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QTcpSocket / QTcpServer, two way communication



  • I am trying to sort out what I'm doing wrong...I have an application that uses QTcpServer to listen for connections and this works....Another process uses QTcpSocket to connect to the server and send a message.

    I also want to be able to send messages from the server application to the process using QTcpSocket and its this that I'm having problems with.

    When the client (process using QTcpSocket) sends a message, the server (process using QTcpServer) answers with an Acknowledge message, no problems with either of these and this works perfectly.

    I have a problem sending a message from the server to the client that isn't in response to a message already received, when I tried this using the same socket that is used to receive messages from the client, nothing is written and the result returned from write is -1.

    The server application:

    listen(QHostAddress::Any, uint16Port);
    QList<QHostAddress> lstAddresses = QNetworkInterface::allAddresses();
    QString strIP;
    for( int i=0; i<lstAddresses.size(); i++ ) {
        if ( lstAddresses[i] != QHostAddress::LocalHost
          && lstAddresses[i].toIPv4Address() ) {
            strIP = lstAddresses[i].toString();
            break;
        }
    }
    if ( strIP.isEmpty() == true ) {
        strIP = QHostAddress(QHostAddress::LocalHost).toString();
    }
    Q_ASSERT_X(isListening(), "clsSocketServer::clsSocketServer", strListenFailure.toLatin1().data());
    QObject::connect(this, &QTcpServer::newConnection, this, &clsSocketServer::newConnection);
    #if defined(DEBUG_SOCKETS)
    qdbg() << tr("Listening on: %1:%2\n")
                .arg(strIP).arg(uint16Port);
    #endif
    

    The client:

    QByteArray arybytMsg;
    QDataStream dsOut(&arybytMsg, QIODevice::WriteOnly);
    dsOut.setVersion(clsJSON::mscintQtVersion);
    //Send message to data stream
    dsOut << QJsonDocument(objJSON).toJson(QJsonDocument::Compact);
    //Write message
    qint64 int64Written = write(arybytMsg);
    qdbg() << "sendJSON: " << int64Written;
    

  • Lifetime Qt Champion

    Hi,

    How have you implemented the socket handling server-side ?

    There's likely something wrong there as you should be able to send data back.



  • @SGaist , I'm really not sure why I can't send, I can send an Acknowledge back and this works in response to receiving a message, but when I try to send an unsolicited message it fails.


  • Lifetime Qt Champion

    Hence my question: how did you implement the socket handling server side ?



  • @SGaist , Yes, however it could probably be improved, did you see the edit of my post which has the only signal and slot I'm connecting to on the server?

    When a message is send from the server to the client, the code halts on line 127 of qsocketnotifier.h:

    return lhs.sockfd != rhs.sockfd;
    

  • Lifetime Qt Champion

    Your connect call looks fishy.

    Why are you using this twice in your connect statement ? It looks like you are connecting the signal of the base class to either the same signal in a derived class or that you have a slot of the exact same name as the signal.

    Also, why did you subclass QTcpServer ?



  • @SGaist , because the class is derived from QTcpServer:

        class clsSocketServer : public QTcpServer {
        Q_OBJECT
    
        private:
            static quint16 msuint16nextModulePort;
    
            void discardClient();
    
        protected:
            void incomingConnection(qintptr sfd) override;
    
        public:
            static const QString mscstrCRLF;
            static const quint16 mscuint16port;
    
            explicit clsSocketServer(QObject* pParent = nullptr, quint16 uint16Port = clsSocketServer::mscuint16port);
            ~clsSocketServer();
    
            static quint16 uint16NextModulePort() {
                return ++clsSocketServer::msuint16nextModulePort;
            }
    
        signals:
            void newConnWithSocket(QAbstractSocket* pSocket);
    
        public slots:
            void acceptError(QAbstractSocket::SocketError socketError);
            void newConnection();
        };
    


  • @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    When a message is send from the server to the client, the code halts on line 127 of qsocketnotifier.h:

    Why do you have create a QTcpServer sub-class? Are you doing something special in incomingConnection()?
    Most of the time there is not need to sub-class QTcpServer, by the way, I never had to do it!


