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<!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 11/29/22</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().
Locking can also be enabled / disabled at the table level with with xbDbf::SetAutoLock().
<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>
<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>
<h4>oplocks = no</h4>
<br>
<hr><br>
<p><img src="xbase.jpg"><br><hr>
</BODY>
</HTML>
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