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Using QTcpSocket can I write a Tcp message with zero payload to server ?



  • Hello Everyone,

    I am trying to send a Tcp Packet whose payload is empty to a server. I already tried using the "write()" API's from QTcpSocket with length as zero but I do not observe the TcpPacket over wireshark.

    The reason I want to do that is
    I have a desktop client app running with QTcpSocket connected to server on Raspberry Pi. The server is sometimes powered off by unplugging and I want to detect its state in my Client App. Strangely in this situation the disconnect or stateChanged signals from QTcpSocket do not trigger. So the only way is to write a Tcp packet with payload zero periodically to server to trigger the signal disconnect. I do not want to fill the payload with even a single character as this causes faulty data sent to server.

    So is there a way around to send TCP packet with empty payload to server.

    Thanks in advance for suggestions.



  • EDIT: solution below doesn't work

    Hi I can't test it right now but I seam to remember the correct way to do this is trying to read.
    Instead of writing a 0 payload packet, try reading from it:

    socket->startTransaction();
    socket->read(1);
    socket->rollbackTransaction();
    

    This code does nothing per se but it should trigger the detection that the server is now disconnected and emit the disconnected() signal



  • @VRonin said in Using QTcpSocket can I write a Tcp message with zero payload to server ?:

    Instead of writing a 0 payload packet, try reading from it:
    socket->startTransaction();
    socket->read(1);
    socket->rollbackTransaction();

    This code does nothing per se but it should trigger the detection that the server is now disconnected and emit the disconnected() signal

    Very cool workaround!
    I have no need of this now, but how knows, maybe in future development :)



  • Hi @VRonin i tried using your method, but still do not get any disconnected() signal emitted.
    Also tried setSocketOption with keepAlive parameter and performed read operation but no detection happens.
    So looking for other methods.



  • Do you have access to the source of the server? Can you send a sort of heartbeat between the two?



  • Unfortunately no, i do not have access to change the server side code. So implementing heartbeat is difficult.



  • @sasanka said in Using QTcpSocket can I write a Tcp message with zero payload to server ?:

    Also tried setSocketOption with keepAlive

    I am using socket.setSocketOption(QAbstractSocket::KeepAliveOption, 1);, which works fine for me, but there are some things you have to known:

    • it must be done when socket is connected
    • the probe interval is per default at 12 hours! (or even more, I don't really remember)

    The easiest way I found to use it, was to enabled it "by hand", after my socket was connected:

    • on Linux systems:
            int enableKeepAlive = 1;
            int fd = socket->socketDescriptor();
            setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &enableKeepAlive, sizeof(enableKeepAlive));
    
            // Interval between the last data packet sent and the first keepalive probe in seconds.
            int maxIdle = 15;
            setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &maxIdle, sizeof(maxIdle));
    
            // Number of probes that are sent and unacknowledged before the client considers
            //the connection broken and notifies the application layer.
            int count = 6;
            setsockopt(fd, SOL_TCP, TCP_KEEPCNT, &count, sizeof(count));
    
            // Interval between subsequent keepalive probes in seconds.
            int interval = 5;
            setsockopt(fd, SOL_TCP, TCP_KEEPINTVL, &interval, sizeof(interval));
    
    • for Windows:
    #include <winsock2.h>
    #include <mstcpip.h>
    #include <Ws2tcpip.h>
    #pragma comment(lib, "ws2_32.lib")
    ...
            int fd = socket->socketDescriptor();
            DWORD dwBytesRet = 0;
            tcp_keepalive alive;    // your options for "keepalive" mode
    
            if(WSAIoctl(fd, SIO_KEEPALIVE_VALS, NULL, 0, &alive, sizeof(alive), &dwBytesRet, NULL, NULL) == SOCKET_ERROR) {
                qWarning() << "WSAIotcl(SIO_KEEPALIVE_VALS) read failed with err#" <<  WSAGetLastError();
                return;
            }
            qDebug() << qPrintable(QString("OnOff:%1, KeepAliveTime=%2, KeepAliveInterval=%3")
                                       .arg(alive.onoff).arg(alive.keepalivetime).arg(alive.keepaliveinterval));
            alive.onoff = TRUE;              // turn it on
            alive.keepalivetime = 15000;     // delay (ms) between requests, default is 2h (7200000)
            alive.keepaliveinterval = 5000;  // delay between "emergency" ping requests, their number (6) is not configurable
    
