DBase

  1. TI2821 - dBASE .DBF File Structure

TI2821 - dBASE .DBF File Structure

Sometimes it is necessary to delve into a dBASE table outside the control of the Borland Database Engine (BDE). For instance, if the .DBT file (that contains memo data) for a given table is irretrievably lost, the file will not be usable because the byte in the file header indicates that there should be a corresponding memo file. This necessitates toggling this byte to indicate no such accompanying memo file. Or, you may just want to write your own data access routine.

Below are the file structures for dBASE table files. Represented are the file structures as used for various versions of dBASE: dBASE III PLUS 1.1, dBASE IV 2.0, dBASE 5.0 for DOS, and dBASE 5.0 for Windows.

The data file header structure for dBASE III PLUS table file.

The table file header
Byte Contents Description
01 byte Valid dBASE III PLUS table file (03h without a memo (.DBT file;) 83h with a memo).
1-3 3 bytes Date of last update; in YYMMDD format.
4-7 32-bit number Number of records in the table.
8-9 16-bit number Number of bytes in the header.
10-11 16-bit number Number of bytes in the record.
12-14 3 bytes Reserved bytes.
15-27 13 bytes Reserved for dBASE III PLUS on a LAN.
28-31 4 bytes Reserved bytes.
32-n 32 bytes Field descriptor array (the structure of this array is each shown below)
n+1 1 byte 0Dh stored as the field terminator.
n above is the last byte in the field descriptor array. The size of the array depends on the number of fields in the table file.

Table Field Descriptor Bytes

Byte Contents Description
0-10 11 bytes Field name in ASCII (zero-filled).
11 1 byte Field type in ASCII (C, D, L, M, or N).
12-15 4 bytes Field data address (address is set in memory; not useful on disk).
16 1 byte Field length in binary.
17 1 byte Field decimal count in binary.
18-19 2 bytes Reserved for dBASE III PLUS on a LAN.
20 1 byte Work area ID.
21-22 2 bytes Reserved for dBASE III PLUS on a LAN.
23 1 byte SET FIELDS flag.
24-31 1 byte Reserved bytes.

Table Records

The records follow the header in the table file. Data records are preceded by one byte, that is, a space (20h) if the record is not deleted, an asterisk (2Ah) if the record is deleted. Fields are packed into records without field separators orrecord terminators. The end of the file is marked by a single byte, with the end-of-file marker, an OEM code page character value of 26 (1Ah). You can input OEM code page data as indicated below.

Allowable Input for dBASE Data Types

Data Type Data Input
C (Character) All OEM code page characters.
D (Date) Numbers and a character to separate month, day, and year (stored internally as 8 digits in YYYYMMDD format).
N (Numeric) - . 0 1 2 3 4 5 6 7 8 9
L (Logical) ? Y y N n T t F f (? when not initialized).
M (Memo) All OEM code page characters (stored internally as 10 digits representing a .DBT block number).

Binary, Memo, and OLE Fields And .DBT Files

Memo fields store data in .DBT files consisting of blocks numbered sequentially (0, 1, 2, and so on). The size of these blocks are internally set to 512 bytes. The first block in the .DBT file, block 0, is the .DBT file header.

Memo field of each record in the .DBF file contains the number of the block (in OEM code page values) where the field's data actually begins. If a field contains no data, the .DBF file contains blanks (20h) rather than a number.

When data is changed in a field, the block numbers may also change and the number in the .DBF may be changed to reflect the new location.

This information is from the Using dBASE III PLUS manual, Appendix C.

The data file header structure for dBASE IV 2.0 table file.

File Structure
Byte Contents Description
0 1byte Valid dBASE IV file; bits 0-2 indicate version number, bit 3 the presence of a dBASE IV memo file, bits 4-6 the presence of an SQL table, bit 7 the presence of any memo file (either dBASE III PLUS or dBASE IV).
1-3 3 bytes Date of last update; formattted as YYMMDD.
4-7 32-bit number Number of records in the file.
8-9 16-bit number Number of bytes in the header.
10-11 16-bit number Number of bytes in the record.
12-13 2 bytes Reserved; fill with 0.
14 1 byte Flag indicating incomplete transaction.
15 1 byte Encryption flag.
16-27 12 bytes Reserved for dBASE IV in a multi-user environment.
28 1 bytes Production MDX file flag; 01H if there is an MDX, 00H if not.
29 1 byte Language driver ID.
30-31 2 bytes Reserved; fill with 0.
32-n* 32 bytes each Field descriptor array (see below).
n + 1 1 byte 0DH as the field terminator.
* n is the last byte in the field descriptor array. The size of the array depends on the number of fields in the database file.

