glutSpaceballButtonFunc sets the Spaceball button callback for the current window.
void glutSpaceballButtonFunc(void (*func)(int button, int state));
glutSpaceballButtonFunc sets the Spaceball button callback for the current window. The Spaceball button callback for a window is called when the window has Spaceball input focus (normally, when the mouse is in the window) and the user generates Spaceball button presses. The button parameter will be the button number (starting at one). The number of available Spaceball buttons can be determined with glutDeviceGet(GLUT_NUM_SPACEBALL_BUTTONS). The state is either GLUT_UP or GLUT_DOWN indicating whether the callback was due to a release or press respectively.
Registering a Spaceball button callback when a Spaceball device is not available is ineffectual and not an error. In this case, no Spaceball button callbacks will be generated.
Passing NULL to glutSpaceballButtonFunc disables the generation of Spaceball button callbacks. When a new window is created, no Spaceball button callback is initially registered.