Description: 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.
This information is from the dBASE for Windows Language Reference manual, Appendix C.
Your DBF description is iffy to say the least. I am enclosing a more accurate file layout. Note it include dBaseIII+, dBaseIV, FoxBase, FoxPro upto and including 2.6.
DBF FILE STRUCTURE ~~~~~~~~~~~~~~~~~~
BYTES DESCRIPTION 00 FoxBase+, FoxPro, dBaseIII+, dBaseIV, no memo - 0x03 FoxBase+, dBaseIII+ with memo - 0x83 FoxPro with memo - 0xF5 dBaseIV with memo - 0x8B dBaseIV with SQL Table - 0x8E
01-03 Last update, format YYYYMMDD **correction: it is YYMMDD** 04-07 Number of records in file (32-bit number) 08-09 Number of bytes in header (16-bit number) 10-11 Number of bytes in record (16-bit number) 12-13 Reserved, fill with 0x00 14 dBaseIV flag, incomplete transaction Begin Transaction sets it to 0x01 End Transaction or RollBack reset it to 0x00
15 Encryption flag, encrypted 0x01 else 0x00 Changing the flag does not encrypt or decrypt the records
16-27 dBaseIV multi-user environment use 28 Production index exists - 0x01 else 0x00 29 dBaseIV language driver ID 30-31 Reserved fill with 0x00 32-n Field Descriptor array n+1 Header Record Terminator - 0x0D
FIELD DESCRIPTOR ARRAY TABLE BYTES DESCRIPTION 0-10 Field Name ASCII padded with 0x00 11 Field Type Identifier (see table) 12-15 Displacement of field in record 16 Field length in bytes 17 Field decimal places 18-19 Reserved 20 dBaseIV work area ID 21-30 Reserved 31 Field is part of production index - 0x01 else 0x00
FIELD IDENTIFIER TABLE ASCII DESCRIPTION C Character D Date, format YYYYMMDD F Floating Point G General - FoxPro addition L Logical, T:t,F:f,Y:y,N:n,?-not initialized M Memo (stored as 10 digits representing the dbt block number) N Numeric P Picture - FoxPro addition
Note all dbf field records begin with a deleted flag field. If record is deleted - 0x2A (asterisk) else 0x20 (space) End of file is marked with 0x1A
