Qt Message, what does it mean, is it important ?

SPlatten

In my debug file which I report various bits of information which includes warnings, errors, fatal errors and information. When I launch my application I see at the end of the file and always at the end of the file:

S000000000030E000000001009T15:55:41.939W:QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x60000002de00), parent's thread is clsServer(0x600000278700), current thread is clsThread(0x600002c50800)
S000000000031E000000002603T15:55:43.532W:QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x60000000e720), parent's thread is clsServer(0x6000002642e0), current thread is clsThread(0x600002c50800)
S000000000032E000000002647T15:55:43.577W:QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x600000020160), parent's thread is clsServer(0x600000260cc0), current thread is clsThread(0x600002c50800)

The bit before the message is my own information which I prefix all messages with:

S000000000030E000000001009T15:55:41.939W:

This is the code that constructs the output:

        QString strPrefix(QString("S%1E%2T%3")
                .arg(cint64NextStep, cintSeqNoLength, cintSeqNoBase, QChar('0'))
                .arg(cint64ElapsedTime, cintTimeLength, cintTimeBase, QChar('0'))
                .arg(cstrtimeNow));

The question is are these messages significant and are they anything to worry about? I assume because its always at the end that something is happening when I stop the application, as I'm still in development this usually is a stop from Qt Creator.

Christian Ehrlicher

@SPlatten said in Qt Message, what does it mean, is it important ?:

The question is are these messages significant and are they anything to worry about? I

Cannot create children for a parent that is in a different thread.

I mean - do they look as you can ignore them?
You do something wrong in your threading code - fix it.

JoeCFD

@Christian-Ehrlicher Agree. I will be worried.

SPlatten

@Christian-Ehrlicher , @JoeCFD , not terribly helpful. These are added to the log file when the application is started, not as I originally said when it is terminated, there are just three that go into the log file, no more no less.

These are the classes mentioned in the reported messages, prototypes:

/**
 * File:    clsListener.h
 * Notes:   This file contains the prototypes for the classes:
 *              clsServer and clsListener
 *******************************************************************************
 * Class:               clsServer
 * Inherits:            QThread
 *
 * Members:
 *  msckDescriptor      Socket descriptor
 *  mpsckIncoming       Pointer to incoming socket
 *
 * Methods:
 *  clsServer           Class constructor
 *  ~clsServer          Class destructor
 *  run                 Thread loop
 *
 * Signals:
 *  error               Signal emitted when error occurs
 *
 * Slots:
 *  cleanup             Cleanup socket
 *  onDisconnected      Slot connected to QAbstractSocket::disconnected
 *  onErrorOccurred     Slot connected to QAbstractSocket::errorOccurrred
 *  onReadyRead         Slot connected to QAbstractSocket::readyRead
 *******************************************************************************
 * Class:               clsListener
 * Inherits:            QTcpServer
 *
 * Static Methods:
 *  commonDecode        Common JSON decode
 *  pGetListener        Access method to get listener for IP port
 *  uint16NextPort      Get next port
 *
 * Static Members:
 *  msmpListeners       Map of listeners
 *  msuint16NextPort    Next available port
 *
 * Members:
 *  mpServer            Pointer to instance of QTcpServer
 *  muint16Port         Port assigned to listener
 *
 * Methods:
 *  clsListener         Class constructor
 *  ~clsListener        Class destructor
 *  uint16Port          Access method to get port
 *
 * Slots:
 *  onAcceptError       Slot to accept socket error
 *  onNewConnection     Slot to accept new connection
 *******************************************************************************
 * History: 2020/11/30 Created by Simon Platten
 */
#ifndef CLSLISTENER_H
    #define CLSLISTENER_H

    #include <map>

    #include <QAbstractSocket>
    #include <QDataStream>
    #include <QJsonDocument>
    #include <QJsonObject>
    #include <QQueue>
    #include <QTcpServer>
    #include <QTcpSocket>
    #include <QThread>

    class clsListener;
    typedef std::map<quint16, clsListener*> mpListeners;

    #if !defined(mqueJSON)
        typedef QQueue<QJsonObject> mqueJSON;
    #endif

    class clsServer : public QThread {
    Q_OBJECT

    private:
        qintptr msckDescriptor;
        QTcpSocket* mpsckIncoming;

    public:
        explicit clsServer(qintptr sckDescriptor, QObject* pParent = nullptr);
        ~clsServer();

        void run();

    signals:
        void error(QTcpSocket::SocketError socketerror);

    public slots:
        void cleanup();
        void onDisconnected();
        void onErrorOccurred(QAbstractSocket::SocketError error);
        void onReadyRead();
    };

    class clsListener : public QTcpServer {
    Q_OBJECT

    private:
        static mpListeners msmpListeners;
        static quint16 msuint16NextPort;
        quint16 muint16Port;

    public:
        clsListener(quint16 uint16Port, QObject* pParent = nullptr);
        ~clsListener();

