XmResolvePartOffsets(Xm) UNIX System V
NAME
XmResolvePartOffsets - a function that allows writing of
upward-compatible applications and widgets.
SYNOPSIS
#include <Xm/XmP.h>
void XmResolvePartOffsets (widget_class, offset)
WidgetClasswidget_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 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 following
steps:
⊕ Instead of creating a resource list, the widget creates
an offset resource list. To help you 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 gets put into the
class record as if it were a normal resource list.
Instead of using XtOffset for the offset, it uses
XmPartOffset.
⊕ Instead of putting the widget size in the class record,
the widget puts the widget part 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 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 the offset address 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.
⊕ Instead of accessing fields directly, the widget must
always go through the offset table. You will probably
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 XmField macro helps you access these fields. Because
the XmPartOffset and XmField macros concatenate things
together, you must ensure there is no space before or
after the part argument. For example, the following do
not work because of the space before or after the part
(Label) argument:
XmField(w, offset, Label, text, char *)
XmPartOffset( Label, text).
Therefore, you must not have any spaces before or after
the part (Label) argument, as illustrated here:
XmField(w, offset,Label, text, char *)
The parameters for XmResolvePartOffsets are defined below:
widget_class
Specifies the widget class pointer for the created
widget.
offset Specifies the offset record.
(printed 2/14/90) XmResolvePartOffsets(Xm)