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Q293638: BUG: Header Corruption Causes END TRANSACTION to Discard Appends

Article: Q293638
Product(s): Microsoft FoxPro
Version(s): 5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbDatabase kbvfp500 kbvfp500a kbvfp600 kbGrpDSFox kbDSupport
Last Modified: 22-JAN-2002

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 5.0, 5.0a, 6.0 
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SYMPTOMS
========

An off-by-one record count in the header of a table may cause the END
TRANSACTION command to discard appended records with no error message displayed.

RESOLUTION
==========

If your tables are subject to this type of corruption, use a test as shown below
to compare the physical file size to the logical file size. Run this test when
you open the table.

STATUS
======

Microsoft has confirmed this to be a problem in the Microsoft products that are
listed at the beginning of this article.

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

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

1. Copy the following code into a .prg file and then run it:

  LOCAL nRecCount, nLogicalSize, nPhysicalSize, nHandler, i

  CREATE DATABASE test_db
  CREATE TABLE test_tbl (one_field N(1,0))
  FOR i = 1 TO 5
  	INSERT INTO test_tbl (one_field) VALUES (i)
  ENDfor

  * Check size before corruption.
  SELECT test_tbl
  nRecCount = RECCOUNT()
  nLogicalSize = HEADER() + (nRecCount * RECSIZE()) + 1
  USE IN test_tbl

  nHandler = FOPEN('test_tbl.dbf', 2)
  nPhysicalSize = FSEEK(nHandler, 0, 2)
  FCLOSE(nHandler)

  ? 'BEFORE CORRUPTION'
  ? 'Record count: ', nRecCount
  ? 'Logical size: ', nLogicalSize
  ? 'Physical size:', nPhysicalSize

  * Manually decrease the record count in the table header.
  * Normally it would happen accidentally after some power 
  * or network failures.

  ?
  ? 'Simulating table corruption...'
  nHandler = FOPEN('test_tbl.dbf', 2)
  FSEEK(nHandler, 4)
  FWRITE(nHandler, CHR(4))
  FCLOSE(nHandler)

  * Check size after corruption
  USE test_tbl
  nRecCount = RECCOUNT()
  nLogicalSize = HEADER() + (nRecCount * RECSIZE()) + 1
  USE IN test_tbl

  nHandler = FOPEN('test_tbl.dbf', 2)
  nPhysicalSize = FSEEK(nHandler, 0, 2)
  FCLOSE(nHandler)

  ?
  ? 'AFTER CORRUPTION'
  ? 'Record count: ', nRecCount
  ? 'Logical size: ', nLogicalSize
  ? 'Physical size:', nPhysicalSize

  * I will try inserting a record within a transaction.

  ?
  ? 'Inserting new record...'

  USE test_tbl
  BEGIN TRANSACTION
  INSERT INTO test_tbl VALUES (6)
  END TRANSACTION
  ? 'No error:', ERROR()

  * Check size after insert
  nRecCount = RECCOUNT()
  nLogicalSize = HEADER() + (nRecCount * RECSIZE()) + 1

  USE IN test_tbl

  nHandler = FOPEN('test_tbl.dbf', 2)
  nPhysicalSize = FSEEK(nHandler, 0, 2)
  FCLOSE(nHandler)

  ?
  ? 'AFTER INSERTION'
  ? 'Record count: ', nRecCount
  ? 'Logical size: ', nLogicalSize
  ? 'Physical size:', nPhysicalSize

  ? 'VFP discarded insert operation.'
  CLOSE DATA ALL

Additional query words:

======================================================================
Keywords          : kbDatabase kbvfp500 kbvfp500a kbvfp600 kbGrpDSFox kbDSupport 
Technology        : kbVFPsearch kbAudDeveloper kbVFP500 kbVFP600 kbVFP500a
Version           : :5.0,5.0a,6.0
Issue type        : kbbug
Solution Type     : kbpending

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