glutKeyboardUpFunc man page
glutKeyboardUpFunc — Sets the keyboard key release callback for the current window.
OpenGLUT - input
glutKeyboardUpFunc(void( *callback )( unsigned char key, int x, int y ));
Client hook for ASCII key releases.
This function provides a way to detect the release of a keyboard key. The keys are reported exactly as with glutKeyboardFunc(), 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), glutKeyboardFunc(3), glutSpecialUpFunc(3).