I made further tests. I'm able to crash the program without any Xlib calls at all:

I found out, that writing a simple member variable in (my class) Xvideo break things when its not inside the constructor ...?
I commented out the whole constructor code of Xvideo. I placed
>m_pDisplay = NULL;
as first command in setup(), which is enough to let it crash.

No I've even less ideas.... looks a bit like "we called the function of a deleted object"

relevant code:

class CamWindow :
Qt Code: 
  1. class CamWindow : public QWidget, private Ui::CamWindow
  2. {
  3. 		Q_OBJECT;
  4. 	protected:
  5. 		Xvideo *m_pXvPort;
  6. 	public:
  7. 		CamWindow() : QWidget()
  8. 		{
  9. 			setupUi ( this );
  10. 			Xvideo *m_pXvPort = NULL;
  11. 			try
  12. 			{
  13. 				m_pXvPort = new Xvideo();
  14. 			}
  15. 			catch ( QString strErr )
  16. 			{
  17. 				QMessageBox::warning ( this, tr ( "x video extension" ),
  18. 				                       strErr, QMessageBox::Ok );
  19. 			}
  20.  
  21. 		}
  22. 		~CamWindow()
  23. 		{
  24. 			;
  25. 		}
  26. 		void setup()
  27. 		{
  28. 			m_pXvPort->setup ( winId(), 100, 100 );
  29. 		}
  30. };
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called functions of class Xvideo:
Qt Code: 
  1. Xvideo::Xvideo()
  2. {
  3.  
  4. }
  5.  
  6. bool Xvideo::setup ( Drawable window, int width, int height )
  7. {
  8.     // next command crashes
  9.     m_pDisplay = NULL;
  10.     ....
  11.  
  12.  
  13. }
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"main code":
Qt Code: 
  1. CamWindow *w = new CamWindow();	
  2. 			m_pWorkspace->addWindow(w);
  3. 			w->show();
  4. 			w->setup();
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