https://invisible-island.net/xterm/xtoolkit/
XtOffset, XtOffsetOf, XtNumber - determine the byte offset or number of
array elements
#include <X11/Intrinsic.h>
Cardinal XtOffset(Type pointer_type, Field field_name);
Cardinal XtOffsetOf(Type structure_type, Field field_name);
Cardinal XtNumber(ArrayVariable array);
array Specifies a fixed-size array.
field_name
Specifies the name of the field for which to calculate the
byte offset.
pointer_type
Specifies a type that is declared as a pointer to the struc-
ture.
structure_type
Specifies a type that is declared as a structure.
The XtOffset macro is usually used to determine the offset of various
resource fields from the beginning of a widget and can be used at com-
pile time in static initializations.
The XtOffsetOf macro expands to a constant expression that gives the
offset in bytes to the specified structure member from the beginning of
the structure. It is normally used to statically initialize resource
lists and is more portable than XtOffset, which serves the same func-
tion.
The XtNumber macro returns the number of elements in the specified
argument lists, resources lists, and other counted arrays.
XtGetResourceList(3), XtSetArg(3)
X Toolkit Intrinsics – C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.1.5.2 XtOffset(3)