在客户端更改网络IP后，可以在C(服务器)中发送，但不能从Android (客户端)中接收

Question

编辑:首先，我知道TCP和UDP是什么，以及它们的区别，但是如何解释我可以在客户端从服务器接收，但不能发送，如下所示...

我用C语言做了一个服务器应用程序，用Java做了一个客户端应用程序，我把它部署到Android上。我有一个奇怪的问题，我不能克服。

当我的手机IP改变(例如，当切换到移动网络时，从Wifi，或相反)，C中的应用程序不再能够接收数据，只是发送，即使手机发送了数据，没有任何错误。

重新连接到同一网络时不会发生这种情况。

我尝试了很多东西，最后得到了这个，用C：

void *receive_thread(){
    pthread_detach(pthread_self());

    struct sockaddr_in dest; /* socket info about the machine connecting to us */
    struct sockaddr_in serv; /* socket info about our server */
    int mysocket;            /* socket used to listen for incoming connections */
    socklen_t socksize = sizeof(struct sockaddr_in);

    memset(&serv, 0, sizeof(serv));           /* zero the struct before filling the fields */
    serv.sin_family = AF_INET;                /* set the type of connection to TCP/IP */
    serv.sin_addr.s_addr = htonl(INADDR_ANY); /* set our address to any interface */
    serv.sin_port = htons(PORTNUMRCV);           /* set the server port number */    

    mysocket = socket(AF_INET, SOCK_STREAM, 0);

    //fcntl(mysocket, F_SETFD, O_NONBLOCK);

    /* bind serv information to mysocket */
    bind(mysocket, (struct sockaddr *)&serv, sizeof(struct sockaddr));

    /* start listening, allowing a queue of up to 1 pending connection */
    listen(mysocket, 1);

    int consocket = accept(mysocket, (struct sockaddr *)&dest, &socksize);
    while(consocket){
        //socklen_t fromlen;
        //fromlen = sizeof inet_ntoa(dest.sin_addr);
        char receivemsg[128];

        int rlen = recv(consocket, &receivemsg, 128, 0); //receive data
        int err = errno;
        //printf("recv error: %d\n", err);
        printf("Received string: %s from %s \n", receivemsg, inet_ntoa(dest.sin_addr));
        if(rlen > 0){
            receivemsg[rlen] = '\0';
            if(comparestr(KEY, receivemsg) == 1){
                printf("Local client\n");
                incrementor++;
            } else if(comparestr(KEY, receivemsg) == 2){
                printf("Remote client\n");
                incrementor--;
            } else {
                printf("Wrong key!\n");
            }
        } else {
            printf("Care!!!! Receiving: %d \n", rlen);
        }
        close(consocket);
        consocket = accept(mysocket, (struct sockaddr *)&dest, &socksize);
    }
    printf("Receive thread ended!\n");
    receive_thread_up = 0;
}

void *send_thread(){
    pthread_detach(pthread_self());

    struct sockaddr_in dest; /* socket info about the machine connecting to us */
    struct sockaddr_in serv; /* socket info about our server */
    int mysocket;            /* socket used to listen for incoming connections */
    socklen_t socksize = sizeof(struct sockaddr_in);

    memset(&serv, 0, sizeof(serv));           /* zero the struct before filling the fields */
    serv.sin_family = AF_INET;                /* set the type of connection to TCP/IP */
    serv.sin_addr.s_addr = htonl(INADDR_ANY); /* set our address to any interface */
    serv.sin_port = htons(PORTNUMSND);           /* set the server port number */

    mysocket = socket(AF_INET, SOCK_STREAM, 0);

    /* bind serv information to mysocket */
    bind(mysocket, (struct sockaddr *)&serv, sizeof(struct sockaddr));

    /* start listening, allowing a queue of up to 1 pending connection */
    listen(mysocket, 1);

    int consocket = accept(mysocket, (struct sockaddr *)&dest, &socksize);
    while(consocket){
        //printf("Incoming connection from %s - sending string\n", inet_ntoa(dest.sin_addr));
        int slen = send(consocket, KEY, 3*sizeof(KEY), 0); //send data
        if(slen>0)printf("Sent string: %s to %s \n", KEY, inet_ntoa(dest.sin_addr));
        else {
            printf("Care!!! Sending: %d \n", slen);
        }       
        close(consocket);
        consocket = accept(mysocket, (struct sockaddr *)&dest, &socksize);
    }
    printf("Send thread ended!\n");
    send_thread_up = 0;
}

