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+<!DOCTYPE HTML PUBLIC>
+<HTML>
+<TITLE>Xbase DBMS Chapter 12</TITLE>
+<BODY BGCOLOR=#FFFFFF>
+<H1><p align="center">Record and File Locking</p></H1>
+<p align="center">Chapter Updated 12/13/22</p><hr>
+
+<h3>Locking Overview</h3>
+
+Xbase64 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>
+
+Automatic record locking is on by default in the Xbase64 library. To disable it,
+use method xbXBase::DisableDefaultAutoLock() and to enable it, use method xbXBase::EnableDefaultAutoLock().
+<br><br>
+Locking can also be enabled / disabled at the table level with with xbDbf::SetAutoLock().<br><br>
+If autolocking is disabled and the code base is being used in a multi user environment, it is 
+up to the application program to verify the needed locks are set as there is no checking or
+setting any locks if autolocking is turned off.  It is only safe to turn off the autolocking functionality
+if the library is being used in a single user environment.
+
+<br><br>
+The current Xbase64 record locking logic is modeled after DBase (tm) V7 locking.
+<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>Linux/Windows File Locking Compatibility Issue</h3>
+
+There is a compatibility locking issue to be aware of.   Windows environments allow for the exclusive 
+opening of file handles and Linux/Unix platforms do not.  If you are writing an application that will be
+using a tool like Dbase on a Windows machine, accessing a file on a Linux/Samba configure machine,
+be aware that the file could be opened in exclusive mode by DBase on the Windows system, and the same file could 
+be simultaneously opened with a program on the Unix box.  That could cause some issues.
+
+<br><br>
+In Unix, a program can not lock a file so another process can not access it.<br>
+In Windows, a program can lock a file so another process can not access it.<br>
+DBase(tm) supports routines to open files exclusively, preventing other users from opening a file.<br>
+Locking on the Mac/Apple platform only works on NFS shares.  It does not work with SMB shares.
+
+<br><h3>Samba settings</h3>
+
+If you will be using Samba on Linux/Unix and sharing files between Linux and Windows machines, 
+you will need to disable oplocks. In the smb.conf file, set:<br>
+<br>
+[sharename]<br>
+oplocks = False<br>
+level2 oplocks = False
+
+
+<br><h3>iLockFlavor</h3>
+
+The library was constructed in a manner so that it could be updated to support alternate lock "flavors".
+The 4.x.x library is built to mirror the DBase locking, but the structure is in place to expand to other locking 
+types if needed.
+
+
+
+
+<br><br><br>
+<hr><br>
+<p><img src="xbase.jpg"><br><hr>
+</BODY>
+</HTML>