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| author | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2014-08-02 08:43:31 +0200 |
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| committer | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2014-08-02 08:43:31 +0200 |
| commit | daf17154bf13139d9375f48525d19d6aaba08155 (patch) | |
| tree | e3c08b6c49dc8a8e83f03327591310546675b43d /html/xbc8.htm | |
Imported Upstream version 3.1.2upstream/3.1.2
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| -rwxr-xr-x | html/xbc8.htm | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/html/xbc8.htm b/html/xbc8.htm new file mode 100755 index 0000000..efab841 --- /dev/null +++ b/html/xbc8.htm @@ -0,0 +1,130 @@ +<!DOCTYPE HTML PUBLIC> +<HTML> +<TITLE>Xbase DBMS Chapter 8</TITLE> +<BODY BGCOLOR=#FFFFFF> +<H1><p align="center">Record and File Locking</p></H1> +<p align="center">Chapter Updated 2/1/99</p><hr> + +<h3>Locking Overview</h3> + +Xbase DBMS supports multi-user processing through file and record locks. +Record locking restricts multiple cooperating programs from simultaneously +accessing the same data and corrupting it. Without record and file locking +in a multi-user environment, simultaneous access to the data and index files +can cause the files to become inaccurate and unusable.<br><br> + +Record locking is on by default in the Xbase DBMS library. To disable it, +comment out the XB_LOCKING_ON option in the <em>options.h</em> file in the +xbase/src directory.<br><br> + +The current Xbase DBMS record locking does not co-exist with other Xbase +products and there is not yet support for locking in a DOS/Windows environment. +The locking functions do work correctly for a Xbase DBMS only configuration. +Future version of Xbase DBMS will have enhanced locking features for +co-existing with other Xbase products and also include DOS/Windows support. +<br><br> + +The locking methods return either XB_LOCK_FAILED or XB_NO_ERROR. If they return +XB_LOCK_FAILED the actual reason can be found in the global variable +<em>errno</em> or function <em>perror()</em> can be executed to view the +results. +<br><br> + +The errno field may contain one of the following values if the lock was not +successful.<br><br> +<TABLE BORDER> +<TR VALIGN="BASELINE"> +<TR><TH ALIGN="LEFT">Error Code<TD>Description +<TR><TH ALIGN="LEFT">EBADF<TD>Invalid file descriptor +<TR><TH ALIGN="LEFT">EINVAL<TD>Invalid lock information or file does not support locks +<TR><TH ALIGN="LEFT">EACCESS<BR>EAGAIN<TD>Lock can not be set because it is blocked by an existing lock on the file. +<TR><TH ALIGN="LEFT">ENOLCK<TD>The system is out of lock resources, too many file locks in place. +<TR><TH ALIGN="LEFT">EDEADLK<TD>Deadlock condition +<TR><TH ALIGN="LEFT">EINTR<TD>Process was interrupted by a signal while it was waiting +</TABLE> +<br><br> +<h3>Types of Locks</h3> + +<li><em>Write or Exclusive Locks</em> provide exclusive access to a +particular file location. No other process can lock the same location.<br><br> + +<li><em>Read or Shared Locks</em> prohibit any process from requesting a write +lock on a specified part of the file. Other processes can request +simultaneous read locks.<br><br><br> + +<h3>DBF File Locking Techniques</h3> + +Xbase DBMS uses the following protocol for DBF file and record locking: +<br><br> + +To lock a record - the first byte of the record is locked.<br> +To lock the file - the header bytes of the file are locked.<br><br> + +When a record is being appended to the file, the header bytes are locked.<br> +When a record is being updated, the header bytes and the specific record are +locked.<br><br> +This locking protocol is probably not compatable with other Xbase type products. +However, Xbase can be safely used for multi-user access when it is not +simultaneously updating DBF or NDX files while other products/programs are. +<br><br><br> + +<h3>NDX File Locking Techniques</h3> + +Xbase DBMS locks indexes by locking the first 512 bytes +of the index file. +The entire index is locked because any updates to the index potentially +can modify significant portions of the index tree. +<br><br><br> + +<h3>DBT File Locking Techniques</h3> + +Xbase DBMS locks memo files by locking the first 4 bytes +of the memo file. This effectively locks the entire file. The entire file +is locked because any updates to the free block chain can significantly +change the structure of the file. +<br><br><br> + + +<h3>AutoLocking Features</h3> + +If XB_LOCKING_ON is set in the <em>options.h</em> file, the locking methods +execute any appropriate locking logic. If XB_LOCKING_ON is not set in the +<em>options.h</em> file, all locking methods return XB_NO_ERROR without +performing any actual record or file locking. This enables the application +program to always call locking routines regardless of the XB_LOCKING_ON switch +in the <em>options.h</em> file. +<br><br> +By leaving the autolocking features enabled, the application program does +not need to address record, file or index locking. All locking is handled +automatically by the Xbase routines. However, if access to the locking +routines is required, they are available to the applciation programmer. +<br><br> +When the files are automatically locked by the Xbase routines, the database +file is locked first, then it locks the indexes in alphabetical order. To +avoid deadlock conditions, files and record locks should always be done in +the same order. When the files are unlocked, then indexes are unlocked +first, then the database is unlocked. +<br><br> +Auto-locking works well in an on-line transaction based environment. +However, it does not function efficiently in batch mode. If you +will be writing programs which process files in a batch mode, disabling +auto-lock and locking the entire file at the beginning of the process +and unlocking the file at the end of the process will significantly +reduce process time. On a 586-200 class machine, a file with 45000 records +can be read thru in a few seconds with the file locked in batch mode. +In record-lock mode it takes about six minutes with the same processor. + +<br><br>For processing large files, locking the file instead of locking each +record is far more efficient. This is how you do it.<br><br> + +For reading the file in batch mode:<br> +xbDbf.AutoLockOff();<br> +xbDbf.LockDatabase( F_SETLKW, F_RDLCK, 0L );<br><br> +For updating the file in batch mode:<br> +xbDbf.AutoLockOff();<br> +xbDbf.LockDatabase( F_SETLKW, F_WRLCK, 0L );<br><br> +<br> +<hr><br> +<p><img src="xbase.jpg"><br><hr> +</BODY> +</HTML> |