XmResolvePartOffsets(library call)       XmResolvePartOffsets(library call)




 NAME    [Toc]    [Back]
      XmResolvePartOffsets - A function that allows writing of upwardcompatible
 applications and widgets

 SYNOPSIS    [Toc]    [Back]
      #include <Xm/Xm.h>
      void XmResolvePartOffsets(
      WidgetClass widget_class,
      XmOffsetPtr * offset);

 DESCRIPTION    [Toc]    [Back]
      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




                                    - 1 -       Formatted:  January 24, 2005






 XmResolvePartOffsets(library call)       XmResolvePartOffsets(library call)




         +  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

      widget_class
                Specifies the widget class pointer for the created widget

      offset    Returns the offset record

 RELATED INFORMATION    [Toc]    [Back]
      XmResolveAllPartOffsets(3).


                                    - 2 -       Formatted:  January 24, 2005