linux多播编程运行结果出不来

编译没有问题，但是运行结果不出来，求中国人指教！
服务器代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#define MCAST_PORT 8888
#define MCAST_ADDR "224.0.0.88"
#define MCAST_DATA "BROADCAST TEST DATA"
#define MCAST_INTERVAL 1
int main(int argc,char *argv[])
{
    int s;
    struct sockaddr_in mcast_addr;
    char buf[] ="BROADCAST TEST DATA";
    s = socket(AF_INET,SOCK_DGRAM,0);
    if(s == -1)
    {
        perror("socket()");
        return -1;
    }
    memset(&mcast_addr,0,sizeof(mcast_addr));
    mcast_addr.sin_family=AF_INET;
    mcast_addr.sin_addr.s_addr=inet_addr(MCAST_ADDR);
    mcast_addr.sin_port=htons(MCAST_PORT);
    while(1)
          { 
        int n=sendto(s,buf,strlen(buf),0,(struct sockaddr*)&mcast_addr,sizeof(mcast_addr));
        if(n<0)
        {
            perror("sendto()");
            return -2;
        }
    sleep(MCAST_INTERVAL);
    }
     return 0;
}

客户端代码：

#include<stdio.h>
#include<stdlib.h>  
#include<string.h>     
#include<unistd.h>      
#include<sys/types.h>    
#include<sys/socket.h>     
#include<arpa/inet.h>    
#include<netinet/in.h>    
 
#define MCAST_PORT         8888
#define MCAST_ADDR         "224.0.0.88"     /*一个局部连接多播地址，路由器不进行转发*/
#define MCAST_INTERVAL     5                        /*发送间隔时间*/
#define BUFF_SIZE         256                           /*接收缓冲区大小*/
int main(int argc, char*argv[])
{  
    int s;                                      /*套接字文件描述符*/
    struct sockaddr_in local_addr;              /*本地地址*/
    int err = -1;
   
    s = socket(AF_INET, SOCK_DGRAM, 0);     /*建立套接字*/
    if (s == -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(MCAST_PORT);
   
                                                /*绑定socket*/
    err = bind(s,(struct sockaddr*)&local_addr, sizeof(local_addr)) ;
    if(err < 0)
    {
        perror("bind()");
        return -2;
    }
   
                                                /*设置回环许可*/
    int loop = 1;
    err = setsockopt(s,IPPROTO_IP, IP_MULTICAST_LOOP,&loop, sizeof(loop));
    if(err < 0)
    {
        perror("setsockopt():IP_MULTICAST_LOOP");
        return -3;
    }
   
    struct ip_mreq mreq;                                /*加入多播组*/
    mreq.imr_multiaddr.s_addr = inet_addr(MCAST_ADDR);  /*多播地址*/
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);      /*网络接口为默认*/
                                                        /*将本机加入多播组*/
    err = setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP,&mreq, sizeof
    (mreq));
    if (err < 0)
    {
        perror("setsockopt():IP_ADD_MEMBERSHIP");
        return -4;
    }
   
    int times = 0;
    int addr_len = 0;
    char buff[BUFF_SIZE] = {0};
    int n = 0;
                                        /*循环接收多播组的消息，5次后退出*/
    for(times = 0;times<5;times++)
    {
        addr_len = sizeof(local_addr);
        memset(buff, 0, BUFF_SIZE);                 /*清空接收缓冲区*/
                                                    /*接收数据*/
        n = recvfrom(s, buff, BUFF_SIZE, 0,(struct sockaddr*)&local_addr,
        &addr_len);
        if( n== -1)
        {
            perror("recvfrom()");
        }
                                                    /*打印信息*/
        printf("Recv %dst message from server:%s\n", times, buff);
        sleep(MCAST_INTERVAL);
    }
   
                                                    /*退出多播组*/
    err = setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP,&mreq, sizeof
    (mreq));
       
    close(s);
    return 0;
}

运行截图：