在Java中是这样的：

public static int sendString(String str) throws IOException{
    System.out.println("Sending string: " + str);
    out.println(str);
    return 0;
}

public static String receiveString(){
    String s = " ";
    try {
        s = in.readLine();
    } catch (SocketException e) {
        System.out.println("Socket Exception: " + e);
    } catch (IOException e) {
        System.out.println("IOException on receive: " + e);
    }
    System.out.println("Received string: " + s);
    return s;
}
public static String reverseString(String str){
    String newStr = " ";
    newStr = new StringBuilder(str).reverse().toString();
    return newStr;
}

我的猜测是recv卡住了，所以我尝试添加

fcntl(mysocket, F_SETFD, O_NONBLOCK);

，在不起作用后，我对它进行了评论。也许使用UDP套接字？我可能会遇到消息未到达或不完整之类的问题。也欢迎提出改进我的代码的建议！

以下是C语言的输出：

Thread: No connection, waiting...
Sent string: I4mP4ssw0rd7 to 192.168.1.2
Received string: I4mP4ssw0rd7
 from 192.168.1.2
Local client
Sent string: I4mP4ssw0rd7 to 192.168.1.2
Thread: Local connection!
Received string: I4mP4ssw0rd7
 from 192.168.1.2
Local client
Sent string: I4mP4ssw0rd7 to 192.168.1.2
Sent string: I4mP4ssw0rd7 to 87.edited
Sent string: I4mP4ssw0rd7 to 87.edited
Sent string: I4mP4ssw0rd7 to 87.edited
Sent string: I4mP4ssw0rd7 to 87.edited
Sent string: I4mP4ssw0rd7 to 87.edited

下面是Java的输出：

03-18 22:52:45.831  13899-13899/willymatica.alarmapp1 I/System.out﹕ Connected!
03-18 22:52:49.375  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected locally!
03-18 22:52:49.585  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:52:54.900  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: I4mP4ssw0rd7
03-18 22:52:54.910  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected locally!
03-18 22:52:54.970  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:53:00.265  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: I4mP4ssw0rd7
03-18 22:53:00.285  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected locally!
03-18 22:53:00.315  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:53:05.631  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: I4mP4ssw0rd7
03-18 22:53:05.631  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected remotely!
03-18 22:53:05.851  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:53:11.266  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: 7dr0wss4Pm4I
03-18 22:53:11.286  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected remotely!
03-18 22:53:11.516  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:53:16.912  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: 7dr0wss4Pm4I
03-18 22:53:16.932  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected remotely!
03-18 22:53:17.162  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:53:22.547  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: 7dr0wss4Pm4I
03-18 22:53:22.567  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected remotely!
03-18 22:53:23.198  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7
03-18 22:53:28.933  13899-13920/willymatica.alarmapp1 I/System.out﹕ Sending string: 7dr0wss4Pm4I
03-18 22:53:28.933  13899-13920/willymatica.alarmapp1 I/System.out﹕ Connected remotely!
03-18 22:53:29.113  13899-13920/willymatica.alarmapp1 I/System.out﹕ Received string: I4mP4ssw0rd7

Answer 1

TCP是面向连接的。一旦建立了TCP连接，就会得到所谓的套接字对。套接字对是源ip地址和源端口以及目的ip地址和目的端口的组合。该连接由这4个参数唯一标识。

在整个会话期间，将使用相同的ip地址和端口。

因此，如果一个网络接口不再可用，就会出现问题，因为每个网络接口都会有一个ip地址，如果它不再可用，TCP连接将无法再选择不同的网络接口。

TCP不支持更改连接两端的IP地址。

这将需要相当长的时间来检测到存在问题。您认为仍然可以发送，但是，TCP将多次重新传输，最终您将看到发送失败。这甚至需要几分钟。根据您的套接字是否阻塞，您将注意到，在IP@已更改的错误条件下，send将阻塞。从TCP的角度来看，这就像断线一样。