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-<!DOCTYPE HTML PUBLIC>
-<HTML>
-<TITLE>Xbase DBMS Chapter 10</TITLE>
-<BODY BGCOLOR=#FFFFFF>
-<H1><p align="center">Xbase DBMS Record and File Locking</p></H1>
-<p align="center">Chapter Updated 4/8/98</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 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 LOCK_FAILED or NO_ERROR.  If they return
-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 LOCKING_ON is set in the <em>options.h</em> file, the locking methods 
-execute any appropriate locking logic.  If LOCKING_ON is not set in the 
-<em>options.h</em> file, all locking methods return NO_ERROR without 
-performing any actual record or file locking.  This enables the application
-program to always call locking routines regardless of the 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>
-DBF.AutoLockOff();<br>
-DBF.LockDatabase( F_SETLKW, F_RDLCK, 0L );<br><br>      
-For updating the file in batch mode:<br>
-DBF.AutoLockOff();<br>
-DBF.LockDatabase( F_SETLKW, F_WRLCK, 0L );<br><br>      
-<br> 
-<hr><br>
-
-<h3>Method Table</h3>
-
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"><h3><Xbase Locking Method List</h3></CAPTION>
-<TR VALIGN="BASELINE">
-<TR><TH ALIGN="LEFT">Method<TD>Description
-<TR><TH ALIGN="LEFT">DBF::AutoLockOn<TD>Turns autolocking on
-<TR><TH ALIGN="LEFT">DBF::AutoLockOff<TD>Turns autolocking off
-<TR><TH ALIGN="LEFT">DBF::ExclusiveLock<TD>Lock file and indexes in exclusive mode
-<TR><TH ALIGN="LEFT">DBF::ExclusiveUnlock<TD>Unlock files and indexes 
-<TR><TH ALIGN="LEFT">DBF::LockDatabase<TD>Locks or unlocks a DBF database
-<TR><TH ALIGN="LEFT">NDX::LockIndex<TD>Locks or unlocks an NDX index
-<TR><TH ALIGN="LEFT">NDX::LockMemoFile<TD>Locks or unlocks a DBT memo field file
-</TABLE>
-<BR><HR>
-
-<h4>Method Descriptions</h4>
-
-<h4>Method VOID DBF::AutoLockOn( VOID )</h4><br>
-
-This method turns automatic record locking on.  Auto record locking is on
-by default if LOCKING_ON is set in the options.h file.<br><br>
-
-<h4>Example Program:</h4>
-
-See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of
-how to use this method.
-<hr>
-
-<h4>Method VOID DBF::AutoLockOff( VOID )</h4><br>
-
-This method turns automatic record locking off.  Auto record locking is on
-by default if LOCKING_ON is set in the options.h file.
-<br><br>
-Turning auto locking off will result in slightly better execution speeds
-but should not be used in multi-user environments when multiple users can
-update files simultanteously.  If multiple users are accessing a file which
-is read only then it is safe to turn off auto-locking for a particular file.
-<br><br>
-Turning autolocking off will disable any index file locking which is 
-particularly dangerous in a multi-user environment if updates on the files
-are permitted. 
-
-
-<h4>Example Program:</h4>
-
-See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of
-how to use this method.
-
-<hr>
-<h4>Method SHORT DBF::ExclusiveLock( SHORT WaitOption )</h4>
-<h4>Method SHORT DBF::ExclusiveUnlock( VOID )</h4><br>
-
-ExclusiveLock and ExclusiveUnclock will lock the data file, memo file (if applicable) 
-and any associated indexes in an exclusive mode. They also turn auto-lock 
-on and off as appropriate.<br><br>
-
-WaitOption is either:<br><br>
-<li>F_SETLK - returns immediately regardless if success or failure<br>
-<li>F_SETLKW - waits until lock function executes<br><br>
-
-<h4>Example Program:</h4>
-
-See program <A HREF="/XbaseSamples/sample4.cpp">sample4.cpp</A> for an example of
-how to use this method.
-
-<hr>
-<h3>Method SHORT DBF::LockDatabase( SHORT WaitOption, SHORT LockType, LONG LRecNo )
-</h3><br>
-
-This method locks or unlocks an Xbase (.DBF) file which was previously opened.<br>
-<br>
-WaitOption is either:<br><br>
-<li>F_SETLK - returns immediately regardless if success or failure<br>
-<li>F_SETLKW - waits until lock function executes<br><br>
-
-LockType is one of:<br><br>
-<li>F_RDLCK - Perform a Read or Shared Lock<br>
-<li>F_WRLCK - Perform a Write or Exclusive Lock<br>
-<li>F_UNLCK - Unlock it<br><br>
-
-LRecNo is:<br><br>
-0 - Lock the header section of the file (use this to lock the file)<br>
-1 through n - Lock a particular record<br><br>
-
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"<h4>Method Return Codes</h4></CAPTION>
-<TR><TH ALIGN="LEFT">Return Code<TD>Description
-<TR><TH ALIGN="LEFT">INVALID_RECORD<TD>An invalid record given
-<TR><TH ALIGN="LEFT">LOCK_FAILED<TD>The lock action failed, see errno
-<TR><TH ALIGN="LEFT">NO_ERROR<TD>The lock was successful
-</TABLE>
-
-
-<h4>Example Program:</h4>
-
-See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of
-how to use this method.
-
-<hr>
-
-<h3>Method SHORT DBF::LockIndex( SHORT WaitOption, SHORT LockType )
-</h3><br>
-
-This method locks or unlocks an Index (.NDX) file which was previously opened.<br>
-<br>
-WaitOption is either:<br><br>
-<li>F_SETLK - returns immediately regardless if success or failure<br>
-<li>F_SETLKW - waits until lock function executes<br><br>
-
-LockType is one of:<br><br>
-<li>F_RDLCK - Perform a Read or Shared Lock<br>
-<li>F_WRLCK - Perform a Write or Exclusive Lock<br>
-<li>F_UNLCK - Unlock it<br><br>
-
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"<h4>Method Return Codes</h4></CAPTION>
-<TR><TH ALIGN="LEFT">Return Code<TD>Description
-<TR><TH ALIGN="LEFT">LOCK_FAILED<TD>The lock action failed, see errno
-<TR><TH ALIGN="LEFT">NO_ERROR<TD>The lock was successful
-</TABLE>
-
-<h4>Example Program:</h4>
-See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of
-how to use this method.
-<hr>
-
-<h3>Method SHORT DBF::LockMemoFile( SHORT WaitOption, SHORT LockType )
-</h3><br>
-
-This method locks or unlocks a memo (.DBT) file which was previously opened.
-It is not necessary for an application to call this method as locking is
-handled automatically by other routines.<br><br>
-
-WaitOption is either:<br><br>
-<li>F_SETLK - returns immediately regardless if success or failure<br>
-<li>F_SETLKW - waits until lock function executes<br><br>
-
-LockType is one of:<br><br>
-<li>F_RDLCK - Perform a Read or Shared Lock<br>
-<li>F_WRLCK - Perform a Write or Exclusive Lock<br>
-<li>F_UNLCK - Unlock it<br><br>
-
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"<h4>Method Return Codes</h4></CAPTION>
-<TR><TH ALIGN="LEFT">Return Code<TD>Description
-<TR><TH ALIGN="LEFT">LOCK_FAILED<TD>The lock action failed, see errno
-<TR><TH ALIGN="LEFT">NO_ERROR<TD>The lock was successful
-</TABLE>
-<hr>
-<p><img src="xbase.jpg"><br><hr>
-</BODY>
-</HTML>