Calling X functions on gtk windows

["Mr. T. M. Sheen" <T M Sheen eng abdn ac uk>]

I am having trouble calling an X function directly on a gtk window.
I want to iconify the application. 
I am just guessing that I have the correct X function, 
I want to do something like

IconifyWindow(GtkWidget *w){
  XIconifyWindow(GDK_WINDOW_XDISPLAY(w),GDK_WINDOW_XWINDOW(w),0);
}

 I have also tried

IconifyWindow(GtkWidget *w){
  XIconifyWindow(GDK_DISPLAY(),GDK_WINDOW_XWINDOW(w),0);
}


  Neither of which works. 

The first crashes and the second does nothing. 

I would be grateful for any pointers. 


	Cheers,

		Tim..