Q174530: FIX: TABLEREVERT() with Table Buffering Results in Page Fault

Article: Q174530
Product(s): Microsoft FoxPro
Version(s): MACINTOSH:3.0b; WINDOWS:3.0b
Operating System(s): 
Keyword(s): kbHWMAC kbvfp
Last Modified: 17-AUG-1999

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0b 
- Microsoft Visual FoxPro for Macintosh, version 3.0b 
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SYMPTOMS
========

Closing a form containing a grid with two or more related record sources and
table buffering enabled results in an Windows Application Error or Macintosh
error type 11 and termination of Visual FoxPro.

Under some circumstances, closing a form containing a grid with multiple record
sources and table buffering enabled, results in an application error and
abnormal termination of Visual FoxPro. This article describes the conditions
under which an application error may occur and a workaround that resolves the
problem.

CAUSE
=====

The occurrence of an application error and abnormal termination of the Visual
FoxPro session, under these conditions, is dependent on a specific sequence of
events. This behavior occurs only under the following circumstances:

- A form is created, containing a grid based on two or more related record
  sources.

- The child table is USEd in the form's DataEnvironment, Load, or Init event
  before the Parent table is USEd.

- A many-to-one relationship is established between the child table and the
  parent table.

- The Grid.RecordSourceType Property is set to 1 (ALIAS).

- The Grid.RecordSource Property is the alias of the child table.

- The Grid.ColumnCount is set to a value of at least two.

- The ControlSource Property for at least one column of the Grid is a field in
  the parent table.

- The ControlSource Property for at least one column of the Grid is a field in
  the child table.

- Table buffering on the child table is enabled.

- A TABLEREVERT(.t.,'Child Table Name') is issued.

- The open tables are closed either in the DataEnvironment or by issuing a
  CLOSE TABLES command.

RESOLUTION
==========

Multiple workarounds are available for this condition:

  Insert ThisForm.Grid.RecordSource="" before releasing the form or issuing a
  'CLOSE TABLES' command.

  -or-

  Use a view or SQL Cursor as the record source for the grid.

  -or-

  Base Grids with multiple record sources on the parent table.

  -or-

  Instantiate cursors for parent tables prior to instantiating cursors for child
  tables.

  -or-

  Use row buffering instead of table buffering.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been corrected in Visual FoxPro 5.0.

MORE INFORMATION
================

This behavior is not observed in Visual FoxPro 3.0. It is evident in Visual
FoxPro 3.0b only when table buffering is enabled.

Steps to Reproduce Behavior
---------------------------

1. Use the following code to create two related tables:

        SET MULTILOCKS ON
        IF !FILE('test.dbc')
           CREATE DATABASE test
           CREATE TABLE test1 (var1 C(10), PRIMARY KEY var1 TAG var1)
          CREATE TABLE test2 (var1 c(10), ;
              FOREIGN KEY var1 TAG var1 REFERENCES test1 TAG var1)
        ENDIF

2. Create a form called test.

3. Select the DataEnvironment and add table test2, then add table test1 OR place
  the following code in the form's Load event:

        SET MULTILOCKS ON
        USE test2 IN 0
        USE test1 IN 0
        SELECT test1
        SET ORDER TO TAG var1
        SELECT test2
        SET ORDER TO TAG var1
        SET RELATION TO var1 INTO test1

4. In the INIT event of the form, place the following code:

        =CURSORSETPROP('Buffering',5,'test2')

5. Add a grid to the form with the following properties:

        Form.Grid1.ColumnCount=2
        Form.Grid1.RecordSourceType=1
        Form.Grid1.RecordSource='test2'

        Form.Grid1.Column1.ControlSource=test1.var1
        Form.Grid1.Column2.ControlSource=test2.var1

6. Add a Command button with the following code in the click event:

        =TABLEREVERT(.T.,'test2')
        CLOSE TABLES ALL
        Release ThisForm

7. Save and run the form.

Additional query words: Grid Column ControlSource RecordSource Buffering

======================================================================
Keywords          : kbHWMAC kbvfp 
Technology        : kbHWMAC kbOSMAC kbVFPsearch kbAudDeveloper kbVFP300bMac kbVFP300b
Version           : MACINTOSH:3.0b; WINDOWS:3.0b
Issue type        : kbbug
Solution Type     : kbfix

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