NAME

XtOffsetOf − determine the byte offset of a field within a structure type. 

Synopsis

Cardinal XtOffsetOf(structure_type, field_name)

Inputs

structure_type
Specifies a type that is declared as a structure.

field_name
Specifies the name of a field of the structure.

Returns

The offset in bytes of the specified field from the beginning of the specified structure. 

Availability

Release 4 and later. 

Description

XtOffsetOf() is a macro that expands to a constant expression that gives the offset in bytes to the specified structure member from the beginning of the structure. 

Usage

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 without any knowledge of the instance structure of the widget; it uses just a relative byte offset.  XtOffsetOf() is slightly more portable than XtOffset() which performs the same function but takes a pointer type rather than a structure type. 

Example

XtOffsetOf() is used to declare a resource list for the Athena Label widget:

#define offset(field) XtOffsetOf(LabelRec, field)
static XtResource resources[] = {
    {XtNforeground, XtCForeground, XtRPixel, sizeof(Pixel),
        offset(label.foreground), XtRString, XtDefaultForeground},
    {XtNfont,  XtCFont, XtRFontStruct, sizeof(XFontStruct *),
        offset(label.font),XtRString, XtDefaultFont},
            .
            .

Background

XtOffsetOf() is defined in terms of XtOffset() on many systems:

#ifdef offsetof
#define XtOffsetOf(s_type,field) offsetof(s_type,field)
#else
#define XtOffsetOf(s_type,field) XtOffset(s_type*,field)
#endif

See Also

XtOffset(1). 

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