QTcpSocket / QTcpServer, connection closed

SPlatten

As is probably very obvious, I'm new to using the classes QTcpSocket and QTcpServer. I've been implementing communications between two processes.

Application A is the application that uses QTcpServer and is the master.
Application B is a child of application A, that is A launches B.

As soon as B launches it sends a message to A. I can see from the bytesWritten message that 54 bytes are sent. Then for some reason A closes the connection to B and on the 2nd attempt of B to write to A I get an error:

QAbstractSocket::RemoteHostClosedError

I've looked at the source in A and nothing stands out, what could explain why A is closing the connection to B ?

B does connect again to A, but exactly the same thing happens, 54 bytes are sent then the connection is closed and A never receives the data.

I have a slot connected to readyRead, it never gets to the slot that will receive the data.

KroMignon

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

Thats all my source.

Maybe, but it is not clean coded.
Why did you not call addPendingConnection() in your incomingConnection() implementation as it is recommended in documentation?

JonB

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

QAbstractSocket::RemoteHostClosedError

Assuming from the message it really is that A/the server is closing (not somehow caused by A doing a close, which you could investigate similarly), couldn't you in A put slots on, say, https://doc.qt.io/qt-5/qabstractsocket.html#stateChanged, and other similar signals, to see if you can catch its state changing or closing? Also, I guess, make sure you don't have a socket going out of scope or being deleted.

SPlatten

@JonB , thank you, the instance of my class is created dynamically, presently there is no destuction, I will look at adding the stateChanged signal, see if that helps.

Pablo J. Rogina

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

I'm new to using the classes QTcpSocket and QTcpServer
Application B is a child of application A, that is A launches B.

Given that context, may I suggest that you work with A & B as completely separated applications/executables, even launching B by hand while you develop it and until you have the communication/protocol 100% working fine.

Once you reach that point, next step will be the "easy" task of making A launch B but all the issues of the communication part were resolved beforehand and in a way that's easy to debug.

SPlatten

@JonB , this is my log when debugging:

D00000000000000000018:onConnected
D00000000000000000019:sendJSON: 54
D00000000000000000020:clsModHelper::onBytesWritten:54
D00000000000000000021:Socket error(QAbstractSocket::RemoteHostClosedError) https://www.google.com/search?rls=en&q=socket+error+code+QAbstractSocket::RemoteHostClosedError&ie=UTF-8&oe=UTF-8QAbstractSocket::RemoteHostClosedError.php

The entry sendJSON comes from the function writing to the socket, where it displays the result returned from calling write().

The onBytesWritten is the slot connected to the signal bytesWritten. Then an error occurs I've added the URL to aid lookup.

In application A I've added the call:

QObject::connect(this, &QAbstractSocket::stateChanged, this, &clsSocketClient::onStateChanged);

Which is on the click socket, but I never see any activity in the slot.

KroMignon

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

QAbstractSocket::RemoteHostClosedError

Take a look at Qt documentation ==> https://doc.qt.io/qt-5/qabstractsocket.html#SocketError-enum

The socket on the other side (I guess server) has closed the connection. Maybe socket has been destroyed.

JonB

@SPlatten
I have never done any of this (from Qt), and I don't know what your various variables refer to.

At some point in the server you call QTcpSocket *QTcpServer::nextPendingConnection(), right? It is the socket returned from that, which should start in QAbstractSocket::ConnectedState, that you are trying to monitor at the server side. Is that what you're doing? You should also check for anything else, like errored/destroyed, whatever the Qt signals for those are.

And just BTW: are you using any multiple threading, especially at the server?

Finally, if you're still stuck, you know the drill: write a totally minimal server which listens on the port and just accepts connections, nothing else. Do your clients still get RemoteHostClosedErrors?