            /*
             * So with this config  socket will send keepalive requests every 15
             * seconds after last data transaction when everything is ok.
             *
             * If there is no reply (wire plugged out) it'll send 6 requests with 5s
             * delay between them and then close.
             *
             * As a result we will get disconnect after approximately 45 sec timeout.
             */
            if (WSAIoctl(fd, SIO_KEEPALIVE_VALS, &alive, sizeof(alive), NULL, 0, &dwBytesRet, NULL, NULL) == SOCKET_ERROR) {
                qWarning() << "WSAIotcl(SIO_KEEPALIVE_VALS) write failed with err#" <<  WSAGetLastError();
                return;
            }
    


  • I had to include LIBS += -lws2_32 in the .pro file while using the above code.
    After segregating and adding the above code to my project i get the following Warning/Error at runtime. May be the socket is unable to initialize properly with the WSAIoctl . The message is as follows.

    WSAIotcl(SIO_KEEPALIVE_VALS) read failed with err# 10093



  • @sasanka said in Using QTcpSocket can I write a Tcp message with zero payload to server ?:

    read failed with err# 10093

    What have you done about calling WSAStartup()?



  • @sasanka said in Using QTcpSocket can I write a Tcp message with zero payload to server ?:

    WSAIotcl(SIO_KEEPALIVE_VALS) read failed with err# 10093

    According Microsoft documentation (https://docs.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2), this is WSANOTINITIALISED:

    Successful WSAStartup not yet performed.

    Either the application has not called WSAStartup or WSAStartup failed. The application may be accessing a socket that the current active task does not own (that is, trying to share a socket between tasks), or WSACleanup has been called too many times.

    When did you try to set KeepAlive?
    Was your socket in connected state (socket->state() == QAbstractSocket::ConnectedState)?



  • Hi @JonB Yes I checked through the error codes from Microsoft website and added the WSAStartup() with parameters as provided in their documentation.

    Hi @KroMignon i made sure to invoke KeepAlive with the above mentioned code once the client has connected with the server.
    It now gives the following error message from WSAIotcl.

    WSAIotcl(SIO_KEEPALIVE_VALS) read failed with err# 10014

    From the error codes listed in MS Documentation.
    WSAEFAULT
    10014
    Bad address.
    The system detected an invalid pointer address in attempting to use a pointer argument of a call. This error occurs if an application passes an invalid pointer value, or if the length of the buffer is too small. For instance, if the length of an argument, which is a sockaddr structure, is smaller than the sizeof(sockaddr).

    Maybe the function the parameters have discrepancies
    WSAIoctl(fd, SIO_KEEPALIVE_VALS, &alive, sizeof(alive), NULL, 0, &dwBytesRet, NULL, NULL)



  • @sasanka said in Using QTcpSocket can I write a Tcp message with zero payload to server ?:

    WSAIoctl

    sry below is the function with read failed.
    WSAIoctl(fd, SIO_KEEPALIVE_VALS, NULL, 0, &alive, sizeof(alive), &dwBytesRet, NULL, NULL)



  • @sasanka said in Using QTcpSocket can I write a Tcp message with zero payload to server ?:

    Hi @KroMignon i made sure to invoke KeepAlive with the above mentioned code once the client has connected with the server.
    It now gives the following error message from WSAIotcl.
    WSAIotcl(SIO_KEEPALIVE_VALS) read failed with err# 10014

    I use this code on WindowsXP/Windows7 devices, and it works fine;

        connect(socket, &QTcpSocket::stateChanged, socket, [this]()
                {
                    if(!socket)
                        return;
                    if(socket->state() == QAbstractSocket::ConnectedState)
                        enableKeepAlive(socket);
                } );
    

    Which OS are you targeting?



  • Hi @KroMignon I am using Windows 10 operating system.
    I changed the parameters from WSAIoctl after reading the documentation.
    Upon using the following function after setting alive parameters i am able to receive disconnect signal.

    WSAIoctl(fd, SIO_KEEPALIVE_VALS, &alive, sizeof(alive), NULL, 0, &dwBytesRet, NULL, NULL)

    I tried powering off the server machine and my client application detects the state. In addition I changed the parameters for alive to detect the disconnect little early.

    alive.onoff = TRUE; // turn it on
    alive.keepalivetime = 600; // delay (ms) between requests, default is 2h (7200000)
    alive.keepaliveinterval = 600; // delay between "emergency" ping requests, their number (6) is not configurable

    Thanks a lot finally it works !!!


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