XtOffset(), XtNumber() - determine the byte offset or number of array elements
Cardinal XtOffset (Type pointer_type, Field field_name)
Cardinal XtNumber (ArrayVariable array)
The XtOffset(3) macro is usually used to determine the offset of various resource fields from the beginning of a widget and can be used at compile time in static initializations.
The XtNumber(3) macro returns the number of elements in the specified argument lists, resources lists, and other counted arrays.
XtSetArg()
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface