Why QThread still working, when work is done?

qwe3

Hi,

I have simple app with 2 threads: main and one for heavy calculations.

When the calculations are in progress I close app using X button. So my code goes to MainWindow destructor and I have here:

    thread->quit();

    qInfo()<<"BEFORE WAIT";
    thread->wait();
    qInfo()<<"AFTER WAIT";

I quit second thread, so my second thread exit event loop. But of course it still calculates. On main thread I wait on line thread->wait(). But when the second thread ends calculates ( I see on debuq "END" ), I think the second thread did the work, there is no event loop in second thread, so work for second thread is over. But I still wait on thread->wait(). And I will wait forever... Why this is happening?

My whole code:
MainWindow ( main thread ) :

constructor:

    thread = new QThread(this);
    worker = new Worker;

    connect(this, &MainWindow::start, worker, &Worker::calculate);

    worker->moveToThread(thread);

    thread->start();

    emit start();

destructor:

    thread->quit();

    qInfo()<<"BEFORE WAIT";
    thread->wait();
    qInfo()<<"AFTER WAIT";

Second thread ( heavy calculations ) :

void Worker::calculate()
{
    double z = 21;
    for(int i=0;i<100000000;i++)
    {
        if(i%5000000 == 0) // to see that thread works
        {
            QCoreApplication::processEvents();
            qDebug()<<i;
        }

        z+=qSin(i*0.32)*qCos(i/2*0.31);  // some heavy calculations 
        z+=10.00/(i+1);
    }
    qInfo()<<"END";
}

And my debuq:

0
5000000
10000000
15000000
20000000
25000000
BEFORE WAIT
30000000
35000000
40000000
45000000
50000000
55000000
60000000
65000000
70000000
75000000
80000000
85000000
90000000
95000000
END
13:58:51: The program has unexpectedly finished.  // because I stop program using Qt Creator

JKSH

@qwe3 said in Why QThread still working, when work is done?:

I quit second thread, so my second thread exit event loop.

Calling quit() tells the event loop to stop running. However, it has no effect on your for-loop.

If you want to interrupt the calculation, then you must call break; from inside your for-loop.

qwe3

@JKSH Thank you for answer. I know that I have to use isInterruptionRequested() and requestInterruption(). But now I ask about something different.

I would like ( now ) to have an application, which calculate what I want ( calculate() method in second thread ) even if I close the application. But when it finish calculates, application should close in correct way. And now ( my code ) I have situation that my application wait to finish calculates, but when they are finished, my application still is running ( I wait on thread->wait ).

Maybe when I quit event loop in second thread, this thread doesn't send finished() signal and I will wait in thread->wait() forever?

I ask about it, not to solved a problem, but to understand how quit() works, and why I have to still wait on thread->wait

qwe3

@JKSH The problem is QCoreApplication::processEvents();

When I delete this line from my code - everything is ok. When this line is in my code - I wait in thread->wait() forever. Why?

JonB

@qwe3
Why is your calculation worker thread call processEvents() in the first place?
It's probably hanging in that when the main UI thread has gone away/is no longer running the UI loop.

qwe3

@JonB Using processEvents() doesn't make sense in my code. I tried something different and accidentally add this method. And now I would like to know, what's going on.

Can you explain me what means It's probably hanging in that when the main UI thread has gone away/is no longer running the UI loop.?

I know there is an event loop, which is in the main.cpp file:

return a.exec();

So when I close app and my code is in mainWindow's destructor I can't receive signals, because mainWindow is no longer in a.exec()?

EDIT:

I don't see link between processEvents() and It's probably hanging in that when the main UI thread has gone away/is no longer running the UI loop.

Christian Ehrlicher

This works fine for me:

class Worker : public QObject
{
public:
  void calculate()
  {
    QThread::sleep(20);
    QCoreApplication::processEvents();
  }
};

class MainWindow : public QMainWindow
{
  Q_OBJECT
public:
  MainWindow()
  {
    thread = new QThread(this);
    worker = new Worker;
    worker->moveToThread(thread);
    thread->start();
    QTimer::singleShot(0, worker, &Worker::calculate);
  }
  ~MainWindow()
  {
    thread->quit();
    thread->wait();
  }
private:
  QThread *thread;
  Worker *worker;
};

int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  MainWindow mw;
  mw.show();
  return app.exec();
}
#include "main.moc"

Please provide a minimal compilable example.

qwe3

@Christian-Ehrlicher Your code is ok?

I think I can't add parent to

worker = new Worker(this);

I have error:

QObject::moveToThread: Cannot move objects with a parent

EDIT:
When I delete parent I get the same situation: I wait on thread->wait()

EDIT2:
My code:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    thread = new QThread(this);
    worker = new Worker;
    worker->moveToThread(thread);
    thread->start();
    QTimer::singleShot(0, worker, &Worker::calculate);
}

MainWindow::~MainWindow()
{
    thread->quit();

    qInfo()<<"BEFORE WAIT";
    thread->wait();
    qInfo()<<"AFTER WAIT";

    delete ui;
}

void Worker::calculate()
{
    QThread::sleep(20);
    qInfo()<<"AFTER SLEEP";
    QCoreApplication::processEvents();
}

Christian Ehrlicher

@qwe3 said in Why QThread still working, when work is done?:

@Christian-Ehrlicher Your code is ok?

You're right, it was a slightly outdated version. Adjusted my code but still works fine.
Your code is not compilable at all so I can't test yours. Please provide a minimal compilable example similar to mine without any ui file or other unneeded stuff.

qwe3

This post is deleted!

qwe3

@Christian-Ehrlicher

Code:

#include <QApplication>
#include <QThread>
#include <QTimer>
#include <QMainWindow>
#include <QDebug>
class Worker : public QObject
{
public:
  void calculate()
  {
    qInfo()<<"START SECOND THREAD";
    QThread::sleep(20);
    QCoreApplication::processEvents();
    qInfo()<<"END SECOND THREAD";
  }
};

class MainWindow : public QMainWindow
{
  Q_OBJECT
public:
  MainWindow()
  {
    thread = new QThread(this);
    worker = new Worker;
    worker->moveToThread(thread);
    thread->start();
    QTimer::singleShot(0, worker, &Worker::calculate);
  }
  ~MainWindow()
  {
    thread->quit();
    qInfo()<<"before";
    thread->wait();
    qInfo()<<"after";
  }
private:
  QThread *thread;
  Worker *worker;
};

int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  MainWindow mw;
  mw.show();
  return app.exec();
}
#include "main.moc"

I only add libraries and debuq. So I start app and after 5 second I close app using X button. Now I wait additional 15 seconds and see on debug END SECOND THREAD. And my app is still running. I have to close it in Qt Creator clicking red square. And have information:

21:03:25: The program has unexpectedly finished.

I don't see on debug after

Christian Ehrlicher

Works fine for me with Qt5.15 on linux but have the same effect on windows. Maybe it's worth opening a Qt bug report.

qwe3

@Christian-Ehrlicher Thank you for help, but I don't understand this:

but have the same effect on windows

Which option:

On linux everything is ok, but on windows you have the same problem like me
On linux and windows everything is ok

?

jsulm

@qwe3 Since @Christian-Ehrlicher suggested to open a bug report it works on Linux but not on Windows (in case 2 there would not be any need to file a bug report).

qwe3

@jsulm I think the same, but I wanted to make sure :)