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Cleaning up after QApplication



  • I'm porting a Qt desktop application to Linux (Ubuntu 19.10, 64bit desktop, Qt 5.12.5, gcc version 9.2.1), and am seeing some unexpected threads remaining alive after QApplication is finished.

    Here's the minimum repro:

    #include <QApplication>
    #include <unistd.h>
    int main(int argc, char * argv[]) {
        {
            QApplication app(argc, argv);
            sleep(1);
        }
        sleep(10);
        return 0;
    }
    

    If I debug the application at various points in time, here's what I see:

    • Before the constructor of QApplication is run, just one thread, the main thread, exists, as expected.
    • After the constructor of QApplication is run, 4 additional threads get created:
      • QXcbEventQueue
      • gmain
      • gdbus
      • QDBusConnection
    • After the destructor of QApplication is run, the QXcbEventQueue thread goes away.
    • Even after the second sleep of 10 seconds is done, the 3 other threads, besides the main thead, remain alive:
      • gmain
      • gdbus
      • QDBusConnection

    I'm looking for a way to properly end these threads (and do other clean up that I might not be aware of at the moment) after QApplication has finished (of course, in the real app I call app.exec() and do other things) and has been destroyed.

    This is not an issue in the application itself, which does not seem to have a problem with reaching the end of main with those 3 threads still alive.

    The application is part of a larger suite of libraries / other applications that are thoroughly tested using Google Test, and the tests include some Death Tests, which, if executed after having run some tests that involve running a QApplication, complain about forking an application that has multiple threads, and get stuck and never finish, presumably, for this reason.

    Any hints on how I might be able to get rid of those threads & perform a full cleanup of other stuff (if any) after QApplication?


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