SPlatten

The class derived from QTcpServer:

clsSocketServer::clsSocketServer(QObject* pParent) : QTcpServer(pParent) {
    QString strListenFailure = QString("Cannot listen to port: ") + QString::number(clsSocketServer::mscuint16port);
    listen(QHostAddress::Any, clsSocketServer::mscuint16port);
    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();
    }
    qdbg() << tr("XMLMPAM is listening on: %1:%2\n")
                .arg(strIP).arg(clsSocketServer::mscuint16port);
    connect(this, &QTcpServer::newConnection, this, &clsSocketServer::sayHello);
    Q_ASSERT_X(isListening(), "clsSocketServer::clsSocketServer", strListenFailure.toLatin1().data());
}
/**
 * @brief clsSocketServer::~clsSocketServer - class destructor
 */
clsSocketServer::~clsSocketServer() {
    close();
}
/**
 * @brief clsSocketServer::discardClient
 */
void clsSocketServer::discardClient() {
    QTcpSocket* pClient = (QTcpSocket*)sender();
    pClient->deleteLater();
}
/**
 * @brief clsSocketService::incomingConnection
 * @param sfd : Client Socket Descriptor
 */
void clsSocketServer::incomingConnection(qintptr sfd) {
    clsSocketClient* pClient = new clsSocketClient(sfd, this);
    pClient->setSocketDescriptor(sfd);
}
/**
 * @brief clsSocketServer::sayHello
 */
void clsSocketServer::sayHello() {
    QTcpSocket* pClient = this->nextPendingConnection();

    if ( pClient == nullptr ) {
        return;
    }
    QByteArray arybytMsg;
    QDataStream dsOut(&arybytMsg, QIODevice::WriteOnly);
    dsOut.setVersion(clsJSON::mscintQtVersion);
    QJsonObject objMsg;
    objMsg.insert(clsJSON::mscszModule, clsMainWnd::mscstrTitle);
    dsOut << QJsonDocument(objMsg).toJson(QJsonDocument::Compact);
    connect(pClient, &QAbstractSocket::disconnected
           ,pClient, &QObject::deleteLater);
    sendJSON(*pClient, objMsg);
}
/**
 * @brief clsSocketServer::sendJSON
 * @param objClient : Reference to client socket
 * @param objJSON : Reference to JSON object to transmit
 */
void clsSocketServer::sendJSON(QTcpSocket& objClient, QJsonObject& objJSON) {
    if ( objClient.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
    qint64 int64Written = objClient.write(arybytMsg);

    qdbg() << "sendJSON: " << int64Written;
}

This isn't finished, however once its created which there is only a single instance, it is not destroyed. A client is created by the overloaded function "incomingConnection".

Presently there is no additional management of connections going on, nothing is cleaning up, closing or disconnecting clients.

qdbg is just a macro:

#define qdbg()      qDebug().noquote().nospace()

KroMignon

@SPlatten Please do not just copy/past code found on internet, try also to read documentation to understand what you are doing.

You have reimplemented QTcpServer::incomingConnection(), why not. But if you what to also use signal QTcpServer::newConnection(), you have to add the socket into the pending list with QTcpServer::addPendingConnection().

SPlatten

@KroMignon , What code found on the internet? Thats all my source.

KroMignon

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

Thats all my source.

Maybe, but it is not clean coded.
Why did you not call addPendingConnection() in your incomingConnection() implementation as it is recommended in documentation?

SPlatten

@KroMignon , sorry I clicked the wrong thing, the problem is not solved.

SPlatten

@KroMignon , I've stopped the main application from creating the process, so I can debug it in two instances of Qt Creator and see whats going on.

So for the sake of this past A = the original parent application and B is the other application. A uses QTcpServer and B uses QTcpCSocket.

