QT 5 Application Crashes With No Monitor

Question

I am running a Qt Desktop Application on CentOS 6.4 with Qt 5 Libraries.

When I start the application from VNC, as some of our servers do not have monitors, the application crashes. Starting from:

QApplication a(argc, argv);

I have tracked this down to a core file call qxcbconnection.cpp and the line:

xcb_create_window(m_connection, XCB_COPY_FROM_PARENT,
                  m_connectionEventListener, m_screens.at(0)->root(),
                  0, 0, 1, 1, 0, XCB_WINDOW_CLASS_INPUT_ONLY,
                  m_screens.at(0)->screen()->root_visual, 0, 0);

The problem is, if there is no monitor connected then, m_screens is empty, and therefore causes an "index out of bounds" error.

My question is: Is there a way to get around this, or perhaps mimic a monitor somehow?

Answer 1

You may have found a regression bug in Qt, as I don't find anything relevant in the Qt4 to Qt5 changelog nor in the QtApplication documentation . IMHO it should not crash , and should just do unnecessary work (like in Qt4). However you can choose which type of instance you want to create. The doc gives an minimal example :

QCoreApplication* createApplication(int &argc, char *argv[])
{
    for (int i = 1; i < argc; ++i)
        if (!qstrcmp(argv[i], "-no-gui"))
            return new QCoreApplication(argc, argv);
    return new QApplication(argc, argv);
}

int main(int argc, char* argv[])
{
    QScopedPointer<QCoreApplication> app(createApplication(argc, argv));

    if (qobject_cast<QApplication *>(app.data())) {
       // start GUI version...
    } else {
       // start non-GUI version...
    }

    return app->exec();
}

Ps : note that QApplication underwent a major change behind the curtains, even if the API didn't change as much.