ERR: "Not a Database File"; How to Modify Database Header

WINDOWS             | MS-DOS

The information in this article applies to:

• Microsoft Visual FoxPro for Windows, versions 3.0, 5.0
• Microsoft FoxPro for Windows, versions 2.5x, 2.6
• Microsoft FoxPro for MS-DOS, versions 2.0, 2.5x, 2.6

SUMMARY

When FoxPro returns the error message "Not a Database File," you can sometimes correct the damaged file's header by using a hexadecimal file editor.

WARNING: Before attempting to modify a database in this way, it is STRONGLY recommended that you back up the database.

The structure of the database file header is given below.

MORE INFORMATION

The "Not a database file" error message in FoxPro 2.x may be caused by a record count value in the table header that is greater than the actual number of records. The Hexedit.app, distributed with Visual FoxPro 5.0 may be used in an attempt to correct the record count. In the Hexedit.app, a hexadecimal representation of the record count may be found at Address: 0000000 in columns 4, 5, 6, and 7. Use the calculator in Windows 95 or Windows NT to convert from decimal to hexadecimal and remember to reverse the order of the numbers as per this article.

For information about performing table maintenance using FoxPro low-level file functions, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q135905
   TITLE     : How to Retrieve Records that were Previously Zapped

   ARTICLE-ID: Q139758
   TITLE     : How to Modify the Table Header from FoxPro

The table below identifies the bytes in the first part of the header:

   Byte number   Description
   -------------------------------------------------------------

   1             File-type identifier:
                    03 - DBF without memo (FoxBASE+/FoxPro/dBASE
                         III PLUS/dBASE IV)
                    83 - DBF with memo (FoxBASE+/dBASE III PLUS)
                    8B - dBASE III with memo
                    F5 - FoxPro with memo
   2-4           Date of last update (YYMMDD)
   5-8           Number of records in file*
   9-10          Offset to start of data**
   11-12         Size of record***
   13-28         Not used
   29            Flag for compound index****
   30-32         Not used

   * The number of records in the database is stored in reverse
     format. In other words, if those bytes were equivalent to F4 01
     00 00, there would be 500 records in the database. 1F4h is the
     hexadecimal equivalent of decimal 500.

  ** The offset to the start of the data is computed from the
     beginning of the file to the first data record. It also is stored
     in reverse format. For example, if bytes 9 and 10 are 41 02, the
     first data record would begin at 241h, or 577 decimal bytes from
     the beginning of the file.

 *** The size of the record also is stored in reverse format; the
     number represents the sum of the field sizes plus 1 because of
     the deletion flag.

     structural .CDX file attached to the database, and 00h if there
     is not an associated structural .CDX file.

The field definition is as follows:

   Byte number   Description
   ------------------------------------------------------

   1-11          Field name
   12            Field type
                 Character 43h
                 Date      44h
                 Logical   4Ch
                 Memo      4Dh
                 Numeric   4Eh
   13-14         Offset of field from beginning of record
   15-16         Not used
   17-18         Field length (nonnumeric fields)
   17            Field length (numeric fields)
   18            Field decimals (numeric fields)
   19-32         Not used

Memo, date, and logical fields, which have fixed lengths of 10, 8, and 1, respectively, need only the first byte (17). The remaining bytes are generally nulls. However, keep in mind that there is a terminating byte of 0Dh at the very end of the header.

After the terminating 0Dh byte, the actual database data begins. The first byte of each record is the deletion flag. If the record has been deleted, but not packed, this byte will be 2Ah (an ASCII asterisk); otherwise, it is 20h (an ASCII space).

For DBF files created in Visual FoxPro for Windows, or 2.x DBF files added to a Database Container (DBC), the backlink information is stored between byte 0DH and the beginning of the data.

REFERENCES

"FoxPro Version 2 Programming Guide," Chapter 11, by Michael Antonovich FoxPro for MS-DOS "Developer's Guide," version 2.0, Appendix B FoxPro for MS-DOS "Update," version 2.5, pages 4-20 to 4-21 FoxPro for Windows "Developer's Guide," version 2.5, Appendix A