QT QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread



  • hello,i am getting this notification when i a using readAll or write() function of QTcpSocket.
    here is my code for client implemantation.
    conversation.cpp

    #include "conversation.h"
    
    QTextStream cin(stdin);
    
    QString text;
    conversation::conversation(QTcpSocket *s1,QObject *parent): QObject(parent)
    {
       C_socket=s1;
    }
    conversation::conversation(){
    
    }
    
    void conversation::readdata(QTcpSocket *s ){
    
        qDebug()<<"in readline..";
        qDebug()<< s;
        s->waitForReadyRead(10000);
      while(s->bytesAvailable()){
        qDebug()<<"reading...."<<s->readAll();
       }
    
         // do{
         qDebug()<<"reading complete";
         if(!s->bytesAvailable()){
             emit stringtoRead(s);
         }
    
    }
    void conversation::writedata(QTcpSocket *s ){
        while(true){
        qDebug()<<"to write somthing";
        text=cin.readLine();
        qDebug()<<"in producer :"<<" "<<text;
        if(s->write(text.toStdString().c_str()))
        {
            qDebug()<<"has been written..";
        }
        }
    }
    

    client.cpp

    #include "client.h"
    #include<QtConcurrent/qtconcurrentrun.h>
    client::client(QObject *parent):QObject(parent){
    }
    void client::test(){
       //connect
       m_sock=new QTcpSocket();
        connect(m_sock,SIGNAL(connected()),this,SLOT(connected()));
        connect(m_sock,SIGNAL(disconnected()),this,SLOT(disconnected()));
        qDebug()<<"connecting...";
        m_sock->connectToHost("127.0.0.1",100);
        if(!m_sock->waitForConnected(1000)){
           qDebug()<<"somthing bad happen "<<m_sock->errorString();
        }
    
    }
    void client::connected(){
        qDebug()<<"connected";
    
        connect(&c,&conversation::stringtoRead,&c,&conversation::readdata);
        QFuture<void>f1=QtConcurrent::run(&this->c,&conversation::writedata,m_sock);
        QFuture<void>f2=QtConcurrent::run(&this->c,&conversation::readdata,m_sock);
    
    }
    

    [Added code tags ~kshegunov]


  • Lifetime Qt Champion

    Hi,

    Take a look at the Threaded Fortune Server example to see how to manage QTcpSocket in a multithreaded environment.



  • @SGaist thanks , i have gone through it. But my purpose differs from the Fortune server.
    here in my code of client i'm using different approach then Fortune code. What I can't really understand is where exactly i'm not following QT's format of QObject and QThread.
    My code works fine but I'm getting this notification every time when it writes something using write().
    qDebug()<<"to write somthing";
    text=cin.readLine();
    qDebug()<<"in producer :"<<" "<<text;
    if(m_sock->write(text.toStdString().c_str()))// HERE THAT NOTIFICATION APPEARS
    {
    qDebug()<<"has been written..";
    }


  • Lifetime Qt Champion

    You are passing your QTcpSocket object around different threads. The threaded fortune server shows that the sockets are created in each new thread and handled by them.



  • @SGaist the problem is that i have to pass the socket value (m_sock) as parameter to another class (conversation class) so that it can know which socket needs to implement the Write and Read function.


  • Lifetime Qt Champion

    The thing is that you pass a socket to the constructor, then you want to pass a socket to these function and you want to do reading and writing on the same socket from different threads.

    From the looks of it, you could leverage Qt's asynchronous nature before starting to use so many threads.



  • @SGaist here i made changes in my program, but QT fires that warning everytime write() function is called.

    #include "client.h"
    QTextStream cin(stdin);
    QString text;
    client::client(QObject *parent):QObject(parent)
    {

    }
    void client::startconnection(){
    connect(client_socket,SIGNAL(connected()),this,SLOT(connection()));
    connect(client_socket,SIGNAL(disconnected()),this,SLOT(disconnection()));
    connect(this,SIGNAL(stringtoRead()),this,SLOT(readdata()));
    client_socket->connectToHost("127.0.0.1",100);
    if(!client_socket->waitForConnected(1000)){
    qDebug()<<"Somthing is bad in connection..";
    }
    ForCommunication();
    }

    void client::connection(){
    qDebug()<<"connection is successfull";
    }

    void client::ForCommunication(){
    qDebug()<<"in communication class..";

    QFuture<void>f2=QtConcurrent::run(this,&client::readdata);
    QFuture<void>f1=QtConcurrent::run(this,&client::writedata);
    

    }
    void client::disconnection(){
    qDebug()<<"disconnected..";
    }
    void client::writedata(){
    while(true)
    {
    qDebug()<<"to write somthing";
    text=cin.readLine();
    qDebug()<<"in producer :"<<" "<<text;
    client_socket->write(text.toStdString().c_str());
    qDebug()<<"has been written..";

    }
    

    }

    void client::readdata(){
    qDebug()<<"in readline..";
    client_socket->waitForReadyRead(10000);
    while(client_socket->bytesAvailable()){
    qDebug()<<"reading...."<<client_socket->readAll();
    qDebug()<<"reading complete";
    }

    if(!client_socket->bytesAvailable()){
    emit stringtoRead();
    }
    }


  • Moderators



  • @jsulm In client.h file.

