trusty (3) XmResolvePartOffsets.3.gz
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
RELATED INFORMATION
XmResolveAllPartOffsets(3). XmResolvePartOffsets(3)