        //static void commonDecode(QAbstractSocket* psckIn);
        static clsListener* pGetListener(quint16 uint16Port);
        static quint16 uint16NextPort() { return ++clsListener::msuint16NextPort; }
        quint16 uint16Port() { return muint16Port; }

    signals:

    protected slots:
        void incomingConnection(qintptr sckDescriptor);
    };
#endif // CLSLISTENER_H

Implementations:

/**
 * File:    clsListener.cpp
 * Notes:   Contains implementation of the listener class
 * History: 2020/11/30 Created by Simon Platten
 *
 * To test, copy and paste the line below into a browser:
 *  http://localhost:8123/?{"msgType":"ready","source":"simon"}
 */
#include <utility>

#include "clsDebugService.h"
#include "clsListener.h"
#include "clsModule.h"
/**
 * @brief clsServer::clsServer
 * @param sckDescriptor : Socket descriptor
 * @param pParent : Optional, pointer to parent
 */
clsServer::clsServer(qintptr sckDescriptor, QObject* pParent)
                                            : QThread(pParent)
                                            , msckDescriptor(sckDescriptor)
                                            , mpsckIncoming(nullptr) {
    //Register type for signals
    qRegisterMetaType<QAbstractSocket::SocketError>("QAbstractSocket::SocketError");
}
/**
 * @brief clsServer::~clsServer
 */
clsServer::~clsServer() {
    if ( isRunning() == true ) {
        terminate();
        while( isRunning() == true ) {
    //Wait until thread stops
        }
    }
    if ( mpsckIncoming != nullptr ) {
        cleanup();
    }
}
/**
 * @brief clsServer::cleanup
 */
void clsServer::cleanup() {
    if ( mpsckIncoming != nullptr ) {
        QObject::disconnect(mpsckIncoming, &QAbstractSocket::errorOccurred
                           ,this, &clsServer::onErrorOccurred);
        QObject::disconnect(mpsckIncoming, &QAbstractSocket::disconnected
                           ,this, &clsServer::onDisconnected);
        QObject::disconnect(mpsckIncoming, &QAbstractSocket::readyRead
                           ,this, &clsServer::onReadyRead);
        mpsckIncoming->deleteLater();
        mpsckIncoming = nullptr;
    }
}
/**
 * @brief clsServer::onDisconnected
 */
void clsServer::onDisconnected() {
    qinf() << "Disconnected";
    cleanup();
}
/**
 * @brief clsServer::onErrorOccurred
 * @param error : error that occurred
 */
void clsServer::onErrorOccurred(QAbstractSocket::SocketError error) {
    //Display error
    QString strURLencoded(QUrl::toPercentEncoding(mpsckIncoming->errorString()));
    qwrn() << "Socket error(" << strURLencoded
           << ") https://www.google.com/search?rls=en&q=socket+error+code+"
           << strURLencoded << "&ie=UTF-8&oe=UTF-8"
           << strURLencoded << ".php";
    //Prevent unused parameter warning
    (void)error;
}
/**
 * @brief clsServer::onReadyRead
 */
void clsServer::onReadyRead() {
    if ( mpsckIncoming == nullptr ) {
        return;
    }
    clsModule::manageReceivedData(mpsckIncoming);
}
/**
 * @brief clsServer::run
 */
void clsServer::run() {
    QTcpSocket* pSocket(new QTcpSocket());
    //Set the ID
    if( !pSocket->setSocketDescriptor(msckDescriptor) ) {
    //Something's wrong, we just emit a signal
        emit error(pSocket->error());
        return;
    }
    //Connect socket and signal
    pSocket->setSocketOption(QAbstractSocket::LowDelayOption, 1);
    mpsckIncoming = pSocket;
    QObject::connect(mpsckIncoming, &QAbstractSocket::errorOccurred
                    ,this, &clsServer::onErrorOccurred);
    QObject::connect(mpsckIncoming, &QAbstractSocket::disconnected
                    ,this, &clsServer::onDisconnected);
    QObject::connect(mpsckIncoming, &QAbstractSocket::readyRead
                    ,this, &clsServer::onReadyRead);
    //Make this thread a loop,
    //thread will stay alive so that signal/slot to function properly
    //not dropped out in the middle when thread dies
    exec();
}
//Static initialisation
mpListeners clsListener::msmpListeners;
quint16 clsListener::msuint16NextPort               = 8122; //Will is increment before use
/**
 * @brief clsListener class constructor
 * @param uint16Port : Port to listen on
 * @param pParent : Pointer to parent
 */
clsListener::clsListener(quint16 uint16Port, QObject* pParent)
                    : QTcpServer(pParent), muint16Port(uint16Port) {
    if ( uint16Port == 0 ) {
        throw "Port invalid!";
    }
    if ( clsListener::pGetListener(uint16Port) != nullptr ) {
        throw "Port already in use by another listener!";
    }
    qinf() << QString("Listening to any local IP address on port: %1").arg(muint16Port);
    //Start listening to port
    if ( !listen(QHostAddress::Any, muint16Port) ) {
        qwrn() << "Unable to lisen to port " << muint16Port;
    }
    clsListener::msmpListeners.insert(std::make_pair(muint16Port, this));
}
/**
 * @brief Removes listener
 */
clsListener::~clsListener() {
    if ( clsListener::msmpListeners.find(muint16Port)
            != clsListener::msmpListeners.end() ) {
        clsListener::msmpListeners.erase(muint16Port);
    }
    close();
}
/**
 * @brief clsListener::incomingConnection
 * @param sckDescriptor : Socket descriptor for new connection
 */
void clsListener::incomingConnection(qintptr sckDescriptor) {    
    //Every new connection will be run in a newly created thread
    clsServer* pThread(new clsServer(sckDescriptor, this));
    //Look for module listening to this port
//    clsModule* pModule(pFindModuleListeningToPort(muint16Port));
    //Connect signal/slot
    //Once a thread is not needed, it will be beleted later
    QObject::connect(pThread, &QThread::finished
                    ,pThread, &QThread::deleteLater);
    pThread->start();
}
/**
 * @brief clsListener::pGetListener
 * @param uint16Port : Port to search for listener using
 * @return Pointer to listener or nullptr if not found
 */
clsListener* clsListener::pGetListener(quint16 uint16Port) {
    mpListeners::iterator itrFound(clsListener::msmpListeners.find(uint16Port));
    if ( itrFound != clsListener::msmpListeners.end() ) {
        return itrFound->second;
    }
    return nullptr;
}

