XtTranslateCoordinates() - translate widget coordinates
void XtTranslateCoords (Widget w, Position x, Position y,
Position *rootx_return, *rooty_return)
While XtTranslateCoords(3) is similar to the Xlib XTranslateCoordinates(3) function, it does not generate a server request because all the required information already is in the widget's data structures.
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface