Important: Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

Console application & QTcpSocket



  • I've been having problems in getting a console application to communicate with another Qt application I've written. The other application uses a class I've written derived from QTcpServer.

    The main function does everything and then enters into a.exec() which seems pretty standard.

    The console application does not as there is no GUI interface, instead it calls a function I've written called loopUntilExit():

    while( mblnExit == false ) {
        std::this_thread::sleep_for(std::chrono::milliseconds(clsModHelper::mscintLoopFrequency));
        QCoreApplication::processEvents();
        ....
    }
    

    mcintLoopFrequency is set to 50. I'm wondering if there is anything else this loop needs to include that could explain why I am not getting the sockets communicating?

    I've got the source to both "fortuneserver" and "fortuneclient" and they are quite simple, but different from my application in the way I've described above.

    The main for my console application looks like:

    QApplication a(intArgc, parystrArgv);
    QApplication::setApplicationDisplayName(clsModFileIO::scpszTitle());
    clsModFileIO obj(&a, intArgc, parystrArgv);
    obj.loopUntilExit();
    return obj.intExitCode();
    }
    


  • @SPlatten as @JonB has already written QJsonDocument::toJson() returns a QByteArray not as QString.

    You have to send and read the same thing if you want it to work!

    void clsModFileIO::onDataIn() {
        QDataStream dsIn;
        dsIn.setDevice(this);
        dsIn.setVersion(clsJSON::mscintQtVersion);
    
        QByteArray jsonArray;
        dsIn.startTransaction();
        dsIn >> jsonArray;
    
        QJsonDocument doc;
        if ( jsonArray.isEmpty() != true ) {
            qdbg() << "onDataIn: " << jsonArray;
            doc = QJsonDocument::fromJson(jsonArray);
        }
        if ( dsIn.commitTransaction() == false ) {
            return;
        }
    }
    

    [EDIT] This only works if you also change the send:

    void clsModHelper::sendJSON(QJsonObject& objJSON) {
        if ( isOpen() != true ) {
            return;
        }
        //Associate this TCP socket with the output data stream
        QDataStream dsOut;
        dsOut.setDevice(this);
        dsOut.setVersion(clsJSON::mscintQtVersion);
        //Send message to data stream
        dsOut << QJsonDocument(objJSON).toJson(QJsonDocument::Compact);
    }
    

  • Lifetime Qt Champion

    @SPlatten said in Console application & QTcpSocket:

    while( mblnExit == false ) {
    std::this_thread::sleep_for(std::chrono::milliseconds(clsModHelper::mscintLoopFrequency));
    QCoreApplication::processEvents();
    ....
    }

    WHY?!
    Do it the right way - use Qt event loop.
    https://doc.qt.io/qt-5/qcoreapplication.html



  • @jsulm , because I'm ignorant and learn by experience.



  • I don't understand why do you replace a.exec() with your own loop?
    QApplication is not a console application by the way...



  • @Bonnie , tell my application that, I call it and its working (in a fashion), are they're any checks I could perform that would indicate what I need to do to get QTcpSocket working?



  • @Bonnie , because originally I had a thread to do everything in the background I wanted to do, but then I thought why don't I just have my loop function and put the thread functionality in that...calling the standard qt loop means I need the thread back to do everything else like sending messages.


  • Lifetime Qt Champion

    @SPlatten said in Console application & QTcpSocket:

    are they're any checks I could perform that would indicate what I need to do to get QTcpSocket working?

    What is not working with QCoreApplication?



  • @SPlatten
    QApplication inherits QGuiApplication.
    So what you have is a gui application but without widgets.

    We don't know much about your application so we can't know why it doesn't work.
    Maybe you can provide a minimum example?

    I've tested with a little code snippet because of your previous post.
    This socket can connect and write data.

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
    
        QTcpSocket socket;
    
        QObject::connect(&socket, &QTcpSocket::bytesWritten, [](qint64 bytes){
            qDebug() << bytes << "written";
        });
    
        QObject::connect(&socket, &QTcpSocket::connected, [&](){
            QDataStream stream;
            qDebug() << "connected";
            stream.setDevice(&socket);
            stream.setVersion(QDataStream::Qt_5_14);
            stream << "TEST";
        });
    
        qDebug() << "connecting";
        socket.connectToHost("localhost", 8123);
    
        return a.exec();
    }
    


  • @jsulm , I've just installed the Debug Symbols and I still can't step into QIODevice/write to see what's happening.

