XtRegisterDrawable man page
XtRegisterDrawable — register a drawable with the Intrinsics event dispatcher
void XtRegisterDrawable(Display* display, Drawable drawable, Widget widget);
void XtUnregisterDrawable(Display* display, Drawable drawable);
Specifies the drawable's display.
Specifies the drawable to register.
Specifies the widget to register the drawable for.
XtRegisterDrawable associates the specified drawable with the specified widget so that future calls to XtWindowToWidget with the drawable will return the widget. The default event dispatcher will dispatch future core events that arrive with the drawable to the widget as though the event contained the widget's window, but the event itself will not be changed in any way when being passed to even handler or action procedures.
If the drawable is already registered with another widget, or if the drawable is the window of a widget in the client's widget tree, the results of calling XtRegisterDrawable are undefined.
XtUnregisterDrawable removes an association created with XtRegisterDrawable. If the drawable is the window of a widget in the client's widget tree the results of calling XtUnregisterDrawable are undefined.
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface
XtUnregisterDrawable(3) is an alias of XtRegisterDrawable(3).