NAME

XmResolvePartOffsets — a function that allows writing of upward-compatible applications and widgets.

SYNOPSIS

#include <Xm/XmP.h> void XmResolvePartOffsets (widget_class, offset)
WidgetClass widget_class;
XmOffsetPtr ∗ offset;

DESCRIPTION

The use of offset records requires one extra global variable per widget class.  The variable consists of a pointer to an array of offsets into the widget record for each part of the widget structure.  The XmResolvePartOffsets function allocates the offset records needed by an application to guarantee upward-compatible applications and widgets.  These offset records are used by the widget to access all of the widget’s variables.  A widget needs to take the following steps:

•Instead of creating a resource list, the widget creates an offset resource list.  To help you accomplish this, use the XmPartResource structure and the XmPartOffset macro.  The XmPartResource data structure looks just like a resource list, but instead of having one integer for its offset, it has two shorts.  This gets put into the class record as if it were a normal resource list.  Instead of using XtOffset for the offset, it uses XmPartOffset. 

•Instead of putting the widget size in the class record, the widget puts the widget part in the same field. 

•Instead of putting XtVersion in the class record, the widget puts XtVersionDontCheck in the class record. 

•The widget defines a variable to point to the offset record.  This can be part of the widget’s class record or a separate global variable. 

•In class initialization, the widget calls XmResolvePartOffsets, passing it the offset address and the class record.  This does several things:

Adds the superclass (which, by definition, has already been initialized) size field to the part size field. 

Allocates an array based upon the number of superclasses. 

Fills in the offsets of all the widget parts with the appropriate values, determined by examining the size fields of all superclass records. 

Uses the part offset array to modify the offset entries in the resource list to be real offsets, in place. 

•Instead of accessing fields directly, the widget must always go through the offset table.  You will probably define macros for each field to make this easier.  Assume an integer field “xyz”: #define BarXyz(w) (∗(int ∗)(((char ∗) w) + offset[BarIndex] + \ XtOffset(BarPart,xyz)))

The XmField macro helps you access these fields.  Because the XmPartOffset and XmField macros concatenate things together, you must ensure there is no space before or after the part argument.  For example, the following do not work because of the space before or after the part (Label) argument:

XmField(w, offset, Label, text, char ∗)

XmPartOffset( Label, text).  Therefore, you must not have any spaces before or after the part (Label) argument, as illustrated here:

XmField(w, offset,Label, text, char ∗)

The parameters for XmResolvePartOffsets are defined below:

widget_classSpecifies the widget class pointer for the created widget. 

offsetSpecifies the offset record. 

7th Edition