Exit QThread Cleanly

Gwal Ashish

I am currently developing a GUI front-end application for DVR. After completing the coding part of it, I have tested it many times and it is working fine.

I am using QThread in my code for checking the state of the backend server 24X7. I do not have any proper solution for closing second thread cleanly when my main thread i.e. GUI thread quits.

Here is the code which I have been using right now for second thread.

@void thread::run()
{

forever
{
forever
{
checking some condition........
emit <signal>;
break;
}
forever
{
checking some condition........
emit <signal>;
break;
}
}
}@

Please suggest any solution for the above scenario.

jazzycamel

Before I answer your question I will first say that, despite examples given in the docs, it is generally considered a bad idea to subclass QThread and instead create a worker object that is moved into a thread. To stop a QThread you need to call quit() and, if that fails, terminate(). The issue here is that you're running a blocking method and no event loop so these methods will not work on there own. What you therefore need is a flag that is checked on every iteration of the forever loop and set by a signal/slot call from your main thread when the GUI exits. The following files form a minimal working example:

widget.h
@
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class QThread;

class Widget : public QWidget
{
Q_OBJECT

public:
explicit Widget(QWidget *parent = 0);
~Widget();

private:
QThread *thread;

signals:
void stopThread();

public slots:
void start();
void tick();
};

#endif // WIDGET_H
@

widget.cpp
@
#include "widget.h"
#include "threaded.h"

#include <QVBoxLayout>
#include <QPushButton>
#include <QThread>
#include <QDebug>

Widget::Widget(QWidget *parent) :
QWidget(parent)
{
QVBoxLayout *l=new QVBoxLayout(this);

QPushButton *startButton=new QPushButton("Start", this);
connect(startButton, SIGNAL(clicked()), this, SLOT(start()));
l->addWidget(startButton);

}

Widget::~Widget()
{
if(thread->isRunning()){
emit stopThread();
thread->quit();
if(!thread->wait(5000)){
thread->terminate();
if(!thread->wait(5000)) qDebug() << "Failed to terminate!";
}
}
}

void Widget::start(){
thread=new QThread();
Threaded *threaded=new Threaded();
connect(this, SIGNAL(stopThread()), threaded, SLOT(stop()));
connect(threaded, SIGNAL(tick()), this, SLOT(tick()));
connect(thread, SIGNAL(started()), threaded, SLOT(work()));
threaded->moveToThread(thread);
thread->start();
}

void Widget::tick(){
qDebug() << "tick";
}
@

threaded.h
@
#ifndef THREADED_H
#define THREADED_H

#include <QObject>

class Threaded : public QObject
{
Q_OBJECT
public:
explicit Threaded(QObject *parent = 0);

private:
bool mStop;

signals:
void tick();

public slots:
void work();
void stop();
};

#endif // THREADED_H
@

threaded.cpp
@
#include "threaded.h"

#include <QMutex>
#include <QWaitCondition>
#include <QCoreApplication>

Threaded::Threaded(QObject *parent) :
QObject(parent), mStop(false)
{}

void Threaded::work(){
QMutex dummy;
QWaitCondition waitCondition;

forever{
    dummy.lock();
    waitCondition.wait(&dummy, 1000);
    emit tick();
    dummy.unlock();
    QCoreApplication::processEvents();
    if(mStop) break;
}

}

void Threaded::stop(){
mStop=true;
}
@

main.cpp
@
#include <QtGui/QApplication>
#include "widget.h"

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();

return a.exec&#40;&#41;;

}
@

Hope this helps ;o)