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authorJörg Frings-Fürst <debian@jff-webhosting.net>2022-12-07 13:17:14 +0100
committerJörg Frings-Fürst <debian@jff-webhosting.net>2022-12-07 13:17:14 +0100
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-<!DOCTYPE HTML PUBLIC>
-<HTML>
-<TITLE>Xbase DBMS Chapter 2</TITLE>
-<BODY BGCOLOR=#FFFFFF>
-<H2><p align="center">Database Overview</p></H2>
-<p align="center">Chapter Updated 3/18/04</p><hr>
-
-The objective of this chapter is to provide information regarding how
-the database files are utilized and document the various record structures.
-With the exception of the brief section on the record buffer, the
-information presented in this chapter is not required to use the
-Xbase library. It is mainly information describing internal file
-structures utilized by the Xbase routines.<br><br>
-
-Xbase files are comprised of a variable length header record which stores
-information about the file and describes
-the fixed length record format, followed by a series of fixed length
-data records.
-<br><br>
-
-Each fixed length data record is preceded by a one byte indicator
-which identifiies if the record has been deleted. If the record is
-not deleted, the indicator is a space (0x20). If deleted, the
-indicator contains an asterisk (0x2A). Data fields are stored in records
-without field separators or record terminators.<br><br>
-
-In earlier releases of dBASE, there is an ASCII NULL character
-between the $0D end of header indicator and the start of the data.
-This NULL was removed starting with dBASE III Plus, making a Plus
-header one byte shorter than an identically structured III file.
-The methods documented in the Xbase software and documentation follow
-the more recent version where the NULL character is not included.
-<br><br>
-
-Each database file is comprised of zero, one or many records. A record is
-comprised of fields. Only one record is accessed at a time.<br><br>
-
-Zero, one or many database files can be open simultaneously.<br><br>
-
-<hr>
-
-<h3>The Record Buffer</h3>
-
-When using the Xbase routines, each open data file has a record buffer
-which is manipulated by calling the database, index and field routines.
-
-Updates are committed from the record buffer to the database when a write,
-or append is performed. The library will automatically write updates
-to the database if the buffer has been updated and the record is
-repositioned or the database is closed.<br><br>
-
-The record buffer is not used for handling the actual data portion of
-memo fields. When working with memo fields, the application program must
-allocate enough buffer space for reading and writing memo fields.<br><br>
-
-Internal to the library, there is an additional record buffer which
-stores the original value of the data record before any changes are made.
-This is used by the index routines for finding and deleting original key
-values from any open indices before adding the new keys. If the key values
-are not changed, no index updates occur.
-
-
-<br><br>
-
-<hr>
-<br>
-<h3>Xbase Database File Header</h3>
-
-The Xbase file header, located at the beginning of the database, describes
-the .DBF database. Knowledge of this structure is not necessary to
-effectively utilize the Xbase libraries.<br><br><br>
-
-<TABLE BORDER>
-<TR valign="BASELINE">
-<TR><TH ALIGN="LEFT">Position<TD>Length<TD>Description
-<TR><TH ALIGN="LEFT">0<TD>1 byte<TD>file version number<br>
- (03H without a .DBT file)<br>
- (83H with a .DBT file)
-<TR><TH ALIGN="LEFT">1-3<TD>3 bytes<TD>date of last update<br>
- (YY MM DD) in binary format
-<TR><TH ALIGN="LEFT">4-7<TD>32 bit number<TD>number of records in data file
-<TR><TH ALIGN="LEFT">8-9<TD>16 bit number<TD>length of header structure
-<TR><TH ALIGN="LEFT">10-11<TD>16 bit number<TD>length of the record
-<TR><TH ALIGN="LEFT">12-31<TD>20 bytes<TD>reserved
-<TR><TH ALIGN="LEFT">32-n<TD>32 bytes each<TD>field descriptor record (see below)
-<TR><TH ALIGN="LEFT">n+1<TD>1 byte<TD>0DH as the field terminator
-</TABLE>
-<BR><BR>
-
-<hr>
-<br>
-<h3>Xbase Field Descriptor Record</h3>
-The Xbase field descriptor record stores information about each field in the
-database. Each database has from 1 to 1024 fields.
