Motif Widget Writer's Guide Migration Version

Motif 2.1 Widget Writer's Guide

XmeResolvePartOffsets

Allows writing of upward-compatible applications and widgets

Format

#include <Xm/XmP.h>
void XmeResolvePartOffsetsWidgetClass
widget_classXmOffsetPtr
*offsetXmOffsetPtr
*constraint_offset

DESCRIPTION

The use of offset records requires two extra global variables per widget class. The variables consist of pointers to arrays of offsets into the widget record and constraint record for each part of the widget structure. The XmeResolvePartOffsets function allocates the offset records needed by an application to guarantee upward-compatible access to widget instance and constraint records by 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 steps described in the following paragraphs.

Instead of creating a resource list, the widget creates an offset resource list. To 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 structure is put into the class record as if it were a normal resource list. Instead of using XtOffset for the offset, the widget uses XmPartOffset.

If the widget is a subclass of the Constraint class and it defines additional constraint resources, create an offset resource list for the constraint part as well. Instead of using XtOffset for the offset, the widget uses XmConstraintPartOffset in the constraint resource list.

Do not create parts that do not preserve alignment (for example, a widget part containing a single Boolean).

XmPartResource resources[] = {
        {       BarNxyz, BarCXyz, XmRBoolean, sizeof(Boolean),
                XmPartOffset(Bar,xyz), XmRImmediate, (XtPointer)False }
};
XmPartResource constraints[] = {
        {       BarNmaxWidth, BarNMaxWidth,
                XmRDimension, sizeof(Dimension),
                XmConstraintPartOffset(Bar,max_width),
                XmRImmediate, (XtPointer)100 } };

Instead of putting the widget size in the class record, the widget puts the widget part size in the same field. If the widget is a subclass of the Constraint class, instead of putting the widget constraint record size in the class record, the widget puts the widget constraint part size in the same field.

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

Define a variable of type XmOffsetPtr to point to the offset record. If the widget is a subclass of the Constraint class, define a variable of type XmOffsetPtr to point to the constraint offset record. These can be part of the widget's class record or separate global variables.

In class initialization, the widget calls XmeResolvePartOffsets, passing it pointers to the class record, the address of the offset record, and the address of the constraint offset record. If the widget is not a subclass of the Constraint class, it should pass NULL as the address of the Constraint offset record. This does several things:

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

If the widget is a subclass of the Constraint class, adds the superclass constraint size field to the constraint size field

Allocates an array based upon the number of superclasses

If the widget is a subclass of the Constraint class, allocates an array for the constraint offset record

Fills in the offsets of all the widget parts and constraint parts with the appropriate values, which can be 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

The widget defines a constant that will be the index to its part structure in the offsets array. The value should be 1 greater than the index of the widget's superclass. Constants defined for all Xm widgets can be found in header file Xm/XmP.h.

#define BarIndex (XmBulletinBIndex + 1)

Instead of accessing fields directly, the widget must always go through the offset table. The XmField and XmConstraintField macros help you access these fields. Because the XmPartOffset, XmConstraintPartOffset, XmField, and XmConstraintField macros concatenate things, you must ensure that there is no space after the part argument. For example, the following macros do not work because of the space after the part (Label) argument:

XmField(w, offset, Label, text, char *)
XmPartOffset(Label, text).

You must not have any spaces after the part (Label) argument, illustrated as follows:

XmField(w, offset, Label, text, char *)
XmPartOffset(Label, text).

You can define macros for each field to make this easier. For example, the following shows how to define a macro for an enumerated field xyz in widget Bar:

#define BarXyz(w) \
        XmField(w,offsets,Bar,xyz,XtEnum)

Define a macro for constraint field max_width as follows:

#define BarMaxWidth(w) \
       
XmConstraintField(w,constraint_offsets,Bar,max_width,Dimension)

The arguments for XmeResolvePartOffsets are as follows:

widget_class

Specifies the widget class pointer for the created widget.

offset

Returns the offset record.

constraint_offset

Returns the constraint offset record.

RELATED INFORMATION

XmResolveAllPartOffsets(3).