Christian Ehrlicher

You're long enough here to know how to use a debugger and debug such warnings.. aren't you?
And searching for QT_FATAL_WARNINGS in the forum will reveal more help.

QObject::connect(mpsckIncoming, &QAbstractSocket::errorOccurred
                ,this, &clsServer::onErrorOccurred);

And this was also discussed many times here - your slot is executed in the main thread and so QTcpSocket is accessed from the wrong thread. The forum search is something you should learn to use.

SPlatten

@Christian-Ehrlicher , Yes, thats exactly what I'm doing, just hoping someone might be able to suggest what could be wrong, apart from the logged message, everything seems to be working fine.

SPlatten

@Christian-Ehrlicher , if the error occurs in a thread, what is the correct way to manage this connection?

Christian Ehrlicher

@SPlatten Do you even read my posts? I wrote what you did wrong (except that there is again no need for a thread and when using a thread reading the documentation on how to use it properly seems to be too hard)

SPlatten

@Christian-Ehrlicher , maybe because your posts are very cryptic and its not easy to see what you are trying to say.

"except that there is again no need for a thread and when using a thread reading the documentation on how to use it properly seems to be too hard"

What does that even mean ?

Christian Ehrlicher

@SPlatten said in Qt Message, what does it mean, is it important ?:

What does that even mean ?

you don't need a thread to read from a QTcpSocket but just use it because it's fancy without known what you're actually doing.
you did not read the QThread documentation which clearly explains on how to properly use a QThread and don't care about it.

SPlatten

This post is deleted!

SPlatten

@Christian-Ehrlicher , Its actually not a problem at all, turns out the only time these messages are reported is when the application is terminated by clicking the stop button in Qt Creator.

If I close the application by clicking the close widgets on the window it doesn't exhibit the problem.

SPlatten

@Christian-Ehrlicher , I've tried various things in an attempt to figure out what you were suggesting, but I'm still seeing the same problem, can you be any more descriptive?

JoeCFD

@SPlatten

Do not do this.

class clsServer : public QThread {
}

Instead, use a worker as Christian posted:
https://doc.qt.io/qt-6/qthread.html#details
It is critical.

Another thing Christian pointed out is: why do you add QTcpSocket inside a thread?
What is the reason?

JoeCFD

@SPlatten
In Qt, the GUI elements (widgets) or QObject are not thread-safe, meaning they should
be created and manipulated only in the main thread. If you try to create or manipulate
GUI elements in a thread other than the main thread, you may encounter issues,
including the error message "Cannot create children for a parent that is in a different
thread."

SPlatten

@JoeCFD , thank you, I will certainly take a look into this.