XmResolvePartOffsets - Man Page

#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

widget_class

Specifies the widget class pointer for the created widget

offset

Returns the offset record

XmResolveAllPartOffsets(3)​.