https://invisible-island.net/xterm/xtoolkit/
XtGetClassExtension - locate a class extension record
#include <X11/Intrinsic.h>
XtPointer XtGetClassExtension(WidgetClass object_class, Cardinal
byte_offset, XrmQuark type, long version, Cardinal record_size);
object_class
Specifies the widget class containing the extension list to
be searched.
byte_offset
Specifies the offset in bytes form the base of the widget
class of the extension field to be searched.
type Specifies the name of the class extension to be located.
version Specifies the minimum acceptable version of the class exten-
sion required for a match.
record_size
Specifies the minimum acceptable length of the class exten-
sion record required for match, or 0.
The list of extension records at the specified offset in the specified
widget_class will be searched for a match on the specified type, a ver-
sion greater than or equal to the specified version, and a record size
greater than or equal the specified record_size if it is nonzero.
XtGetClassExtension returns a pointer to a matching extension record or
NULL if no match is found. The returned extension record is owned by
the widget class and must not be modified or freed by the caller,
except possibly to be modified by the widget class owner.
X Toolkit Intrinsics – C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.1.5.2 XtGetClassExtension(3)