[Interest] Terminating a QThread

[Etienne Sandré-Chardonnal]

The best is still not to terminate threads if not absolutely necessary.

Even without an event loop, you can poll a flag that asks the thread to
return from the run function.



For instance:


void RenderThread::run()

{

while(!stopRequested)

	{

		//Do some intensive calculation here...

	}

//Clean resources here...

}

The "stopRequested" flag being set to "true" from outside the thread, when
we want to quit. That's not perfect (as it is a kind of polling, so if the
loop takes about 1ms to execute, you will need to wait at most 1ms to stop
the thread), but has virtually no overhead, is clean.

Never terminate threads. Or at least, use a mechanism that asks the thread
to quit cleanly, and terminate it if it has not quit after a given timeout,
issuing warnings that termination occured).
 There always a solution for avoiding it.


2013/6/21 francois cellier <f_cellier at yahoo.fr>

> Dear all,
>
> Even if I know that it can be dangerous to terminate a thread, I need to
> do it for my application.
> The function that run into the thread is like a long linear sequence of
> code that has not been designed in a signal/slot way. Moreover, in my
> thread, I need an eventloop as I use in it some TCP or UDP  Qt sockets.
>
> I have tried the worker approach with the following code and I did not
> find a way to terminate the thread :
>
> For my thread class :
>
>    1. ThreadEx::ThreadEx(QObject <http://qt-project.org/doc/QObject.html>
>    *parent) :
>    2.     QThread <http://qt-project.org/doc/QThread.html>(parent)
>    3. {
>    4. }
>    5.
>    6. void ThreadEx::run()
>    7. {
>    8.     QThread <http://qt-project.org/doc/QThread.html>::
>    setTerminationEnabled(true);
>    9.     exec();
>    10. }
>
>
>
> For my worker class:
>
>    1. ObjectInThread::ObjectInThread(QObject<http://qt-project.org/doc/QObject.html>
>    *parent) :
>    2.     QObject <http://qt-project.org/doc/QObject.html>(parent)
>    3. {
>    4. }
>    5.
>    6.
>    7. void ObjectInThread::run()
>    8. {
>    9.    int compteur = 0;
>    10. //here i am using a loop instead of the long linear flow  but it
>    is not a loop  in the real code
>    11.     while(1)
>    12.     {
>    13.         qDebug() <<compteur;
>    14.         compteur++;
>    15.         Sleep(1000);
>    16.     }
>    17. }
>
>
> And for my main object that starts the thread
>
>    1. #include "mainobject.h"
>    2. #include <QDebug>
>    3.
>    4. MainObject::MainObject()
>    5. {
>    6.     m_thread = new ThreadEx();
>    7.     m_object = NULL;
>    8. }
>    9.
>    10.
>    11. void MainObject::start()
>    12. {
>    13.     if(m_object)
>    14.         delete m_object;
>    15.
>    16.     m_object = new ObjectInThread();
>    17.     m_object->moveToThread(m_thread);
>    18.     QObject <http://qt-project.org/doc/QObject.html>::connect(
>    m_thread,SIGNAL(started()),m_object,SLOT(run()));
>    19.     m_thread->start();
>    20. }
>    21.
>    22.
>    23. void MainObject::stop()
>    24. {
>    25.     qDebug()<<"I try to stop\n";
>    26.     m_thread->terminate();
>    27. }
>
> The start and stop functions are slots called via a simple GUI.
> Using the stop slot does nothing.
>
> Moreover I tried the other way of using QThread that consists in
> inheriting the QThread class and overriding the run method with the code :
>
> void ThreadEx::run()
> {
>     int compteur = 0;
>     while(1)
>     {
>          qDebug() <<compteur;
>          compteur++;
>          sleep(1);
>     }
> }
>
> In that case terminate works but I do not have an eventloop as I did not
> call the exec method.
>
> Is this the expected behaviour ?
>
> I am using Qt5.1 on centOS / RedHat 6.2. I have also tried this on Windows
> with no more success.
>
> Thanks for your help,
> François
>
>
>
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>
>
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