glutSpecialUpFunc - Man Page
Sets the special key release callback for the current window
OpenGLUT - input
glutSpecialUpFunc(void( *callback )( int key, int x, int y ));
Client hook for special key releases.
This function provides a way to detect the release of a keyboard
The keys are reported exactly as with glutSpecialFunc(), save that the
registered via this function is used to report the event.
This callback is bound to the
Windows created via glutCreateMenuWindow() always cascade keyboard and mouse events to their parent.
glutCreateMenuWindow(3), glutKeyboardUpFunc(3), glutSpecialFunc(3).