Function XPGetWidgetGeometry 

pub unsafe extern "C" fn XPGetWidgetGeometry(
    inWidget: XPWidgetID,
    outLeft: *mut c_int,
    outTop: *mut c_int,
    outRight: *mut c_int,
    outBottom: *mut c_int,
)

XPGetWidgetGeometry

This routine returns the bounding box of a widget in global coordinates. Pass NULL for any parameter you are not interested in.