  • Lifetime Qt Champion

    So first question: why do you feel the need to subclass QTcpServer ? 95% of the time it's not necessary, always think composition over inheritance first.

    I know that the usual way to build a Qt application is to create a class derived from QWidget/QMainWindow/QDialog and that you will have to do that for custom widgets as well, but you usually do that on a specific set of classes to then do composition with other elements unless you actively need some custom behaviour.

    Next: why do you name a slot with exactly the same name a signal already existing in the base class ? That's actively searching for troubles. A recommandation for slot names: they usually match an action. Here for example: onNewConnection would make it clear that you are acting on a new connection.



  • The only purpose of this application is to act as a gateway / server between another application and the various modules it creates, I sub-classed QTcpServer because the main class and purpose is to be a server, I see no problem in doing this and its cleaner.

    I agree with you on naming slots with a prefix of 'on' and this is exactly what I usually do.



  • @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    purpose is to be a server, I see no problem in doing this and its cleaner.

    But why did you reimplement incomingConnection() ?
    Don't understand my wrong, it is not a judgement in any form, I only wonder why you have to do it.



  • @KroMignon , My incomingConnection:

    void clsSocketServer::incomingConnection(qintptr sfd) {
        clsSocketClient* pClient = new clsSocketClient(this);
        pClient->setSocketDescriptor(sfd);
        addPendingConnection(pClient);   
    }
    

    This part of the application is working ok, in that an external application can send messages to the server, but its sending messages from the server to the external application that is failing...all except the Acknowledge message which does make to the external application.



  • @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    its sending messages from the server to the external application that is failing...all except the Acknowledge message which does make to the external application.

    Just be sure we are talking about the same issue:
    You have a TCP client which is connected to the TCP server, the connected client has an instance of clsSocketClient (which seems to be a subclass of QTcpSocket). Let's call the instance mClient.
    When you do mClient.write(<something>), you've got an error.

    Is that your problem?



  • @KroMignon , there is no error sending data from the client to the server application, it regularly sends a message, a heartbeat message, which the server responds with an acknowledge. This acknowledge is also sent from the server with no error.

    However if the server attempts to send any other message to the client, this fails and is not written, the return is -1.



  • @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    However if the server attempts to send any other message to the client, this fails and is not written, the return is -1.

    I don't understand this, what do you mean with "server attempts to send to client"?
    The server TCP socket can not send/receive anything, it can only accept/reject connection requests, which are transferred to a new TCP socket. This is TCP basics, or did I misunderstood something?

    Can you be more explicit or show a code extract?



  • @KroMignon , the intention is for the server to be able to send messages to the client. However when I've tried to do this is always fails, yet a response to a message received by the server is always successful..


  • Lifetime Qt Champion

    Not knowing your code base it's difficult to answer since we do not know what happens client side and we do not know how you handle the sending of message back to the client.

    One thing however is that it looks like you are making things more complicated than necessary. From your description, you seem to need a QTcpServer to get the connections and then a list of QTcpSocket for the exchange of messages. So I fail to see the need to subclass both of them in order to implement your proxy.



  • @SGaist , subclass or not subclass, the implementation would be the same?

    The clients all send the server regular heartbeats, if a client does not receive an ack in response to the heartbeat then it can determine that the server is no longer running and self terminate.

    From time to time the server will issue commands to the clients, its these that do not work.

    In trying to fix this I've managed to break everything so once I get it back up I will publish some source.


  • Lifetime Qt Champion

    @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    @SGaist , subclass or not subclass, the implementation would be the same?

    Not necessarily, it depends on what your code does.



  • @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    From time to time the server will issue commands to the clients, its these that do not work.
    In trying to fix this I've managed to break everything so once I get it back up I will publish some source.

    It looks to me as you want to reimplement TCP KeepAlive feature.
    Why do not simply activate KeepAlive option on TCP socket?

    But perhaps I don't understand very well your explanation.
    When you are talking about "server" and "client" do you mean the application or the socket?



  • @KroMignon , by server I mean the application using QTcpServer that is in effect the master because it is the application that starts the other processes, listens for incoming messages and issues commands to the other processes.

    The clients use QTcpSocket to connect to the server application. If the server goes away then the clients will self terminate. I don't know about the KeepAlive feature so I will read up, but basically the heartbeat is an alternative to pinging the server to see if it is active and responding. Can I self terminate if the server doesn't respond using the KeepAlive?



  • @SPlatten said in QTcpSocket / QTcpServer, two way communication:

    Can I self terminate if the server doesn't respond using the KeepAlive?

    TCP KeepAlive feature, send "non data" packet between two endpoints to check if counterpart is still present/reachable. If not, the connection is closed (cf. https://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html).

    To enable KeepAlive: socket->setSocketOption(QAbstractSocket::KeepAliveOption, 1);

    But, there are always some backdrawn. With Qt, you can only switch on the KeepAlive feature for the socket (must be done when socket is connected, not before!).
    According to your OS, you also have to setup the KeepAlive parameters:

    • IDLE interval before starting KeepAlive
    • how many KeepAlive retries before deconnection
    • interval between each KeepAlive

    For example for Linux (maybe Unix?) system, the TCP settings can be found on /proc/sys/net/ipv4:

    • tcp_keepalive_time: interval between the last data packet sent and the first keepalive probe in seconds.
    • tcp_keepalive_intvl: interval between subsequent keepalive probes in seconds.
    • tcp_keepalive_probes: number of probes that are sent and unacknowledged before the client considers the connection broken and notifies the application layer


  • Whats the live cycle of a connection with QTcpSocket?

    My client connects to the application which is listening, here is the client connection code:

    setSocketOption(QAbstractSocket::LowDelayOption, 1);
    //Get the I/P address
    QList<QHostAddress> lstAddresses = QNetworkInterface::allAddresses();
    QString strIP;
    for( int i=0; i<lstAddresses.size(); i++ ) {
        if ( lstAddresses[i] != QHostAddress::LocalHost
          && lstAddresses[i].toIPv4Address() ) {
            strIP = lstAddresses[i].toString();
            break;
        }
    }
    if ( strIP.isEmpty() == true ) {
        strIP = QHostAddress(QHostAddress::LocalHost).toString();
    }
    //Connect to the Application
    qdbg() << "Connecting to: " << strIP << ":" << muint16XMLMPAMport;
    connectToHost(strIP, muint16XMLMPAMport);
    

    The port is 8124. This does connect to the server and sends a message with:

    void clsSocketClient::sendJSON(const QJsonObject& crobjJSON) {
        if ( isOpen() != true ) {
            return;
        }
        //Associate this TCP socket with the output data stream
        QByteArray arybytMsg;
        QDataStream dsOut(&arybytMsg, QIODevice::WriteOnly);
        dsOut.setVersion(clsJSON::mscintQtVersion);
        //Send message to data stream
        dsOut << QJsonDocument(crobjJSON).toJson(QJsonDocument::Compact);
    
        //Write message
    #if defined(DEBUG_SOCKETS)
        qint64 int64Written =
    #endif
        write(arybytMsg);    
    #if defined(DEBUG_SOCKETS)    
        QJsonDocument objDoc(crobjJSON);
        qdbg() << "clsSocketClient::sendJSON"
               << "[" << int64Written << "]:"
               << QString(objDoc.toJson(QJsonDocument::Compact));
    #endif
    }
    

