QSocketNotifier can not enable from an other thread.

[dabiabilus]

I have a two classes derived from QThread

First class emits a signal to the second class when an event occurs. I connected these signal and slot with queud connection explicitly. Second class try to write to a tcp socket created in the run block of the second class. but it gives the error "QSocketNotifier can not enable from an other thread." I coul not understand why this happen.

Code is beriefly below :

connect(firstClass,SIGNAL(send(int )),secondClass,SLOT(response(int )),Qt::QueuedConnection);
 
firstClass::emitSignal()
{
  emit send(1);
}
 
secondClass::run()
{
    m_socket = new QTcpSocket();
    ....
    ...
}
secondClass::response(int  a_data)
{
     m_socket->write(".....");
}

connect(firstClass,SIGNAL(send(int )),secondClass,SLOT(response(int )),Qt::QueuedConnection);

firstClass::emitSignal()
{
  emit send(1);
}

secondClass::run()
{
    m_socket = new QTcpSocket();
    ....
    ...
}
secondClass::response(int  a_data)
{
     m_socket->write(".....");
}

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[jacek]

The problem is that QThread object lives in the thread that created it, not the thread it represents. So if you connect something through a queued connection to QThread object, signals will be queued in wrong place.

You have to either split your implementation into a thread and a task that lives in it or move that thread to itself (see QObject::moveToThread()).

[dabiabilus]

I did not understanda at all.

Both thread class lives in main thread. but when I connect both thread with direct connection than, as I know , slot will invoked in the caller thread event loop. But , it does not have permission to use socket object , created in the second thread.

[jpn]

Try to print QThread::currentThread() in the slot and you'll see the problem. I'd recommend reading the Multithreading presentation by Bradley T. Hughes from TT DevDays 2007. Especially the chapter starting from page 32 is important to understand.

[spirit]

Try to print QThread::currentThread() in the slot and you'll see the problem. I'd recommend reading the Multithreading presentation by Bradley T. Hughes from TT DevDays 2007. Especially the chapter starting from page 32 is important to understand.

cool article!