XmResolvePartOffsets(library call) XmResolvePartOffsets(library call)

NAME

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

SYNOPSIS

#include <Xm/Xm.h>
void XmResolvePartOffsets(
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 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:

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

+o Allocates an array based upon the number of superclasses

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

+o 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

XmResolveAllPartOffsets(3).

XmResolvePartOffsets(library call)