use XGrabKeyboard(...)

I have KDE 3,X 4 and the following code

.....

  XSelectInput ( prDisplay, nWnd, ExposureMask | KeyPressMask );


  XGrabKeyboard (prDisplay, nWnd, True, GrabModeSync, GrabModeSync, 
CurrentTime);


  XMapWindow ( prDisplay, nWnd );



  while ( 1 ) {
    XNextEvent ( prDisplay, &rEvent );

.....

But my application has not recieved any key events in focus out
Why do it?