Why QThread still working, when work is done?

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 :)