Qt5 App crashes on close (QWidget::show)

Question

So I have been stuck at this problem for a while now. For some reason, my application always crashes on close and this is the error message I get:

The inferior stopped because it triggered an exception.

Stopped in thread 0 by: Exception at 0x50c15a08, code: 0xc0000005: read access violation at: 0x0, flags=0x0 (first chance)

The stack shows the following:

QWidget::show Line: 7030

And this is what my main.cpp currently looks like:

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

I have no clue why the crash is occurring.

A little background: This is a multi-threaded application (using both QThread and QtConcurrent). When the threads are NOT started, the application closes just fine. Hence I added the following check to ensure that the thread is being terminated properly but this is not helping:

    void MainWindow::closeEvent(QCloseEvent *event)
    {
            if (thread->isRunning()) {
                thread->terminate();
                while(!thread->isFinished()){
                }
                event->accept();
            }
    }

EDIT: How the new thread was created

QThread *myThread = new QThread(this);
        Worker *worker  = new Worker(this, user, p);
        worker->moveToThread(myThread);

        connect(myThread, SIGNAL(started()), worker, SLOT(doWork()));
        connect(worker, SIGNAL(workDone()), this, SLOT(deleteThread()));
        myThread->start();

...and this is how I am handling the deletion of the thread after the download completes:

void MainWindow::deleteThread(){
    worker->deleteLater();
    myThread->quit();
    while(!myThread->isFinished()){
    }
    myThread->deleteLater();
}

Do you create the threads with valid parents so that Qt can handle deleting them properly? — Bowdzone, Nov 05 '14 at 06:58
What is the type of `thread` here? Does it inherit from `QThread`? — Nejat, Nov 05 '14 at 07:04
To answer @Bowdzone and @Nejat's questions together, yes the thread does have valid parent and it is inherited from QThread. `QThread thread = new QThread(this)` where `this` is `MainWindow` class — BurninatorDor, Nov 05 '14 at 14:21

score 1 · Answer 1 · answered Nov 06 '14 at 08:22

1

You should either listen for terminated signal from the thread or wait for the thread to terminate. Telling the thread to terminate doesn't block, so if you then immediately accept the close event you will get your exception.

This is described in more detail in the documentation for the terminate slot

answered Nov 06 '14 at 08:22

Klathzazt

2,415
19
27

If you check the last block of code posted in the question, I do NOT call `event->accept` immediately after I call terminate. The program loops until `myThread.isFinished` returns true. – BurninatorDor Nov 06 '14 at 14:54
1

@burninatorDor: finished, not running, and terminated are all different. If the thread is not terminated you will get an exception. [The first post here is excellent in describing QThread lifecycle](http://stackoverflow.com/questions/17045368/qthread-emits-finished-signal-but-isrunning-returns-true-and-isfinished-re) and is generally a good discussion on the topic – Klathzazt Nov 06 '14 at 17:07

IGHOR · Answer 2 · 2014-11-06T18:43:03.443

1

Proper QThread stop code is:

myThread->quit();
myThread->wait();

Also you have used moveToThread(this);

You need to moveToThread back to parent thread first.

For example your code is:

moveToThread(this);
start();

You need to save currentThread() before moveToThread() like this:

QThread *savedThread=currentThread();

moveToThread(this);
start();

And before quit or terminate you need to restore it:

moveToThread(savedThread);

quit();
wait();

You can put it on destructor of course.

edited Nov 06 '14 at 18:43

answered Nov 06 '14 at 13:20

IGHOR

691
6
22

I am sorry but this does not help – BurninatorDor Nov 06 '14 at 20:29
@BurninatorDor what happening if you comment this line? while(!myThread->isFinished()){} If you can send me project I'll fix it. – IGHOR Nov 06 '14 at 23:05

Qt5 App crashes on close (QWidget::show)

2 Answers2