-Knowledge of this structure is not necessary to
-effectively utilize the Xbase libraries.<br><br><br>
-
-<TABLE BORDER>
-<TR VALIGN="BASELIGN">
-<TR><TH ALIGN="LEFT">Position<TD>Length<TD>Description
-<TR><TH ALIGN="LEFT">0-10<TD>11 bytes<TD>field name in ASCII zero-filled
-<TR><TH ALIGN="LEFT">11<TD>1 byte<TD>field type in ASCII (C N L D or M)
-<TR><TH ALIGN="LEFT">12-15<TD>32 bit number<TD>field data address
-<TR><TH ALIGN="LEFT">16<TD>1 byte<TD>field length in binary
-<TR><TH ALIGN="LEFT">17<TD>1 byte<TD>field decimal count in binary
-<TR><TH ALIGN="LEFT">18-31<TD>14 bytes<TD>reserved bytes (version 1.00)
-</TABLE>
-<BR><BR>
-<hr>
-<br>
-<h3>Field Data Format</h3>
-Data are stored in ASCII format in the database as follows:<br><br>
-<TABLE BORDER>
-<TR VALIGN="BASELIGN">
-<TR><TH ALIGN="LEFT">DATA TYPE<TD>DATA RECORD STORAGE
-<TR><TH ALIGN="LEFT">Character<TD>ASCII characters, left justified, right blank filled
-<TR><TH ALIGN="LEFT">Date<TD>(8 digits in YYYYMMDD format, such as<BR>
- 19601007 for October 7, 1960)
-<TR><TH ALIGN="LEFT">Logical<TD>? Y y N n T t F f (? when not initialized)
-<TR><TH ALIGN="LEFT">Memo<TD>10 digits representing a .DBT block number
-<TR><TH ALIGN="LEFT">Numeric<TD>. 0 1 2 3 4 5 6 7 8 9 + -, right justified, left blank filled
-<TR><TH ALIGN="LEFT">Float<TD>. 0 1 2 3 4 5 6 7 8 9 + -, right justified, left blank filled
-</TABLE>
-<BR><BR>
-
-<hr>
-<h3>Memo Fields</h3>
-
-Memo fields store variable length data elements in a seperate .DBT file.
-The main .DBF file maintains a ten byte field which is used by the Xbase
-routines for determining the location of the data in the .DBT file.
-<br><br>
-
-Xbase DBMS supports both dBASE III+ and dBASE IV version memo files.
-The version IV files are somewhat more efficient in that they reuse
-unused memo space when data are deleted or freed from use. With version
-III files, all new updates are appended to the end of the file and the
-unused space is not reclaimed until the datafiles are packed.
-<br><br>
-
-Memo fields can be used for storing a variety of date type. However,
-type 3 files are limited to storing textual data because most internal
-memo field processing in a type 3 file relies on two contiguous 0x1a
-charaters. <br><br>
-
-Type 4 memo fields can be used for storing BLOB (binary large object)
-data reliably, as the internal file structure does not rely on any
-special characters embedded in the data.<br><br>
-
-A special note on storing string data in a memo field. For those users
-that are new to C/C++ programming, string fields typically end with
-a null (0x00) terminator character. As a general rule of thumb when using
-the library, add one to the length of any string when
-specifying the length of the data. This stores the null terminating byte
-with the data. For example, when storing string "This is a string"
-specified size should be 17, not 16.
-
-
-<h4>Technical memo file information</h4>
-
-The following info on memo fields is for the curious.
-It is not required
-reading if you don't need to know the internals.<br><br>
-
-<li>Memo files are made up of one or more blocks
-<li>For version III files, the block size is 512
-<li>For version IV files, the block size is a multiple of 512
-<li>The minimum amout of space necessary to store one memo field is
-one block or 512 bytes.
-<li>The default block size can be adjusted by manipulating the
-XB_DBT_BLOCK_SIZE macro in the options.h file.
-
-
-<li>The main .DBF file maintains a ten byte numeric field which is blank if
-no memo data exists for a given field. Otherwise it contains a number, which
-when multiplied by the block size, points to the offset in the file of the head
-block in the file/
-<br><br>
-
-For version 3 memo field files, there are two fields in the head block of
-the file, <em>NextBlockNo</em> and <em>Version</em>. Depending on the
-Xbase software, some vendors products update these two fields, some do not.
-The Xbase library keeps the fields updated, but does not rely on them to
-be valued with correct data. This helps to support maximum compatibility
-amoungst all Xbase tools available.<br><br>
-
-For version 4 memo field files,
-the first block in the .DBT file is a header block which is comprised of
-8 bytes of data which maintain the file's block size and the next free
-block available in the file. Blocks two through n contain the actual
-memo data. A chain of empty blocks is maintained within the file for
-potential future use. When an add or update routine executes, it first
-attempts to find a spot in a set of blocks which were earlier allocated,
-but not currently in use for the data. If no free spot is found, data are
-appended to the end of the file.
