hpux man page for xtoffset

XtOffset()																XtOffset()

Name
  XtOffset - determine the byte offset of a field within a structure pointer type.

Synopsis
  Cardinal XtOffset(pointer_type, field_name)

Inputs
  pointer_type
	     Specifies a type that is declared as a pointer to the structure.

  field_name Specifies the name of the field in the structure pointed to by pointer_type.

Returns
  The offset in bytes of the named field from the beginning of the structure type pointed to by pointer_type.

Description
  XtOffset()  is a macro that computes the offset of a named field in a structure, given the field name and a type which points to the struc-
  ture.

Usage
  XtOffset() has been superseded by XtOffsetOf() which takes the structure type itself rather than a pointer to the structure, and is defined
  in a more portable way.

  XtOffset()  is  usually  used to determine the location of an variable within a structure when initializing a resource list.	It is used by
  widget writers, and anyone who needs to fetch application resources with XtGetApplicationResources().  Resource fields are defined in terms
  of offsets from a base address from the beginning of a widget.  Thus, a resource value can be kept up to date by the Resource Manager with-
  out any knowledge of the instance structure of the widget; it uses just a relative byte offset.

Background
  XtOffset() is a macro defined as follows for most architectures:

     #define XtOffset(p_type,field)	((Cardinal) (((char *) (&(((p_type)NULL)->field))) - ((char *) NULL)))

See Also
  XtOffsetOf(1).

Xt - Utilities																XtOffset()