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diff --git a/html/xbc2.htm b/html/xbc2.htm deleted file mode 100755 index ac4800e..0000000 --- a/html/xbc2.htm +++ /dev/null @@ -1,276 +0,0 @@ -<!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> - |