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_nameSpecifies 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 structure. 

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XtOffset() has been superseded by XtOffsetOf() which takes the structure type itself rather than a pointer to the structure, and is defined in a more portable way.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 without 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) \ #define XtOffset(p_type,field) \  ((Cardinal) (((char ∗) (&(((p_type)NULL)->field))) - ((char ∗) NULL)))

SEE ALSO

XtOffsetOfUNIX SYSTEM V/68.