    This works and I can see that int64Written is > 0 in the Application Output. The server application receives this message and sends and Ack:

    bool blnDecodeHeartbeat(const QJsonObject& crobjJSON) {
        QJsonObject::const_iterator citrFound = crobjJSON.find(clsJSON::mscszModule);
    
        if ( citrFound == crobjJSON.end() ) {
            return false;
        }
        QString strModuleName = citrFound.value().toString();
        clsModule* pModule = clsModule::pGetModule(strModuleName);
    
        if ( pModule == nullptr ) {
            return false;
        }
        pModule->updateHearbeat();
        //Send acknowledge back to module
        QJsonObject objJSON;
        objJSON.insert(clsJSON::mscszModule, strModuleName);
        objJSON.insert(clsJSON::mscszMsgType, clsJSON::mscszAck);
        //Cast the socket to the required type
        emit pModule->sendJSON(objJSON);
        return true;
    }
    

    This decode is called the instance the message arrives from the client and the sending of the Ack message is always successful. The server later tries to send a message to the client which is not the result of receiving a message:

    void clsScriptHelper::notify(const QJsonObject& crobjModule
                                ,QJsonObject objCmds) {
        QJsonObject::const_iterator citrFound = crobjModule.find(clsJSON::mscszModule);
        QString strModuleName;
        clsModule* pModule;
    
        if ( citrFound == crobjModule.end() ) {
            return;
        }
        strModuleName = citrFound.value().toString();
        //Make sure the module is set in the message
        objCmds.insert(clsJSON::mscszModule, strModuleName);
        //Set command type
        objCmds.insert(clsJSON::mscszMsgType, clsJSON::mscszCmdNotify);
        //Look up the socket for the module
        pModule = clsModule::pGetModule(strModuleName);
    
        if ( pModule == nullptr || pModule->blnTxAllowed() != true ) {
            clsModule::sendLater(strModuleName, objCmds);
            return;
        }
        //Convert object into byte array for transmission
        emit pModule->sendJSON(objCmds);
    }
    

    The code to send Later:

    void clsModule::sendLater(const QString& crstrModule, const QJsonObject& crobjJSON) {
        mpSendLater::iterator itrList = clsModule::msmpSendLaterLst.find(crstrModule);
        QJsonObject* pobjJSON;
    
        if ( itrList != clsModule::msmpSendLaterLst.end() ) {
            pobjJSON = itrList->second;
        } else {
            pobjJSON = new QJsonObject();
    
            if ( pobjJSON != nullptr ) {
                clsModule::msmpSendLaterLst.insert(std::make_pair(crstrModule, pobjJSON));
            }
        }
        if ( pobjJSON != nullptr ) {
            *pobjJSON = crobjJSON;
    #if defined(DEBUG_SOCKETS)
            QString strExpoded(clsJSON::strExplodeJSON(*pobjJSON, 0));
            qdbg() << "clsModule::sendLater: " << strExpoded.toLatin1().data();
    #endif
        }
    }
    

    The purpose of this function is that if the client is not connected or not ready then it inserts the message into a map that is supposed to be transmitted when the client next connects.

    When the client is connected the waiting messages are sent using the clsSocketClient::sendJSON as posted above, and this fails 100% of the time. Why?

    Its as if the client isn't ready to receive a message. Typical output in Application Output:

    D00000000000000000028T000000000915:clsJSON::commonDecode [50]: {"module":"mdFileIO","msgType":"init","port":8124}
    W00000000000000000029T000000001334:QProcess::setProgram: Process is already running
    D00000000000000000030T000000001334:Process: mdFileIO started, PID: 1046
    D00000000000000000031T000000002816:clsJSON::commonDecode [49]: {"PID":"1046","module":"mdFileIO","msgType":"hb"}
    D00000000000000000032T000000002816:clsSocketClient::sendJSON[41]:{"module":"mdFileIO","msgType":"ack"}
    D00000000000000000033T000000002816:clsSocketClient::onBytesWritten:41
    D00000000000000000034T000000004718:clsJSON::commonDecode [49]: {"PID":"1046","module":"mdFileIO","msgType":"hb"}
    D00000000000000000035T000000004718:clsSocketClient::sendJSON[41]:{"module":"mdFileIO","msgType":"ack"}
    D00000000000000000036T000000004718:clsSocketClient::onBytesWritten:41
    

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