ReplayKit2 有线投屏项目-反向Socket实现

一、需求

　　我们在使用RTMP协议进行推流的时候，底层仍然采用的是TCP协议或者QUICK协议，有客户端主动发起请求。但是在有线投屏中，需要PC端向手机发起请求建立连接

二、实现

　　在客户端主动发起请求之前，我们首先启动一个socket监听来自PC的连接，如果连接成功，那么我们使用这个已经建立好的连接，继续后面的流程

　　在实现中，我们需要设计一个超时的机制，一般socket可以对send和recv设置超时，当然超时都是对同步的socket生效的。

　　正常设置如下：

   struct timeval tv, recv_timeout;
   tv.tv_sec = timeout / 1000;
   tv.tv_usec = static_cast<int>(timeout % 1000 * 1000);
   int ret = ::setsockopt(m_nRealServerSocket, SOL_SOCKET, SO_RCVTIMEO, (const char*) &tv, sizeof(tv));
   ret = ::setsockopt(m_nRealServerSocket, SOL_SOCKET, SO_SNDTIMEO, (const char*) &tv, sizeof(tv));

　　但是这个设置对accept在iOS平台是不生效的，必须采用select的方式

　　select监听描述符如下所示

#define MYPORT 1937    // the port users will be connecting to
#define BACKLOG 1     // how many pending connections queue will hold
#define BUF_SIZE 200
int fd_A[BACKLOG];     // accepted connection fd
int conn_amount;    // current connection amount

- (void)startSelectDemo2
{
    int sock_fd, new_fd;  // listen on sock_fd, new connection on new_fd
    struct sockaddr_in server_addr;    // server address information
    struct sockaddr_in client_addr; // connector's address information
    socklen_t sin_size;
    int yes = 1;
    char buf[BUF_SIZE];
    int ret;
    int i;

    if ((sock_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
        NSLog(@"socket");
        exit(1);
    }

    if (setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {
        NSLog(@"setsockopt");
        exit(1);
    }

    server_addr.sin_family = AF_INET;         // host byte order
    server_addr.sin_port = htons(MYPORT);     // short, network byte order
    server_addr.sin_addr.s_addr = INADDR_ANY; // automatically fill with my IP
    memset(server_addr.sin_zero, '\0', sizeof(server_addr.sin_zero));

    if (bind(sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        NSLog(@"bind");
        exit(1);
    }

    if (listen(sock_fd, BACKLOG) == -1) {
        NSLog(@"listen");
        exit(1);
    }

    NSLog(@"listen port %d\n", MYPORT);

    fd_set fdsr;
    int maxsock;
    struct timeval tv;

    conn_amount = 0;
    sin_size = sizeof(client_addr);
    maxsock = sock_fd;
    while (1) {
        // initialize file descriptor set
        FD_ZERO(&fdsr);
        FD_SET(sock_fd, &fdsr);

        // timeout setting
        tv.tv_sec = 5;
        tv.tv_usec = 0;

        // add active connection to fd set
        for (i = 0; i < BACKLOG; i++) {
            if (fd_A[i] != 0) {
                FD_SET(fd_A[i], &fdsr);
            }
        }

        ret = select(maxsock + 1, &fdsr, NULL, NULL, &tv);
        if (ret < 0) {
            NSLog(@"select");
            break;
        } else if (ret == 0) {
            NSLog(@"timeout\n");
            continue;
        }

        // check every fd in the set
        for (i = 0; i < conn_amount; i++) {
            if (FD_ISSET(fd_A[i], &fdsr)) {
                ret = recv(fd_A[i], buf, sizeof(buf), 0);
                if (ret <= 0) {        // client close
                    NSLog(@"client[%d] close\n", i);
                    close(fd_A[i]);
                    FD_CLR(fd_A[i], &fdsr);
                    fd_A[i] = 0;
                } else {        // receive data
                    if (ret < BUF_SIZE)
                        memset(&buf[ret], '\0', 1);
                    NSLog(@"client[%d] send:%s\n", i, buf);
                }
            }
        }

        // check whether a new connection comes
        if (FD_ISSET(sock_fd, &fdsr)) {
            new_fd = accept(sock_fd, (struct sockaddr *)&client_addr, &sin_size);
            if (new_fd <= 0) {
                NSLog(@"accept");
                continue;
            }
            else
            {
                const char *hello = "hello from clent";
                send(new_fd, hello, strlen(hello), 0);

                char recvbuf[4096];
                int retry = 0;
                while (retry++ < 3) {

                    memset(recvbuf, 0, 4096);

                    int ret = recv(new_fd, recvbuf, 4096 - 1, 0);

                    if(ret > 0)
                    {
                        NSLog(@"app recv:%s\n",recvbuf);
                    }
                    else
                    {
                        NSLog(@"app recv error ret = %d\n",ret);
                    }
                }

            }

            // add to fd queue
            if (conn_amount < BACKLOG) {
                fd_A[conn_amount++] = new_fd;
                NSLog(@"new connection client[%d] %s:%d\n", conn_amount,
                       inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
                if (new_fd > maxsock)
                    maxsock = new_fd;
            }
            else {
                NSLog(@"max connections arrive, exit\n");
                send(new_fd, "bye", 4, 0);
                close(new_fd);
                break;
            }
        }

        int i;
        NSLog(@"client amount: %d\n", conn_amount);
        for (i = 0; i < BACKLOG; i++) {
            NSLog(@"[%d]:%d  ", i, fd_A[i]);
        }
        NSLog(@"\n\n");    }

    // close other connections
    for (i = 0; i < BACKLOG; i++) {
        if (fd_A[i] != 0) {
            close(fd_A[i]);
        }
    }

    exit(0);
}

　　在成功收到连接之后，我们确保连接变成同步方式

unsigned long ul = 0;
ioctl(Sock, FIONBIO, &ul)

　　并且在recv的时候，最后一个参数也可以决定是否是同步还是异步的方式，0 表示默认方式 ； 0x80表示异步的方式

ssize_t    recv(int, void *, size_t, int) __DARWIN_ALIAS_C(recv);

　　

三、总结

　　对于理解网络底层的编程，理解select的工作原理还需要继续加深。

四、代码

https://github.com/liqiushui/iOSSocketAcceptTimeout

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