-
-The free block chain is sorted in block number order. When blocks of
-data are freed and added to the free block chain, the routines will attempt
-to concatonate free block chains togethor where possible. When a delete
-occurs, or an update which requires less space occurs, the new free space
-is added to the free block chain.
-
-<br><br>
-
-<h3>Various Memo File Block Types</h3>
-
-<TABLE BORDER>
-<TR VALIGN="BASELIGN">
-<TR><TH ALIGN="LEFT">Valid Block Types
-<TR><TH ALIGN="LEFT">Head Block
-<TR><TH ALIGN="LEFT">Only data block for memo field
-<TR><TH ALIGN="LEFT">First of several contiguous data block set
-<TR><TH ALIGN="LEFT">2-n of contiguous data block set
-<TR><TH ALIGN="LEFT">Only data block in free chain (version IV only)
-<TR><TH ALIGN="LEFT">First of several contiguous free block set (version IV only)
-<TR><TH ALIGN="LEFT">2-n of contiguous free block set (type 4 only)
-</TABLE>
-<BR><BR>
-
-<h3>Head Block Structure</h3>
-<TABLE BORDER>
-<TR VALIGN="BASELIGN">
-<TR><TH ALIGN="LEFT">1-4<TD>LONG<TD>Next Block ID
-<TR><TH ALIGN="LEFT">5-8<TD>LONG<TD>Not used all 0x00's
-<TR><TH ALIGN="LEFT">9-16<TD>CHAR(8)<TD>Filename (Version IV Only)
-<TR><TH ALIGN="LEFT">17<TD>CHAR<TD>Version (0x03 = Version III, 0x00 = Version IV)
-<TR><TH ALIGN="LEFT">18-20<TD>CHAR(3)<TD>Not used all 0x00's
-<TR><TH ALIGN="LEFT">21-22<TD>SHORT<TD>Block Size (Version IV only )
-<TR><TH ALIGN="LEFT">23-Remainder of block<TD>CHAR<TD>Not used
-</TABLE>
-<BR><BR>
-
-
-<h3>Version IV Head Data Block Structure</h3>
-<TABLE BORDER>
-<TR VALIGN="BASELIGN">
-<TR><TH ALIGN="LEFT">xbShort<TD>0-1<TD>-1
-<TR><TH ALIGN="LEFT">xbShort<TD>2-3<TD>Starting position of data (always 8 ?)
-<TR><TH ALIGN="LEFT">xbLong<TD>4-7<TD>Length of data includes first 8 bytes
-<TR><TH ALIGN="LEFT">char (9) - Blocksize<TD>8-15<TD>Data
-</TABLE>
-<BR><BR>
-
-<h3>Version IV Head Free Block Structure</h3>
-<TABLE BORDER>
-<TR VALIGN="BASELIGN">
-<TR><TH ALIGN="LEFT">xbLong<TD>0-3<TD>Next free block in the free block chain
-<TR><TH ALIGN="LEFT">xbLong<TD>4-7<TD>Number of free blocks in this contiguous free
- block set
-</table>
-<br><br>
-Version 3 and 4 memo fields are terminated with two contiguous 0x1A bytes of data.
-<br><br>
-<hr>
-<h3>64 Bit File Support</h3>
-Beginning with Release 3.0 of the library, Xbase was renamed to Xbase64 and
-64 bit file support was incorporated into the library.
-<br><br>
-This functionality currently works on Linux/Unix platforms which support
-64 bit file structures. 64 bit support was not tested with Windows
-because the author didn't have access to a compiler which creates 64
-bit Windows apps. If you need this functionality for Windows, consider
-donating a Windows 64 bit compiler to the project and I'll get 64 bit
-support working for Windows.
-<br><br>
-It is not advisable to use Xbase 64 bit file support in combination with
-32 bit Xbase applications such as older versions of DBase, Clipper and Foxpro.
-Xbase64 can extend the files well beyond any limits in the older 32 bit
-application development environments.
-<br><br>
-If Xbase is being built with glibc, verify it is at least version 2.2.3.
-<br><br><hr>
-<p><img src="xbase.jpg"><hr>
-</BODY>
-</HTML>
-