Using QWidget::update() from non-GUI thread

Question

Sometimes my application crashes in QWidget::update() that is performing in non-GUI thread.

I am developing an application in which receives video frames from remote

Answer 1

The question is: is the rule "GUI operations outside the main thread are not allowed" applicable for QWidget::update()? Does this operation belongs to "GUI operations"?

Yes. Update belongs to GUI operations. According to the documentation, all QWidget and derived classes can only be used by the main thread. This is general, and specific functions may state that they are thread safe, but in this case update() does not, so it is not safe to call from other threads.

The signal/slot mechanism works because Qt will (unless told otherwise) use events to allow slots in one thread to be triggered by signals in another. If you were to use signals/slots and tell Qt not to do the special thread handling, the crashes would reappear.