<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
<!-- This component and the accompanying materials are made available under the terms of the License 
"Eclipse Public License v1.0" which accompanies this distribution, 
and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
<!-- Initial Contributors:
    Nokia Corporation - initial contribution.
Contributors: 
-->
<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-FA120B3F-4EC4-5A0A-8A36-D5CD032CF1B0" xml:lang="en"><title>Using
Mutexes</title><shortdesc>This document describes how to use mutexes.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Mutexes provide serialised access to shared resources. They are Kernel
objects and, as such, are managed by the Kernel.</p>
<p>A mutex can be used by threads across any number of processes. If a resource
is only shared between the threads within the same process, it can be more
efficient to use a critical section.</p>
<p>Access to a mutex is through an RMutex handle.</p>
<p>Mutexes are similar to semaphores in that they have a <codeph>TInt</codeph> count
value that is incremented by calling the <codeph>Signal()</codeph> member
function of the mutex handle and decremented by calling the <codeph>Wait()</codeph> member
function of the mutex handle. A mutex with a negative value implies that a
thread must wait for access to the shared resource.</p>
<p>Unlike a semaphore, the count value of a mutex is always set to one when
it is constructed.</p>
<p>The creator of a shared resource uses the <codeph>CreateLocal()</codeph> or <codeph>CreateGlobal()</codeph> member
functions of <codeph>RMutex</codeph> to create a mutex and to open the handle
to it. Any thread wishing to access the resource first calls <codeph>Wait()</codeph>;
that thread then calls <codeph>Signal()</codeph> after completing its access.</p>
<p>The first thread to call <codeph>Wait()</codeph> returns immediately, and
is free to continue execution, but any other threads that call <codeph>Wait()</codeph> will
wait in a queue maintained by the mutex. Waiting threads are released on a
first-in first-out basis when the thread currently accessing the resource
calls <codeph>Signal()</codeph>.</p>
<p>The nature of the shared resources should be such that any access completes
in a relatively short time so that threads do not wait for extensive periods
on the mutex.</p>
<p>In many cases, it may be better to use servers to serialize access to a
shared resource (for example, the window server) rather than use a mutex. </p>
</conbody></concept>