    The console is connected to the server, but it doesn't seem to be writing data.



  • @SPlatten said in Console application & QTcpSocket:

    Tell my application that, I call it and its working (in a fashion), are they're any checks I could perform that would indicate what I need to do to get QTcpSocket working?

    I am sorry to write it but you are doing the hole thing wrong :(
    It is certainly because you have a leak of knowledge about how Qt works.
    Qt is an strongly asynchronous based framework.
    Please take time to acquire this knowledge, there are basic things to know:

    There are many ways to make work an application, but not all are recommended.

    Some basic thing to take care:

    • avoid to lock the used thread, this will "kill" signals/slots handling by the event loop
    • avoid processEvents() usage, there may be case where you need to do it, but in general this is a good indicator for bad implementation.

    If you want to wait an signal to be emitted, you should use a local QEventLoop

    QEventLoop myLoop;
    
    // exit loop after 30 seconds with exit code -1 (timeout)
    QTimer::singleShot(30000, &myLoop, [&myLoop]() { myLoop.exit(-1); });
    // signal which will stop wait
    connect(&myObject, &WorkingClass::done, &myLoop, &QEventLoop::quit);
    
    int result = myLoop.exec(); // wait until signal is emitted or timeout
    if(result < 0)
        qDebug() << "Timeout!!";
    
    


  • @Bonnie , @KroMignon , in my console application what I want to achieve is the following:

    • At a fixed frequency send a heartbeat message to a QTcpServer
    • Call an derived body function do perform any other required activities

    The above is currently done in my loopUntilExit function, but the message isn't being written and I never get a bytesWritten signal raised.


  • Lifetime Qt Champion

    @SPlatten said in Console application & QTcpSocket:

    At a fixed frequency send a heartbeat message to a QTcpServer

    Qtimer

    "Call an derived body function do perform any other required activities" - call it, or is there problems doing so?

    "I've just installed the Debug Symbols and I still can't step into QIODevice/write to see what's happening." - isn't this from your other thread and not relevant for this one? Please don't mix topics. To step into Qt code you have to install Qt source code and set the path to it in QtCreator. Debug symbols only help to get meaningful stack traces.



  • @SPlatten said in Console application & QTcpSocket:

    in my console application what I want to achieve is the following:

    • At a fixed frequency send a heartbeat message to a QTcpServer
    • Call an derived body function do perform any other required activities

    To complete the application done by @Bonnie :

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
    
        QTcpSocket socket;
        QTimer heartBeat;
    
        heartBeat.setSingleShot(false);
        heartBeat.setInterval(5000); // 5 seconds
        QObject::connect(heartBeat, &QTimer::timeout, [&](){
            QDataStream stream;
            qDebug() << "heartBeat";
            stream.setDevice(&socket);
            stream.setVersion(QDataStream::Qt_5_14);
            stream << "heartBeat";
        };
        
        QObject::connect(&socket, &QTcpSocket::bytesWritten, [&](qint64 bytes){
            qDebug() << bytes << "written";
        });
    
        QObject::connect(&socket, &QTcpSocket::connected, [&](){
            QDataStream stream;
            qDebug() << "connected";
            stream.setDevice(&socket);
            stream.setVersion(QDataStream::Qt_5_14);
            stream << "TEST";
            heartBeat.start();
        });
        
        QObject::connect(&socket, &QTcpSocket::disconnected, [&]() {
             qDebug() << "Bye bye";
            a.exit();
        };
        
        qDebug() << "connecting";
        socket.connectToHost("localhost", 8123);
    
        return a.exec();
    }
    
    


  • @Bonnie ,@jsulm ,@KroMignon , odd results now, I've modified the console app main to:

    QCoreApplication a(intArgc, parystrArgv);
    QCoreApplication::setApplicationName(clsModFileIO::scpszTitle());
    clsModFileIO obj(&a, intArgc, parystrArgv);
    return a.exec();
    

    In the clsModFileIO constructor:

    QObject::connect(this, SIGNAL(connected()), this, SLOT(onConnected()));
    QObject::connect(this, SIGNAL(connected()), this, SLOT(onSendModuleStartupMsg()));
    QObject::connect(this, SIGNAL(connected()), this, SLOT(onStartHeartbeat()));
    QObject::connect(this, SIGNAL(disconnected()), this, SLOT(onDisconnected()));
    QObject::connect(this, &QAbstractSocket::errorOccurred, this, &clsModHelper::onErrorOccurred);
    QObject::connect(this, &QIODevice::readyRead, this, &clsModHelper::onDataIn);
    QObject::connect(this, SIGNAL(bytesWritten(qint64)), this, SLOT(onBytesWritten(qint64)));        
    

    This class is derived from QTcpSocket. OnConnected is a pure virtual slot. I am getting a receipt in onDataIn, but the encoding looks messed up, I am sending on JSON in my messages, what I get back looks like Japanese:

    笢浯摵汥∺≘䵌䵐䅍≽
    

  • Lifetime Qt Champion

    @SPlatten said in Console application & QTcpSocket:

    what I get back looks like Japanese

    Can't comment on that as I don't know what you are sending back and what encoding you're using.



  • @jsulm, I haven't called any encoding methods, what is the default?


  • Lifetime Qt Champion

    @SPlatten For QString you can find this information in the documentation: https://doc.qt.io/qt-5/qstring.html



  • @SPlatten said in Console application & QTcpSocket:

    This class is derived from QTcpSocket. OnConnected is a pure virtual slot. I am getting a receipt in onDataIn, but the encoding looks messed up, I am sending on JSON in my messages, what I get back looks like Japanese:
    笢浯摵汥∺≘䵌䵐䅍≽

    Again this depends how you have written data on socket and how you read them on the other side.
    You should know that for Qt QString internally always stored in UTF-8, but when you send it "outside" you have to take care about which string format is to be used.

    Please show the JSON write to socket and JSON read from socket code.



  • @SPlatten
    I remember you're sending a QByteArray ( from QJsonDocument::toJson() ) by QDataStream.
    Do you also use QDataStream to read to a QByteArray in onDataIn?



  • @Bonnie , @jsulm , @KroMignon , this is my send function:

    void clsModHelper::sendJSON(QJsonObject& objJSON) {
        if ( isOpen() != true ) {
            return;
        }
        //Associate this TCP socket with the output data stream
        QByteArray arybytMsg;
        QDataStream dsOut(&arybytMsg, QIODevice::WriteOnly);
        //dsOut.setDevice(this);
        dsOut.setVersion(clsJSON::mscintQtVersion);
        //Send message to data stream
        dsOut << QJsonDocument(objJSON).toJson(QJsonDocument::Compact);
        //Write message
        write(arybytMsg);
    }
    

    And receiving:

    void clsModFileIO::onDataIn() {
        QDataStream dsIn;
        dsIn.setDevice(this);
        dsIn.setVersion(clsJSON::mscintQtVersion);
    
        QString strRx;
        dsIn.startTransaction();
        dsIn >> strRx;
    
        if ( strRx.isEmpty() != true ) {
            qdbg() << "onDataIn: " << strRx;
        }
        if ( dsIn.commitTransaction() == false ) {
            return;
        }
    }
    

    clsModFileIO is derived from clsModHelper. clsJSON::mscintQtVersion is defined as:

    const int clsJSON::mscintQtVersion      = QDataStream::Qt_5_14;
    


  • @SPlatten
    But you are sending a QJsonDocument::toJson() which returns a QByteArray and receiving a QString? So....



  • @SPlatten as @JonB has already written QJsonDocument::toJson() returns a QByteArray not as QString.

    You have to send and read the same thing if you want it to work!

    void clsModFileIO::onDataIn() {
        QDataStream dsIn;
        dsIn.setDevice(this);
        dsIn.setVersion(clsJSON::mscintQtVersion);
    
        QByteArray jsonArray;
        dsIn.startTransaction();
        dsIn >> jsonArray;
    
        QJsonDocument doc;
        if ( jsonArray.isEmpty() != true ) {
            qdbg() << "onDataIn: " << jsonArray;
            doc = QJsonDocument::fromJson(jsonArray);
        }
        if ( dsIn.commitTransaction() == false ) {
            return;
        }
    }
    

    [EDIT] This only works if you also change the send:

    void clsModHelper::sendJSON(QJsonObject& objJSON) {
        if ( isOpen() != true ) {
            return;
        }
        //Associate this TCP socket with the output data stream
        QDataStream dsOut;
        dsOut.setDevice(this);
        dsOut.setVersion(clsJSON::mscintQtVersion);
        //Send message to data stream
        dsOut << QJsonDocument(objJSON).toJson(QJsonDocument::Compact);
    }
    


  • This post is deleted!


  • @KroMignon , thank you, now its working!


Log in to reply