Workload in a QThread blocks main application's event loop ?

[0xBulbizarre]

Do you actually start that second thread?

Yes (of course), the second's thread run() method is :

void run()
{
  exec();
}

void run()
{
  exec();
}

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Do you want some test code to experiment with, do you need more infos ?

[jacek]

Yes (of course), the second's thread run() method is :

That's the run() method, but how do you start that thread?

[0xBulbizarre]

That's the run() method, but how do you start that thread?

I start it from gui thread with

otherThread = new OtherThread(this);
otherThread->start();

otherThread = new OtherThread(this);
otherThread->start();

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OtherThread's declaration:

class OtherThread : public QThread

class OtherThread : public QThread

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[jacek]

otherThread = new OtherThread(this);
otherThread->start();

Looks OK, did you try with 0 instead of "this"?

[0xBulbizarre]

Looks OK, did you try with 0 instead of "this"?

yes, but nothing changes (apparently).


I think I found something... The slot executed by the singleShot timer shall be executed by the worker thread, but is in fact executed by the GUI thread. So, I deduce that the connection created by the singleShot timer is a DirectConnection instead of a QueuedConnection. That would explain why the slot is executed by the emitting thread.

Here is how I create the worker thread, and how I create the singleShot() timer

// code from the GUI thread
otherThread = new OtherThread(this);
otherThread->start();
QTimer::singleShot(0, otherThread, SLOT(startThread(void)));
cout << "In GUI Thread, Thread=" << currentThread() << " ThreadID=" << currentThreadId() << endl;
QCoreApplication::processEvents(); // process the timer event

// code from the GUI thread
otherThread = new OtherThread(this);
otherThread->start();
QTimer::singleShot(0, otherThread, SLOT(startThread(void)));
cout << "In GUI Thread, Thread=" << currentThread() << " ThreadID=" << currentThreadId() << endl;
QCoreApplication::processEvents(); // process the timer event

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here is my startThread() function (from the worker thread)

void OtherThread::startThread(void) {
  cout << "In startThread() Thread=" << currentThread() << " ThreadID=" << currentThreadId() << endl;
  // do some job...
}

void OtherThread::startThread(void) {
  cout << "In startThread() Thread=" << currentThread() << " ThreadID=" << currentThreadId() << endl;
  // do some job...
}

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outputs where we can see that the GUI thread executes the slot instead of the worker thread:
In GUI Thread, Thread=0x804f198 ThreadID=3078543040
In startThread() Thread=0x804f198 ThreadID=3078543040

Where is my mistake ? should QTimer::singleShot()'s receiver be something else that my otherThread ?

[0xBulbizarre]

I saw in the docs that:

By default, QObject::connect() establishes a direct connection if the sender and the receiver live in the same thread, and a queued connection if they live in different threads.

If we look at my code above (reproduced below)... we can see that both the sender and the receiver live in the same thread. The sender is the singleShot timer, the receiver is a thread object, also living in the GUI thread.

otherThread = new OtherThread(this);
otherThread->start();
QTimer::singleShot(0, otherThread, SLOT(startThread(void)));

otherThread = new OtherThread(this);
otherThread->start();
QTimer::singleShot(0, otherThread, SLOT(startThread(void)));

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is it correct ? how to solve that problem ?

[jacek]

QThread object lives in the thread that created it. You could split that class into two --- the thread itself and an object that lives within that thread.

[0xBulbizarre]

Ok, I now understand why a QThread should'nt do any business logic directly. I'll try that tonight!

thanks for what I learned in this (forum) Thread !

[madrich]

Try this

otherThread = new OtherThread(this);
otherThread->moveToThread(otherThread);
otherThread->start();

that should work ... Regards Madrich