https://invisible-island.net/xterm/xtoolkit/
XtReservePropertyAtom, XtReleasePropertyAtom - maintain a cache of property atoms
#include <X11/Intrinsic.h> Atom XtReservePropertyAtom(Widget w); void XtReleasePropertyAtom(Widget w, Atom atom);
w Specifies the widget used to reserve the atom. atom Specifies the atom whose reservation is to be released.
XtReservePropertyAtom returns an atom that may be used for properties in conjunction with conversion requests from widget w. The atom returned will be unique for the display of the widget specified.
X Toolkit Intrinsics – C Language Interface Xlib - C Language X Interface X Version 11 libXt 1.1.5.2 XtReservePropertyAtom(3)