XmResolvePartOffsets(library call) XmResolvePartOffsets(library call)

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

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

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 access to widget instance 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.

XmPartResource resources[] = {
  { BarNxyz, BarCXyz, XmRBoolean,
    sizeof(Boolean), XmPartOffset(Bar,xyz),
    XmRImmediate, (XtPointer)False }

Instead of putting the widget size in the class record, the widget puts the widget part size 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, of type XmOffsetPtr, 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 a pointer to contain the address of the offset record 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

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 XmP.h.

#define BarIndex (XmBulletinBIndex + 1)

Instead of accessing fields directly, the widget must always go through the offset table. The XmField macro helps you access these fields. Because the XmPartOffset and XmField macros concatenate things together, 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)

Therefore, you must not have any spaces after the part (Label) argument, as illustrated here:

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

You can 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 parameters for XmResolvePartOffsets are

Specifies the widget class pointer for the created widget
Returns the offset record