# UDP组播

组播和广播类似，只是在设置时需要将Socket加入某个组中而已，服务端程序：

```
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <arpa/inet.h>
#include <unistd.h> //usleep


#define checkError(ret) do{if(-1==ret){printf("[%d]err:%s\n", __LINE__, strerror(errno));exit(1);}}while(0)


int main(int argc, char const *argv[])
{
    printf("udp multicast server\n");


    int iSocketFd = socket(AF_INET, SOCK_DGRAM, 0);
    checkError(iSocketFd);


    struct sockaddr_in server_addr; 
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;  
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    server_addr.sin_port = htons(7878);
    checkError(bind(iSocketFd, (struct sockaddr*)&server_addr, sizeof(server_addr)));


    struct ip_mreq multi_addr;
    memset(&multi_addr, 0, sizeof(multi_addr));
    multi_addr.imr_multiaddr.s_addr = inet_addr("239.227.227.227");//multicast addr
    multi_addr.imr_interface.s_addr = htonl(INADDR_ANY);


    int re = 1;
    checkError(setsockopt(iSocketFd, SOL_SOCKET, SO_REUSEADDR, &re, sizeof(re)));
    checkError(setsockopt(iSocketFd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*)&multi_addr, sizeof(multi_addr)));


    char aRecv[2048] = {0};
    const char *aSend = "This is udp multicast server";
    struct sockaddr_in client_addr;//create a client addr
    memset(&client_addr, 0, sizeof(client_addr));
    socklen_t client_len = sizeof(client_addr);


    while(1)
    {
        printf("wait client data...\n");
        memset(aRecv, 0, sizeof(aRecv));
        int irecv = recvfrom(iSocketFd, aRecv, sizeof(aRecv), 0, (struct sockaddr*)&client_addr, &client_len);
        if(-1 == irecv){
            printf("recvfrom err:%s\n", strerror(errno));
            if(errno == EAGAIN){
                usleep(100);
                continue;
            } else {
                exit(1);
            }
        }
        printf("client ipaddr:%s, data:%s\n", inet_ntoa(client_addr.sin_addr), aRecv);
        checkError(sendto(iSocketFd, aSend, strlen(aSend), 0, (struct sockaddr*)&client_addr, sizeof(client_addr)));
        sleep(1);
    }
    return 0;
}
```

客户端在发送数据后直接接收即可：

```
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <arpa/inet.h>
#include <unistd.h> //usleep
#define checkError(ret) do{if(-1==ret){printf("[%d]err:%s\n", __LINE__, strerror(errno));exit(1);}}while(0)


int main(int argc, char const *argv[])
{
    printf("this is udp multicast client\n");
    int iSocketFd = socket(AF_INET, SOCK_DGRAM, 0);
    checkError(iSocketFd);


    struct sockaddr_in server_addr; 
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;  
    server_addr.sin_addr.s_addr = inet_addr("239.227.227.227");
    server_addr.sin_port = htons(7878);


    char aRecv[2048] = {0};
    const char *aSend = "Hello Multicast Server";
    while(1)
    {
        checkError(sendto(iSocketFd, aSend, strlen(aSend), 0, (struct sockaddr*)&server_addr, sizeof(server_addr)));


        memset(aRecv, 0, sizeof(aRecv));
        checkError(recv(iSocketFd, aRecv, sizeof(aRecv), 0));
        printf("Recv Server Data:%s\n", aRecv);
        sleep(1);
    }


    return 0;
}
```
