UDP组播：同一台机器上的多个客户端

服务器代码：

#define EXAMPLE_PORT 6000
#define EXAMPLE_GROUP "239.0.0.1"

int main(int argc, char **argv) {

    struct sockaddr_in addr;
    int sock;
    ssize_t cnt;
    struct ip_mreq mreq;
    char message[50];
    socklen_t addrlen;

    /* set up socket */
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0) {
        perror("socket");
        return(1);
    }
    memset((char *)&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(EXAMPLE_PORT);
    addrlen = sizeof(addr);

    /* send */
    addr.sin_addr.s_addr = inet_addr(EXAMPLE_GROUP);
    while (1) {
        time_t t = time(0);
        sprintf(message, "time is %-24.24s", ctime(&t));
        printf("sending: %s\n", message);
        cnt = sendto(sock, message, sizeof(message), 0,
                     (struct sockaddr *) &addr, addrlen);
        if (cnt < 0) {
            perror("sendto");
            return (1);
        }
        sleep(5);
    }

    return 0;
}

客户端代码：

#define EXAMPLE_PORT 6000
#define EXAMPLE_GROUP "239.0.0.1"

int main(int argc, char **argv) {

    if (argc < 2)
        return 1;

    const char *ip_addr = argv[1];

    struct sockaddr_in addr;
    int sock;
    socklen_t addrlen;
    ssize_t cnt;
    struct ip_mreq mreq;
    char message[50];

    /* set up socket */
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0) {
        perror("socket");
        return(1);
    }
    memset((char *)&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(ip_addr); // *
    addr.sin_port = htons(EXAMPLE_PORT);
    addrlen = sizeof(addr);

    /* receive */
    if (bind(sock, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        perror("bind");
        return (1);
    }
    mreq.imr_multiaddr.s_addr = inet_addr(EXAMPLE_GROUP);
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);
    if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                   &mreq, sizeof(mreq)) < 0) {
        perror("setsockopt mreq");
        return (1);
    }
    while (1) {
        cnt = recvfrom(sock, message, sizeof(message), 0,
                       (struct sockaddr *) &addr, &addrlen);
        if (cnt < 0) {
            perror("recvfrom");
            return (1);
        } else if (cnt == 0) {
            break;
        }
        printf("%s: message = \"%s\"\n", inet_ntoa(addr.sin_addr), message);
    }

    return 0;
}

问：我可以在同一台机器上运行多个客户端吗？现在我不能这样做，因为第二个客户端的启动以错误结束address already in use。

(*) 之前想用特定接口的IP地址绑定socket，是这样的：

addr.sin_addr.s_addr = htonl(INADDR_ANY);