event loop already running

[prkhr4u]

I am trying to run a QApplication on button click from Main window.
However I am getting this error:

QCoreApplication::exec: The event loop is already running

My code inside button click is this:

if (!g_thread_supported ())
      g_thread_init (NULL);
 
    gst_init (NULL, NULL);
    QApplication app(NULL, NULL);
    app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));
 
    // prepare the pipeline
 
    GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    //assert(jpegdec);
    gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    gst_element_link_many(src,jpegdec, sink);
 
    g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    // prepare the ui
 
    QWidget window;
    window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
    window.move(0,400);
    window.setFixedSize(320, 200);
    window.show();
 
    WId xwinid = window.winId();
    gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
 
    // run the pipeline
 
    GstStateChangeReturn sret = gst_element_set_state (pipeline,
        GST_STATE_PLAYING);
    if (sret == GST_STATE_CHANGE_FAILURE) {
      gst_element_set_state (pipeline, GST_STATE_NULL);
      gst_object_unref (pipeline);
      // Exit application
      QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    }
 
    int ret = app.exec();
 
    window.hide();
    gst_element_set_state (pipeline, GST_STATE_NULL);
    gst_object_unref (pipeline);

if (!g_thread_supported ())
      g_thread_init (NULL);

    gst_init (NULL, NULL);
    QApplication app(NULL, NULL);
    app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));

    // prepare the pipeline

    GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    //assert(jpegdec);
    gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    gst_element_link_many(src,jpegdec, sink);

    g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    // prepare the ui

    QWidget window;
    window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
    window.move(0,400);
    window.setFixedSize(320, 200);
    window.show();

    WId xwinid = window.winId();
    gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);

    // run the pipeline

    GstStateChangeReturn sret = gst_element_set_state (pipeline,
        GST_STATE_PLAYING);
    if (sret == GST_STATE_CHANGE_FAILURE) {
      gst_element_set_state (pipeline, GST_STATE_NULL);
      gst_object_unref (pipeline);
      // Exit application
      QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    }

    int ret = app.exec();

    window.hide();
    gst_element_set_state (pipeline, GST_STATE_NULL);
    gst_object_unref (pipeline);

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I am able to run this code,if I put it after

ui->setupUi(this);

ui->setupUi(this);

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in the MainWindow

How can I resolve this problem?

[Lesiok]

From QApplication doc : For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time. You can not have multiple QApplication objects.

[anda_skoa]

How does your main() function look like? Do you create and run a QApplication in there already?

Cheers,
_

[prkhr4u]

My main() function is like this:

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

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

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I have created 1 QApplication object there also

[anda_skoa]

Then this is enough.
Just remove the QApplication stuff from your slot.

Cheers,
_

[prkhr4u]

I have removed the QApplication stuff from my code.
But now nothing is being shown.No error also.
Here is the modified code:

if (!g_thread_supported ())
      g_thread_init (NULL);
 
    gst_init (NULL, NULL);
    //QApplication app(NULL, NULL);
    //a.connect(&a, SIGNAL(lastWindowClosed()), &a, SLOT(quit ()));
 
    // prepare the pipeline
 
    GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    gst_element_link_many(src,jpegdec, sink);    
    g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    // prepare the ui
 
    QWidget window;
    window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
    window.move(1000,480);
    window.setFixedSize(320, 200);
    window.show();
 
    WId xwinid = window.winId();
    gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
 
    // run the pipeline
 
    GstStateChangeReturn sret = gst_element_set_state (pipeline,
        GST_STATE_PLAYING);
    if (sret == GST_STATE_CHANGE_FAILURE) {
      gst_element_set_state (pipeline, GST_STATE_NULL);
      gst_object_unref (pipeline);
      // Exit application
      //QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    }
 
    //int ret = a.exec();
 
    window.hide();
    gst_element_set_state (pipeline, GST_STATE_NULL);
    gst_object_unref (pipeline);

if (!g_thread_supported ())
      g_thread_init (NULL);

    gst_init (NULL, NULL);
    //QApplication app(NULL, NULL);
    //a.connect(&a, SIGNAL(lastWindowClosed()), &a, SLOT(quit ()));

    // prepare the pipeline

    GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    gst_element_link_many(src,jpegdec, sink);    
    g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    // prepare the ui

    QWidget window;
    window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
    window.move(1000,480);
    window.setFixedSize(320, 200);
    window.show();

    WId xwinid = window.winId();
    gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);

    // run the pipeline

    GstStateChangeReturn sret = gst_element_set_state (pipeline,
        GST_STATE_PLAYING);
    if (sret == GST_STATE_CHANGE_FAILURE) {
      gst_element_set_state (pipeline, GST_STATE_NULL);
      gst_object_unref (pipeline);
      // Exit application
      //QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    }

    //int ret = a.exec();

    window.hide();
    gst_element_set_state (pipeline, GST_STATE_NULL);
    gst_object_unref (pipeline);

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[Lesiok]

I have removed the QApplication stuff from my code.
But now nothing is being shown.No error also.

Because in line 41 You hide new window.

[d_stranz]

Because in line 41 You hide new window.

Worse than that. The window is created on the stack in line 19, so as soon as control exits the if() clause starting on line 1, the window goes out of scope and is destroyed. Poof! Now you see it, now you don't!