Field Descriptor Array
Byte Contents Description
0-10 11 bytes Field name in ASCII (zero-filled).
11 1 byte Field type in ASCII (C, D, F, L, M, or N).
12-15 4 bytes Reserved.
16 1 byte Field length in binary.
17 1 byte Field decimal count in binary.
18-19 2 bytes Reserved.
20 1 byte Work area ID.
21-30 10 bytes Reserved.
31 1 byte Production MDX field flag; 01H if field has an index tag in the production MDX file, 00H if not.

Database records:

The records follow the header in the database file. Data records are preceded by one byte; that is, a space (20H) if the record is not deleted, an asterisk (2AH) if the record is deleted. Fields are packed into records without field separators or record terminators. The end of the file is marked by a single byte, with the end-of-file marker an ASCII 26 (1AH) character.

Allowable Input for dBASE Data Types:


Data  Type           Data Input
----  ----------     -----------------------------------------------------
C     (Character)    All OEM code page characters.
D     (Date)         Numbers and a character to separate month, day, and
                     year (stored internally as 8 digits in YYYYMMDD
                     format).
F     (Floating      - . 0 1 2 3 4 5 6 7 8 9
      point binary
      numeric)
N     (Binary        - . 0 1 2 3 4 5 6 7 8 9
      coded decimal
      numeric)   
L     (Logical)      ? Y y N n T t F f (? when not initialized).
M     (Memo)         All OEM code page characters (stored internally as 10
                     digits representing a .DBT block number).

Memo Fields And .DBT Files

Memo fields store data in .DBT files consisting of blocks numbered sequentially (0, 1, 2, and so on). SET BLOCKSIZE determines the size of each block. The first block in the .DBT file, block 0, is the .DBT file header.

Each memo field of each record in the .DBF file contains the number of the block (in OEM code page values) where the field's data actually begins. If a field contains no data, the .DBF file contains blanks (20h) rather than a number.

When data is changed in a field, the block numbers may also change and the number in the .DBF may be changed to reflect the new location.

This information is from the dBASE IV Language Reference manual, Appendix D.

The data file header structure for dBASE 5.0 for DOS table file.

The table file header:

Byte  Contents    Description

0     1 byte      Valid dBASE for Windows table file; bits 0-2 indicate
                  version number; bit 3 indicates presence of a dBASE IV
                  or dBASE for Windows memo file; bits 4-6 indicate the
                  presence of a dBASE IV SQL table; bit 7 indicates the
                  presence of any .DBT memo file (either a dBASE III PLUS
                  type or a dBASE IV or dBASE for Windows memo file).
1-3   3 bytes     Date of last update; in YYMMDD format.
4-7   32-bit      Number of records in the table.
      number
8-9   16-bit      Number of bytes in the header.
      number
10-11 16-bit      Number of bytes in the record.
      number
12-13 2 bytes     Reserved; filled with zeros.
14    1 byte      Flag indicating incomplete dBASE transaction.
15    1 byte      Encryption flag.
16-27 12 bytes    Reserved for multi-user processing.
28    1 byte      Production MDX flag; 01h stored in this byte if a prod-
                  uction .MDX file exists for this table; 00h if no .MDX
                  file exists.
29    1 byte      Language driver ID.
30-31 2 bytes     Reserved; filled with zeros.
32-n  32 bytes    Field descriptor array (the structure of this array is
      each        shown below)
n+1   1 byte      0Dh stored as the field terminator.
n above is the last byte in the field descriptor array. The size of the array depends on the number of fields in the table file.

Table Field Descriptor Bytes

Byte  Contents    Description
----- --------    --------------------------------------------------------
0-10  11 bytes    Field name in ASCII (zero-filled).
11    1 byte      Field type in ASCII (B, C, D, F, G, L, M, or N).
12-15 4 bytes     Reserved.
16    1 byte      Field length in binary.
17    1 byte      Field decimal count in binary.
18-19 2 bytes     Reserved.
20    1 byte      Work area ID.
21-30 10 bytes    Reserved.
31    1 byte      Production .MDX field flag; 01h if field has an index
                  tag in the production .MDX file; 00h if the field is not
                  indexed.

Table Records

The records follow the header in the table file. Data records are preceded by one byte, that is, a space (20h) if the record is not deleted, an asterisk (2Ah) if the record is deleted. Fields are packed into records without field separators orrecord terminators. The end of the file is marked by a single byte, with the end-of-file marker, an OEM code page character value of 26 (1Ah). You can input OEM code page data as indicated below.

Allowable Input for dBASE Data Types

Data Type      Data Input
C (Character)  All OEM code page characters.
D (Date)       Numbers and a character to separate month, day, and year
               (stored internally as 8 digits in YYYYMMDD format).
F (Floating    - . 0 1 2 3 4 5 6 7 8 9
  point binary
  numeric)
N (Numeric)    - . 0 1 2 3 4 5 6 7 8 9
L (Logical)    ? Y y N n T t F f (? when not initialized).
M (Memo)       All OEM code page characters (stored internally as 10
               digits representing a .DBT block number).

