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.TH "XmResolvePartOffsets" "library call"

.SH "NAME"

\fBXmResolvePartOffsets\fP \(em A function that allows writing of upward-compatible applications and widgets

.iX "XmResolvePartOffsets"

.SH "SYNOPSIS"

.PP

.nf

#include <Xm/Xm\&.h>

\fBvoid \fBXmResolvePartOffsets\fP\fR(

\fBWidgetClass \fBwidget_class\fR\fR,

\fBXmOffsetPtr \fB* offset\fR\fR);

.fi

.SH "DESCRIPTION"

.PP

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 \fBXmResolvePartOffsets\fP 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\&.

.PP

Instead of creating a resource list, the widget creates an offset

resource list\&.

To accomplish this, use the \fBXmPartResource\fR

structure and the \fBXmPartOffset\fP macro\&.

The \fBXmPartResource\fR 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 \fBXtOffset\fP for the offset, the widget uses

\fBXmPartOffset\fP\&.

.PP

.nf

\f(CWXmPartResource resources[] = {

  { BarNxyz, BarCXyz, XmRBoolean,

    sizeof(Boolean), XmPartOffset(Bar,xyz),

    XmRImmediate, (XtPointer)False }

};\fR

.fi

.PP

.PP

Instead of putting the widget size in the class record, the widget puts the

widget part size in the same field\&.

.PP

Instead of putting \fBXtVersion\fP in the class record, the widget puts

\fBXtVersionDontCheck\fP in the class record\&.

.PP

The widget defines a variable, of type \fBXmOffsetPtr\fR, to point to

the offset record\&.

This can be part of the widget\&'s class record or a separate global

variable\&.

.PP

In class initialization, the widget calls \fBXmResolvePartOffsets\fP,

passing it a pointer to contain the address of the offset

record and the class record\&.

This does several things:

.IP "   \(bu" 6

Adds the superclass (which, by definition, has already been initialized)

size field to the part size field

.IP "   \(bu" 6

Allocates an array based upon the number of superclasses

.IP "   \(bu" 6

Fills in the offsets of all the widget parts with the appropriate

values, determined by examining the size fields of all superclass

records

.IP "   \(bu" 6

Uses the part offset array to modify the offset entries in the resource

list to be real offsets, in place

.PP

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 \fBXm\fP

widgets can be found in \fBXmP\&.h\fP\&.

.PP

.nf

\f(CW#define BarIndex (XmBulletinBIndex + 1)\fR

.fi

.PP

.PP

Instead of accessing fields directly, the widget must always go through

the offset table\&.

The \fBXmField\fP macro helps you access these fields\&.

Because the \fBXmPartOffset\fP and \fBXmField\fP

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:

.PP

.nf

\f(CWXmField(w, offset, Label, text, char *)

XmPartOffset(Label, text)\fR

.fi

.PP

.PP

Therefore, you must not have any spaces after the part (Label)

argument, as illustrated here:

.PP

.nf

\f(CWXmField(w, offset, Label, text, char *)\fR

.fi

.PP

.PP

You can define macros for each field to make this easier\&.

Assume an integer field \fIxyz\fP:

.PP

.nf

\f(CW#define BarXyz(w) (*(int *)(((char *) w) + \e

        offset[BarIndex] + XtOffset(BarPart,xyz)))\fR

.fi

.PP

.PP

The parameters for \fBXmResolvePartOffsets\fP are

.IP "\fIwidget_class\fP" 10

Specifies the widget class pointer for the created widget

.IP "\fIoffset\fP" 10

Returns the offset record

.SH "RELATED INFORMATION"

.PP

\fBXmResolveAllPartOffsets\fP(3)\&.

...\" created by instant / docbook-to-man, Sun 22 Dec 1996, 20:29