct ip_mreq mreq; // 多播地址结构体
// 加入多播组，相当于创建一个QQ群，某人加入此群
mreq.imr_multiaddr.s_addr = inet_addr(group); // 多播地址，类似于 QQ 群号
mreq.imr_interface.s_addr = htonl(INADDR_ANY);// 将本机加入多播组，类似于某人加入此群
// 加入多播组
err = setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
if (err < 0)
{
perror("[50]setsockopt():IP_ADD_MEMBERSHIP");
return -4;
}
int times = 0;
int addr_len = 0;
char buff[256] = {0};
int n = 0;
printf("加入多播组成功，准备接收数据\n");
//接收数据
while(1)
{
addr_len = sizeof(local_addr);
memset(buff, 0, sizeof(buff));
// 接收数据
n = recvfrom(sockfd, buff, sizeof(buff), 0,(struct sockaddr*)&local_addr, &addr_len);
if( n == -1)
{
perror("recvfrom()");
close(sockfd);
return -1;
}
printf("Recv message from server: %s\n", buff);
}
// 退出广播组
err = setsockopt(sockfd, IPPROTO_IP, IP_DROP_MEMBERSHIP,&mreq, sizeof(mreq));
if (err < 0)
{
perror("[79]setsockopt():IP_DROP_MEMBERSHIP");
return -4;
}
close(sockfd);
return 0;
}
在ubuntu编译运行时，出现如下的错误：
查询相关资料得到的答案如下：
It means that the tool is trying to use multicast but the network interface doesn't support it There are two likely causes:
·Your machine doesn't have multicast support enabled. For example, on Linux and FreeBSD it is possible to compile a kernel which doesn't support multicast.
·You don't have a route for multicast traffic. Some systems don't add this by default, and you need to run：
route add -net 224.0.0.0 netmask 255.255.255.255 eth0(or similar). If you wish to use RAT in unicast mode only, it is possible to add the multicast route on the loopback interface.
这主要和当前的网络配置有关，因为多播IP地址没有加入到路由表中。
解决方法：把需要用到的多播地址(如本例的224.0.0.88)加入到路由表中，命令如下：
sudo route add -net 224.0.0.88 netmask 255.255.255.255 eth0
224.0.0.88：为当前使用的多播IP地址
eth0：为当前使用的有效网卡
其它辅助命令：
sudo route del -net 224.0.0.88 netmask 255.255.255.255 eth0 //把224.0.0.88从路由表中删除
route -n //查看路由表信息
具体操作过程如下图：