    0_1524635085296_clientH.png
    Before i put this line start connection function but still i was getting same error.so i put it on header file to check does it make any change or not and i did not get any changes


  • Moderators

    @JadeN001 You create the socket in a different thread than the two threads where it is used with QtConcurrent::run.
    So, you still use same socket in different threads.
    Why do you use threads at all? As @SGaist suggested you can simply use assynchronous Qt APIs...



  • @jsulm thanks
    I'm confused with the phrase "different thread" .Am I supposed to create new QTcpSocket for each thread!! and if so, where should it be declared?
    I'll go through the assynchronus API . but Is it possible to get rid of this warning by using thread concept !


  • Moderators

    @JadeN001 To get rid of this message you have to do multi-threading in the correct way.
    But the question is: why do you need threads? With asynchronous Qt APIs there is usually no need for threads for such use cases. Why do you think you need threads?



  • Hier a little helper, code is untested, so you'll may have to modify /bugfix it:

    asynchronus and threaded.

    //main.cpp
    
    #include <QApplication>
    #include <QThread>
    #include "client.h"
    
    int main(int argc, char *argv[])
    {
        QApplication a(argc, argv);
    
        QThread *thread = new QThread ();
        client *m_client = new client();
        m_client->moveToThread(thread);
        QObject::connect(thread, &QThread::started, m_client, &client::startconnection);
        QObject::connect(thread, &QThread::finished, m_client, &client::deleteLater);
        QObject::connect(thread, &QThread::finished, thread, &QThread::deleteLater);
        thread->start();
    
        return a.exec();
    }
    
    
    #include "client.h"
    
    client::client(QObject *parent):QObject(parent)
    {
    
    }
    void client::startconnection()
    {
        QTcpSocket client_socket = new QTcpSocket(this);
        
        connect(client_socket, &QTcpSocket::bytesWritten, this, &client::BytesWritten);
        connect(client_socket, &QTcpSocket::readyRead, this, &client::ReadyRead);
        
        connect(client_socket,SIGNAL(connected()),this,SLOT(connection()));
        connect(client_socket,SIGNAL(disconnected()),this,SLOT(disconnection()));
        
        client_socket->connectToHost("127.0.0.1",100);
    }
    
    void client::BytesWritten(const quint64 &bytes){
        qDebug() << bytes << "Bytes written";
    }
    
    void client::ReadyRead(){
        qDebug() << Q_FUNC_INFO << client_socket->readAll();
    }
    
    void client::connection(){
        qDebug()<<"connection is successfull, now write test data";
        writedata("TestString");
    }
    
    void client::disconnection(){
        qDebug()<<"disconnected..";
    }
    void client::writedata(const QString &str){
        qDebug()<<"to write somthing";
        client_socket->write(str.c_str());
    }
    


  • @jsulm I'm trying to create a chat application kind of program without using QWidgets .
    So I need to make program work parallelly in aspect of sending and receiving messages without being blocked. I tried it without using QThread and QConcurrent (meaning I used signals and SLOTs only).
    So in that code , the readyRead() gets called only after I write something. It means even if there's data to read , I can't get that data till I write something first.
    So what I want here is when a client is sending some data, the other client should not get blocked into writing mode and it should be able to read the data and display. So the problem that i am facing is ,when readyRead() is called data gets stored into buffer and after write() function completed it display whole messages which was in buffer.



  • @J.Hilk thanks , I'll definitely look into it. Thanks again.



  • @J.Hilk thnaks , after doing some changes to this code , I no longer see those warnings in client code...but client doesn't read untill I write something(using cin.readline() function). Is there a way through which client doesn't wait to write something and reads the message from other client, in short it doesn't get blocked in writing.



  • @JadeN001 ready read is emitted as soon as data is available at the socket.

    The fact that you only get data when you write something has nothing to do with qt, but with the server you connect to.

    Change the server you have, so that he sends without beeing triggered first. A Timer for example.


  • Lifetime Qt Champion

    @JadeN001 said in QT QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread:

    text=cin.readLine();

    Isn't that a blocking operation ? If so, the event loop is blocked until you get something from stdin.



  • @J.Hilk My server is just sending message when it arrives from client.i think at client side, i have to change the logic of calling write() function only when if there is no data in buffer for reading..I have tried to use bytesavailable().but it is not working.Do you have any idea about it?



  • @SGaist yes , that too is causing trouble. can you suggest any nonblocking userinput method? I'm really stuck at this point , SocketNotifier warning and this cin.readLine().


  • Lifetime Qt Champion

    It would rather be: do your blocking input in one thread and manage your network in another one.



  • @SGaist thanks very much
    I followed your suggestion and now it seems like , I'm getting this notification only when I write() something. @SGaist Is there a way to get the current SocketNotifier and handle it ?


  • Lifetime Qt Champion

    You might be interested by this small example for reading std using Qt.



  • @SGaist thanks very much
    I tried this way too, but here I must pass the socketDescriptor as a parameter in QSocketNotifier. Its a bit confusing on client side how we can get socketDescriptor as in my knowledge we can only get socketdescriptor at serverside because of its Incommingconnection(). now, One more thing....Is it possible to get a warning of socket Notifier because reading and writing functions are being called at the same time ??
    0_1524828143661_op.png


  • Lifetime Qt Champion

    The socket notifier was only for stdin. As for your client side network connection, do everything in your dedicated worker object.


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