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
----- -------- --------------------------------------------------------
0 1 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 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
12-14 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 Meaning
------- ---------- -------------------------------------------------
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.
The field descriptor array:
Byte Contents Meaning
------- ------------ --------------------------------------------------
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.