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Q263902: PRB: Table/dbf File Record Count Incorrect Based on File Size

Article: Q263902
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbServer kbSQL kbvfp300 kbvfp300b kbvfp500 kbvfp500a kbvfp600 KbDBFDBC kbGrpDSFox kbDSu
Last Modified: 13-MAY-2002

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a, 6.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

A table (.dbf) file shows that it has a number of records that is greater than
or less than the number that is expected based on the file's size.

RESOLUTION
==========

In most cases, the preceding symptoms can be overcome by modifying the header of
the table/dbf file to reflect a calculated number of records. This process is
offered as a workaround. Its usefulness is limited to a situation where all of
the following are true:

- The expected data is contained within the file.
- The file size is consistent with the amount of data that it contains.
- The file's Header Information, apart from the bytes that store the number of
  records, is all valid.

Microsoft provides programming examples for illustration only, without warranty
either expressed or implied, including, but not limited to, the implied
warranties of merchantability and/or fitness for a particular purpose. This
article assumes that you are familiar with the programming language being
demonstrated and the tools used to create and debug procedures. Microsoft
support professionals can help explain the functionality of a particular
procedure, but they will not modify these examples to provide added
functionality or construct procedures to meet your specific needs. If you have
limited programming experience, you may want to contact a Microsoft Certified
Partner or the Microsoft fee-based consulting line at (800) 936-5200. For more
information about Microsoft Certified Partners, please visit the following
Microsoft Web site:

  http://www.microsoft.com/partner/referral/

For more information about the support options that are available and about how
to contact Microsoft, visit the following Microsoft Web site:

  http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

The following demonstrates how to calculate the number of records that a table
should contain.

1. Create a program named RECCALC.PRG and add the following code:

  IF EMPTY(ALIAS())
     =MESSAGEBOX("No table is open in the selected work area.")
     RETURN
  ENDIF
  lnFile = ADIR(laFile,DBF())
  IF lnFile > 0
     lnFileSize = laFile(2)
     lnRecords = INT((lnFileSize - HEADER())/RECSIZE())
  ELSE
     lnRecords = RECCOUNT()
  ENDIF
  RETURN lnRecords

2. Open a table in the currently selected work area.

3. To show the calculated number of records printed to the Visual FoxPro screen,
  run the program from the Command window as follows:

  ?RECCALC()

In the event that the table header has been written or modified to reflect that
the table contains any number of records different from what is actually
contained in the file, the following code can be used to modify the header to
reflect the calculated number of records. This program uses low-level file
input/output (I/O) to modify the table header programmatically.

1. Create a program named RECHEADR.PRG and add the following code:

  IF EMPTY(ALIAS())
     =MESSAGEBOX("No table is open in the selected work area.")
     RETURN
  ENDIF

  lnCalcRecords = RECCALC()

  IF RECCOUNT() <> lnCalcRecords
     lcFileName = DBF()
     USE
     lnHandle = FOPEN(lcFileName, 2)

     IF lnHandle > 0
        lnHeaderData = FREAD(lnHandle,32)
        BKUP_lnHeaderData = lnHeaderData

        lnHeaderByte8 = CHR(INT(lnCalcRecords / (256^3)))
        lnHeaderByte7 = CHR(INT(lnCalcRecords / (256^2)))

        lnHeaderByte6 = CHR(INT(lnCalcRecords / 256) - ;
           IIF(lnCalcRecords / 256 > 256, ;
               (INT(INT(lnCalcRecords / 256) / 256) * 256), ;
               0))

        lnHeaderByte5 = CHR(MOD(lnCalcRecords,256))

        lnHeaderData = SUBSTR(lnHeaderData, 1, 4) + ;
           lnHeaderByte5 + lnHeaderByte6 + lnHeaderByte7 + ;
           lnHeaderByte8 + SUBSTR(lnHeaderData, 9)

        =FSEEK(lnHandle, 0)
        =FWRITE(lnHandle, lnHeaderData)
        =FCLOSE(lnHandle)
     ENDIF
     USE (lcFileName)
     RETURN .T.  && Program modified the table header
  ELSE
     RETURN .F.  && Program did not modify the table header
  ENDIF

2. Open a table in the currently selected work area.

3. To modify the table header to reflect the calculated number of records, run
  the program from the Command window as follows:

  ?RECHEADR()

4. If a modification was made, note that a logical '.T.' (True) is printed to
  the Visual FoxPro screen. Otherwise, a logical '.F.' (False) is printed.


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

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

It is possible to use a hexidecimal editing tool to modify the header of a table.
The header bytes that contain the record count are located in columns 4, 5, 6,
and 7 of address 00000000 (topmost row). In versions of Visual FoxPro 5.0 and
later, a hexidecimal editing tool is located at
HOME()+"tools\hexedit\hexedit.app".

NOTE: The following steps should be taken only on a table/dbf file that is of NO
IMPORTANCE.

1. Use a hexidecimal editing tool and open a .dbf file.

2. At address 00000000, change the information in columns 4, 5, 6, and 7 to read
  as 00 (double zero).

3. Save the changes and close the file.

4. In Visual FoxPro, open the modified .dbf file as a table, and note that the
  record count is zero.

REFERENCES
==========

For additional information about using low-level file I/O to modify a table
header, click the article number below to view the article in the Microsoft
Knowledge Base:

  Q135905 How to Retrieve Records That Were Previously Zapped

For additional informationabout situations than can result in a table with data
but zero records, click the article number below to view the article in the
Microsoft Knowledge Base:

  Q263914 PRB: CREATEOFFLINE() Results in No Records

Additional query words:

======================================================================
Keywords          : kbServer kbSQL kbvfp300 kbvfp300b kbvfp500 kbvfp500a kbvfp600 KbDBFDBC kbGrpDSFox kbDSupport kbCodeSnippet kbSQLProg 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300 kbVFP300b kbVFP500 kbVFP600 kbVFP500a
Version           : WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Issue type        : kbprb

=============================================================================

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