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Thread: event loop already running

  1. #1
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    Default event loop already running

    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:
    Qt Code:
    1. if (!g_thread_supported ())
    2. g_thread_init (NULL);
    3.  
    4. gst_init (NULL, NULL);
    5. QApplication app(NULL, NULL);
    6. app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));
    7.  
    8. // prepare the pipeline
    9.  
    10. GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    11. GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    12. GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    13. GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    14. //assert(jpegdec);
    15. gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    16. gst_element_link_many(src,jpegdec, sink);
    17.  
    18. g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    19. // prepare the ui
    20.  
    21. QWidget window;
    22. window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
    23. window.move(0,400);
    24. window.setFixedSize(320, 200);
    25. window.show();
    26.  
    27. WId xwinid = window.winId();
    28. gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
    29.  
    30. // run the pipeline
    31.  
    32. GstStateChangeReturn sret = gst_element_set_state (pipeline,
    33. GST_STATE_PLAYING);
    34. if (sret == GST_STATE_CHANGE_FAILURE) {
    35. gst_element_set_state (pipeline, GST_STATE_NULL);
    36. gst_object_unref (pipeline);
    37. // Exit application
    38. QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    39. }
    40.  
    41. int ret = app.exec();
    42.  
    43. window.hide();
    44. gst_element_set_state (pipeline, GST_STATE_NULL);
    45. gst_object_unref (pipeline);
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    I am able to run this code,if I put it after
    Qt Code:
    1. ui->setupUi(this);
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    in the MainWindow

    How can I resolve this problem?

  2. #2
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    Default Re: event loop already running

    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.

  3. #3
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    Default Re: event loop already running

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

    Cheers,
    _

  4. #4
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    Default Re: event loop already running

    My main() function is like this:
    Qt Code:
    1. int main(int argc, char *argv[])
    2. {
    3. QApplication a(argc, argv);
    4. MainWindow w;
    5. w.show();
    6. return a.exec();
    7. }
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    I have created 1 QApplication object there also

  5. #5
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    Default Re: event loop already running

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

    Cheers,
    _

  6. #6
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    Default Re: event loop already running

    I have removed the QApplication stuff from my code.
    But now nothing is being shown.No error also.
    Here is the modified code:
    Qt Code:
    1. if (!g_thread_supported ())
    2. g_thread_init (NULL);
    3.  
    4. gst_init (NULL, NULL);
    5. //QApplication app(NULL, NULL);
    6. //a.connect(&a, SIGNAL(lastWindowClosed()), &a, SLOT(quit ()));
    7.  
    8. // prepare the pipeline
    9.  
    10. GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    11. GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    12. GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    13. GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    14. gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    15. gst_element_link_many(src,jpegdec, sink);
    16. g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    17. // prepare the ui
    18.  
    19. QWidget window;
    20. window.setWindowFlags(Qt::Window|Qt::FramelessWindowHint);
    21. window.move(1000,480);
    22. window.setFixedSize(320, 200);
    23. window.show();
    24.  
    25. WId xwinid = window.winId();
    26. gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
    27.  
    28. // run the pipeline
    29.  
    30. GstStateChangeReturn sret = gst_element_set_state (pipeline,
    31. GST_STATE_PLAYING);
    32. if (sret == GST_STATE_CHANGE_FAILURE) {
    33. gst_element_set_state (pipeline, GST_STATE_NULL);
    34. gst_object_unref (pipeline);
    35. // Exit application
    36. //QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    37. }
    38.  
    39. //int ret = a.exec();
    40.  
    41. window.hide();
    42. gst_element_set_state (pipeline, GST_STATE_NULL);
    43. gst_object_unref (pipeline);
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  7. #7
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    Default Re: event loop already running

    Quote Originally Posted by prkhr4u View Post
    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.

  8. #8
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    Default Re: event loop already running

    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!

  9. The following user says thank you to d_stranz for this useful post:

    prkhr4u (12th February 2014)

  10. #9
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    Default Re: event loop already running

    @Lesiok I agree with your fact that only one QApplication object can be formed , but can you explain how in this case I am able to make 2 QApplication object and run it.
    Here is the code:

