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<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>