Memo Fields And .DBT Files

Memo fields store data in .DBT files consisting of blocks numbered sequentially (0, 1, 2, and so on). SET BLOCKSIZE determines the size of each block. The first block in the .DBT file, block 0, is the .DBT file header.

Each memo field of each record in the .DBF file contains the number of the block (in OEM code page values) where the field's data actually begins. If a field contains no data, the .DBF file contains blanks (20h) rather than a number.

When data is changed in a field, the block numbers may also change and the number in the .DBF may be changed to reflect the new location.

Unlike dBASE III PLUS, if you delete text in a memo field, dBASE 5.0 for DOS may reuse the space from the deleted text when you input new text. dBASE III PLUS always appends new text to the end of the .DBT file. In dBASE III PLUS, the .DBT file size grows whenever new text is added, even if other text in the file is deleted.

This information is from the dBASE for DOS Language Reference manual, Appendix C.

The data file header structure for dBASE 5.0 for Windows table file.

The table file header:

Byte  Contents    Description

0     1 byte      Valid dBASE for Windows table file; bits 0-2 indicate
                  version number; bit 3 indicates presence of a dBASE IV
                  or dBASE for Windows memo file; bits 4-6 indicate the
                  presence of a dBASE IV SQL table; bit 7 indicates the
                  presence of any .DBT memo file (either a dBASE III PLUS
                  type or a dBASE IV or dBASE for Windows memo file).
1-3   3 bytes     Date of last update; in YYMMDD format.
4-7   32-bit      Number of records in the table.
      number
8-9   16-bit      Number of bytes in the header.
      number
10-11 16-bit      Number of bytes in the record.
      number
12-13 2 bytes     Reserved; filled with zeros.
14    1 byte      Flag indicating incomplete dBASE IV transaction.
15    1 byte      dBASE IV encryption flag.
16-27 12 bytes    Reserved for multi-user processing.
28    1 byte      Production MDX flag; 01h stored in this byte if a prod-
                  uction .MDX file exists for this table; 00h if no .MDX
                  file exists.
29    1 byte      Language driver ID.
30-31 2 bytes     Reserved; filled with zeros.
32-n  32 bytes    Field descriptor array (the structure of this array is
      each        shown below)
n+1   1 byte      0Dh stored as the field terminator.
n above is the last byte in the field descriptor array. The size of the array depends on the number of fields in the table file.

Table Field Descriptor Bytes

Byte  Contents    Description

0-10  11 bytes    Field name in ASCII (zero-filled).
11    1 byte      Field type in ASCII (B, C, D, F, G, L, M, or N).
12-15 4 bytes     Reserved.
16    1 byte      Field length in binary.
17    1 byte      Field decimal count in binary.
18-19 2 bytes     Reserved.
20    1 byte      Work area ID.
21-30 10 bytes    Reserved.
31    1 byte      Production .MDX field flag; 01h if field has an index
                  tag in the production .MDX file; 00h if the field is not
                  indexed.

Table Records

The records follow the header in the table file. Data records are preceded by one byte, that is, a space (20h) if the record is not deleted, an asterisk (2Ah) if the record is deleted. Fields are packed into records without field separators orrecord terminators. The end of the file is marked by a single byte, with the end-of-file marker, an OEM code page character value of 26 (1Ah). You can input OEM code page data as indicated below.

Allowable Input for dBASE Data Types

Data Type      Data Input

B (Binary)     All OEM code page characters (stored internally as 10
               digits representing a .DBT block number).
C (Character)  All OEM code page characters.
D (Date)       Numbers and a character to separate month, day, and year
               (stored internally as 8 digits in YYYYMMDD format).
G (General     All OEM code page characters (stored internally as 10
               digits or OLE) representing a .DBT block number).
N (Numeric)    - . 0 1 2 3 4 5 6 7 8 9
L (Logical)    ? Y y N n T t F f (? when not initialized).
M (Memo)       All OEM code page characters (stored internally as 10
               digits representing a .DBT block number).

Binary, Memo, and OLE Fields And .DBT Files

Binary, memo, and OLE fields store data in .DBT files consisting of blocks numbered sequentially (0, 1, 2, and so on). SET BLOCKSIZE determines the size of each block. The first block in the .DBT file, block 0, is the .DBT file header.

Each binary, memo, or OLE field of each record in the .DBF file contains the number of the block (in OEM code page values) where the field's data actually begins. If a field contains no data, the .DBF file contains blanks (20h) rather than a number.

When data is changed in a field, the block numbers may also change and the number in the .DBF may be changed to reflect the new location.

Unlike dBASE III PLUS, if you delete text in a memo field (or binary and OLE fields), dBASE for Windows (unlike dBASE IV) may reuse the space from the deleted text when you input new text. dBASE III PLUS always appends new text to the end of the .DBT file. In dBASE III PLUS, the .DBT file size grows whenever new text is added, even if other text in the file is deleted.

This information is from the dBASE for Windows Language Reference manual, Appendix C.

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