    //main.cpp
    Qt Code:
    1. QApplication a(argc, argv);
    2. MainWindow w;
    3. w.show();
    4. return a.exec();
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    //mainwindow.cpp
    Qt Code:
    1. ui->setupUi(this);
    2. if (!g_thread_supported ())
    3. g_thread_init (NULL);
    4.  
    5. gst_init (NULL, NULL);
    6. QApplication app(NULL, NULL);
    7. app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));
    8.  
    9. // prepare the pipeline
    10.  
    11. GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    12. GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    13. GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    14. GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    15. //assert(jpegdec);
    16. gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    17. gst_element_link_many(src,jpegdec, sink);
    18. g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    19. // prepare the ui
    20.  
    21. QWidget window;
    22. window.move(0,0);
    23. window.resize(320, 240);
    24. window.show();
    25.  
    26. WId xwinid = window.winId();
    27. gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
    28.  
    29. // run the pipeline
    30.  
    31. GstStateChangeReturn sret = gst_element_set_state (pipeline,
    32. GST_STATE_PLAYING);
    33. if (sret == GST_STATE_CHANGE_FAILURE) {
    34. gst_element_set_state (pipeline, GST_STATE_NULL);
    35. gst_object_unref (pipeline);
    36. // Exit application
    37. QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
    38. }
    39.  
    40. int ret = app.exec();
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    I am able to run successfully and able to get the desired output.



    Added after 12 minutes:


    I have made some changes and am still unable to get the QWidget window.Here is my code on button click event:
    Qt Code:
    1. gst_init (NULL, NULL);
    2. // prepare the pipeline
    3.  
    4. GstElement *pipeline = gst_pipeline_new ("xvoverlay");
    5. GstElement *src = gst_element_factory_make ("souphttpsrc",NULL);
    6. GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
    7. GstElement *jpegdec = gst_element_factory_make("jpegdec",NULL);
    8.  
    9. gst_bin_add_many (GST_BIN (pipeline), src,jpegdec, sink, NULL);
    10. gst_element_link_many(src,jpegdec, sink);
    11. gst_element_set_state(sink, GST_STATE_READY);
    12. g_object_set (src, "location", "http://169.254.75.39/video2.mjpg", NULL);
    13.  
    14. // prepare the ui
    15. QWidget widg1;
    16. widg1.move(0,0);
    17. widg1.setFixedSize(320,200);
    18. widg1.show();
    19.  
    20. WId xwinid = widg1.winId();
    21. QApplication::syncX();
    22. gst_x_overlay_set_window_handle (GST_X_OVERLAY (sink), xwinid);
    23.  
    24. GstStateChangeReturn sret = gst_element_set_state (pipeline,
    25. GST_STATE_PLAYING);
    26. if (sret == GST_STATE_CHANGE_FAILURE) {
    27. gst_element_set_state (pipeline, GST_STATE_NULL);
    28. gst_object_unref (pipeline);
    29. }
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    After debugging, I found that an empty QWidget window is created after QApplication::syncX(),but is destroyed as it exits the function.
    Also the State is changing successfully as it does not enter the if condition and directly exit.

    What should I do to retain the window and successfully overlay the gstreamer stream over it??
    Last edited by prkhr4u; 7th February 2014 at 07:22. Reason: duplicate contents

  11. #10
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    Default Re: event loop already running

    What you really want to do ?

  12. #11
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    Default Re: event loop already running

    Quote Originally Posted by prkhr4u View Post
    What should I do to retain the window and successfully overlay the gstreamer stream over it??
    C++ basics: if you want an object to live across a block scope boundary, create it on the heap.

    As d_stranz already wrote but which you seem to have ignored: you create widg1 on the stack inside the block scope of an if(). It is destroyed when the scope ends.

    Cheers,
    _

  13. The following user says thank you to anda_skoa for this useful post:

    prkhr4u (12th February 2014)

  14. #12
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    Default Re: event loop already running

    @Lesiok,I want to create a QWidget to play a video stream using gstreamer on button click event.
    But pl tell how are 2 QApplication objects created in the above example ??

    @anda_skoa, point noted. Sorry I neglected the stack point
    I will try to create the QWidget on the heap and see if it persists.

  15. #13
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    Default Re: event loop already running

    Quote Originally Posted by prkhr4u View Post
    @Lesiok,I want to create a QWidget to play a video stream using gstreamer on button click event.
    But pl tell how are 2 QApplication objects created in the above example ??
    So create QWidget on heap (i.e. with new opeartor) and show them. That is all. You don't need next QApplication.
    You are making a mistake like many others here : trying to use an extensive library (Qt) without knowing the basics of programming language (C++).

  16. The following user says thank you to Lesiok for this useful post:

    prkhr4u (12th February 2014)

  17. #14
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    Default Re: event loop already running

    Thank You all,the issue has been solved.
    I used declared QWidget like this:
    Qt Code:
    1. QWidget *wid = new QWidget(this);
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