When A starts up:

clsSocketServer::clsSocketServer(QObject* pParent) : QTcpServer(pParent) {
    QString strListenFailure = QString("Cannot listen to port: ") + QString::number(clsSocketServer::mscuint16port);
    listen(QHostAddress::Any, clsSocketServer::mscuint16port);
    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();
    }
    qdbg() << tr("XMLMPAM is listening on: %1:%2\n")
                .arg(strIP).arg(clsSocketServer::mscuint16port);
    connect(this, &QTcpServer::newConnection, this, &clsSocketServer::sayHello);
    Q_ASSERT_X(isListening(), "clsSocketServer::clsSocketServer", strListenFailure.toLatin1().data());
}

The slot connected to newConnection:

void clsSocketServer::sayHello() {
    clsSocketClient* pClient = new clsSocketClient(nextPendingConnection());

    if ( pClient == nullptr ) {
        return;
    }
    QByteArray arybytMsg;
    QDataStream dsOut(&arybytMsg, QIODevice::WriteOnly);
    dsOut.setVersion(clsJSON::mscintQtVersion);
    QJsonObject objMsg;
    objMsg.insert(clsJSON::mscszModule, clsMainWnd::mscstrTitle);
    dsOut << QJsonDocument(objMsg).toJson(QJsonDocument::Compact);
    connect(pClient, &QAbstractSocket::disconnected
           ,pClient, &QObject::deleteLater);
    pClient->sendJSON(objMsg);
}

When B starts up:

clsModHelper::clsModHelper(QObject* pParent, int intArgc, char* parystrArgv[]
                          ,const char* cpszTitle, double dblVersion)
            : QTcpSocket(pParent)            
            , mdblVersion(dblVersion)
            , mfpDbgLog(nullptr)
            , mint64AppPID(QCoreApplication::applicationPid())
            , mstrTitle(cpszTitle)
            , muint16ModulePort(0), muint16LauncherPID(0)
            , muint16XMLMPAMport(0) {
    if ( intArgc < CLA_LAUNCHER_PID ) {
        std::cout << "Insufficient arguments, aborting!" << std::endl;
        exit(EXIT_FAILURE);
    }
    if ( mspThis == nullptr ) {
        mspThis = this;
    }
    muint16XMLMPAMport = (quint16)atoi(parystrArgv[CLA_XMLMPAM_PORT]);
    muint16ModulePort = (quint16)atoi(parystrArgv[CLA_MODULE_PORT]);
    muint16LauncherPID = (quint16)atoi(parystrArgv[CLA_LAUNCHER_PID]);
    setSocketOption(QAbstractSocket::LowDelayOption, 1);
    //Connect up the signals
    QObject::connect(QCoreApplication::instance(), &QCoreApplication::aboutToQuit
                    ,this, &clsModHelper::onExitModule);
    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)));        
    QObject::connect(this, &clsModHelper::terminateModule, this, &clsModHelper::onExitModule);
    qdbg() << state();
    //Install custom message handler
    clsDebugService::serviceDebugQueue(cpszTitle, true, true);
    //Build up title for output to the console
    qdbg() << QString::asprintf("%s Version: %.2lf", cpszTitle, mdblVersion);
    connectToXMLMPAM();
}

The function connectToXMLMPAM():

/**
 * @brief clsModHelper::connectToXMLMPAM
 */
void clsModHelper::connectToXMLMPAM() {
    //Start of one-shot timer to connect to XMLMPAM
    QTimer::singleShot(clsModHelper::mscintModuleCycleFrequency, this, SLOT(onConnectToXMLMPAM()));
}

If there is anything else I haven't posted, please let me know, I'm not seeing anything received by A, yet B keeps logging that its sent data and there are no errors.

KroMignon

@SPlatten did you remove clsSocketServer::incomingConnection()?

SPlatten

@KroMignon yes

KroMignon

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

yes

I am not sure you are understanding what you are doing, and that's bad.

The protected virtual function QTcpServer::incomingConnection() is called by QTcpServer on each new connection request.
The default implementation of this function will create a QTcpSocket instance with the connection informations and use the QTcpServer as parent and finally add the new instance to the connection queue with addPendingConnection().

