XmResolvePartOffsets(library call) XmResolvePartOffsets(library call)
NAMEXmResolvePartOffsets — A function that allows writing of upward-compat‐
ible 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 XmRe‐
solvePartOffsets 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 sep‐
arate 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 ini‐
tialized) 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 struc‐
ture 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 argu‐
ment. 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) argu‐
ment, 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 INFORMATIONXmResolveAllPartOffsets(3).
XmResolvePartOffsets(library call)
