NAME¶
XmResolvePartOffsets — A function that allows writing of
upward-compatible applications and widgets "XmResolvePartOffsets"
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:
- •
- 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).