These interfaces get and set the user ID of a property hanging on a window. Refer to the XTSOLgetPropUID(3) and XTSOLsetPropUID(3) man pages.
Status XTSOLgetPropUID(Display *display, Window winID, Atom property, uid_t *uidp); void XTSOLsetPropUID(Display *display, Window win, Atom property, uid_t *uidp);