With this default implementation, all clients will run in same thread as QTcpServer, because QObject with a parent cannot be move to another thread, only the parent QObject.
The call addPendingConnection() will also trigger a signal QTcpServer::newConnection().
And finally you can retrieve to instance you have add to queue with QTcpServer::nextPendingConnection().

This is why QTcpServer::incomingConnection() exists and how it has to be used.

So again, please take time to read documentation of the classes you are using.
The documentation is not so bad!

To summarize: in your first code, you are subclassing incomingConnection() but not add the new connection to pending queue with addPendingConnection() and finally waiting for signal QTcpServer::newConnection() which will never be triggered.

SPlatten

@KroMignon , What I'm struggling to get to grips with is getting the QTcpServer and QTcpSocket to work. I'm using the examples to prod and learn. I have written lots of socket based applications in the past, not Qt.

I am an experience programmer, the samples FortuneServer and FortuneClient do not use incomingConnection directly either, they are very lean examples. The examples to not call addPendingConnection either.

Initialisation of QTcpServer in FortuneServer:

    tcpServer = new QTcpServer(this);
    if (!tcpServer->listen()) {
        QMessageBox::critical(this, tr("Fortune Server"),
                              tr("Unable to start the server: %1.")
                              .arg(tcpServer->errorString()));
        close();
        return;
    }
//! [0]
    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();
    // use the first non-localhost IPv4 address
    for (int i = 0; i < ipAddressesList.size(); ++i) {
        if (ipAddressesList.at(i) != QHostAddress::LocalHost &&
            ipAddressesList.at(i).toIPv4Address()) {
            ipAddress = ipAddressesList.at(i).toString();
            break;
        }
    }
    // if we did not find one, use IPv4 localhost
    if (ipAddress.isEmpty())
        ipAddress = QHostAddress(QHostAddress::LocalHost).toString();
    statusLabel->setText(tr("The server is running on\n\nIP: %1\nport: %2\n\n"
                            "Run the Fortune Client example now.")
                         .arg(ipAddress).arg(tcpServer->serverPort()));

The slot that is connected to the newConnection signal:

void Server::sendFortune()
{
    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_5_10);
    out << fortunes[QRandomGenerator::global()->bounded(fortunes.size())];

    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    qDebug() << "Server::sendFortune clientConnection->isOpen(): " << clientConnection->isOpen();
    qDebug() << "Server::sendFortune clientConnection->error(): " << clientConnection->error();

    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    clientConnection->write(block);
    clientConnection->disconnectFromHost();
}

The FortuneClient doesn't use addPendingConnection either.

In my application newConnection is getting called, I've said this before. Its sending data successfully that I'm now looking at as this isn't happening.

KroMignon

@SPlatten said in QTcpSocket / QTcpServer, connection closed:

What I'm struggling to get to grips with is getting the QTcpServer and QTcpSocket to work. I'm using the examples to prod and learn. I have written lots of socket based applications in the past, not Qt.
I am an experience programmer, the samples FortuneServer and FortuneClient do not use incomingConnection directly either, they are very lean examples. The examples to not call addPendingConnection either.

I don't want to hurt you, explain by writing in a foreign language is not so easy, maybe my words are not everytime appropriated.
I also had many problems to start with Qt, even if I was developing embedded software more than 20 years before in C/C++/Assembler.
There is a philosophy to understand and accept.
I am trying to follow this simple rule "If you are fight against the framework, than you are doing something wrong!".

I know there are many examples available, but most of them are outdated, but as Qt is open source, you can always take a look behind the scene and look what the code really does:

void QTcpServer::incomingConnection(qintptr socketDescriptor)
{
#if defined (QTCPSERVER_DEBUG)
    qDebug("QTcpServer::incomingConnection(%i)", socketDescriptor);
#endif

    QTcpSocket *socket = new QTcpSocket(this);
    socket->setSocketDescriptor(socketDescriptor);
    addPendingConnection(socket);
}

My big problem is that I don't really understand what do you want to achieve with your TCP server. I think you have a "bad start". That is normal, s***t happens ;)

I would love to help you, but I am a little lost, because I don't really understand your code and I can't understand the final result you want to have.
Create a TCP server with Qt is very simple, it takes less than 100 line codes.

SPlatten

@KroMignon , I'm starting to think the problem is not source code related but another factor. In the example I am looking at FortuneServer there is a fundamental difference. It gets the port to use automatically, where as in my code I am using a specific port 8123.

I've got the source to both FortuneServer and my own side by side and FortuneServer is much simpler in that it only implements one signal from QTcpServer "newConnection".

When a new client connection in the FortuneServer occurs it calls in the sendFortune slot:

 QTcpSocket *clientConnection = tcpServer->nextPendingConnection();

The difference is that the clientConnection status of isOpen is true, where as in my own slot, it's false.

In my slot that is called on a new Connection:

void clsSocketServer::sayHello() {
    clsSocketClient* pClient = new clsSocketClient(nextPendingConnection());

    if ( pClient != nullptr ) {
    //Construct the message to send
        QJsonObject objMsg;
        objMsg.insert(clsJSON::mscszModule, clsMainWnd::mscstrTitle);
        pClient->sendJSON(objMsg);
    }
}

In the function sendJSON:

void clsSocketClient::sendJSON(QJsonObject& objJSON) {
    //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(objJSON).toJson(QJsonDocument::Compact);
    //Write message
    qint64 int64Written = write(arybytMsg);
    //Make sure the data is written now
    flush();
    qdbg() << "sendJSON: " << int64Written;
}

The call to flush was just to ensure that the written content is not held in a buffer instead of calling disconnect, because I want the connection to remain open. But "int64Written" is -1 and I see in the "Application Output":

device not open

[Edit] Observing something odd...in my constructor for clsSocketClient I output:

qdbg() << __FILE__ << "," << __LINE__ << ",isOpen: " << pClient->isOpen();

This results in:

D00000000000000000020:../clsSocketClient.cpp,28,isOpen: true

Ignore the D number thats just my own message prefix, 28 is the line number, the at this point the socket is open. Then when it returns from the constructor and calls the function sendJSON:

void clsSocketClient::sendJSON(QJsonObject& objJSON) {
    //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(objJSON).toJson(QJsonDocument::Compact);
    //Write message
    qint64 int64Written = write(arybytMsg);
    //Make sure the data is written now
    flush();
    //disconnectFromHost();           //Is this required?
    qdbg() << "sendJSON: " << int64Written;
}

The "Application Output" shows:

W00000000000000000022:QIODevice::write (clsSocketClient): device not open
D00000000000000000023:sendJSON: -1

KroMignon

@SPlatten No this cannot work.

As far as I can understand your code clsSocketClient is as subclass of QTcpSocket.
So simple C++ error: auto pClient = new clsSocketClient(nextPendingConnection()); ~~is calling QTcpSocket copy constructor~~ is using the QTcpSocket as parent.

One solution would be to use incomingConnection to create the instance and save it in the queue:

/**
 * @brief clsSocketService::incomingConnection
 * @param sfd : Client Socket Descriptor
 */
void clsSocketServer::incomingConnection(qintptr sfd) {
    auto pClient = new clsSocketClient(this);
    pClient->setSocketDescriptor(sfd);
    addPendingConnection(pClient);
}

And then:

void clsSocketServer::sayHello() {
    auto pClient = qobject_cast<clsSocketClient*>(nextPendingConnection());

    if ( pClient != nullptr ) {
    //Construct the message to send
        QJsonObject objMsg;
        objMsg.insert(clsJSON::mscszModule, clsMainWnd::mscstrTitle);
        pClient->sendJSON(objMsg);
    }
}