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+++ b/Symbian3/SDK/Source/GUID-FE14D954-4712-5DCE-998E-A39D79500ABF.dita	Thu Jan 21 18:18:20 2010 +0000
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+<?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-FE14D954-4712-5DCE-998E-A39D79500ABF" xml:lang="en"><title>Asynchronous
+service providers</title><shortdesc>This document explains the use of asynchronous service providers.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>In a multiprocessing system, user programs wait for events, for example,
+keyboard input, pointer input, completion of an I/O request, timer events
+etc. These events, and the services associated with them, are provided by <i>asynchronous
+service providers</i>.</p>
+<p>A program has a main loop that:</p>
+<ul>
+<li id="GUID-9F0D109A-9562-52C5-9AA7-B6C99FD4E0DA"><p>calls appropriate service
+providers</p> </li>
+<li id="GUID-5A0790A7-F25C-59C7-868D-AE7DFCF323AB"><p>waits for events</p> </li>
+<li id="GUID-17B548C5-5355-55FB-9F3F-2D229EF9F0BE"><p>acts on completed events,
+resulting, most likely, in further calls to service providers.</p> </li>
+</ul>
+<p>All Symbian platform programs make extensive use of asynchronous services.
+For this reason, Symbian platform provides mechanisms which make them easy
+to use. </p>
+<p>Low-level asynchronous service handling encompasses:</p>
+<ul>
+<li id="GUID-AC93B268-406A-5558-BCA1-AE1EB2448D51"><p>the asynchronous service
+provider, which provides asynchronous services using request functions and
+cancel functions.</p> </li>
+<li id="GUID-CC29D95F-0EE4-5F4C-918A-17AEF2EC6783"><p>the asynchronous <i>request
+status</i> which indicates the completion status of a request.</p> </li>
+<li id="GUID-ED692928-B973-59B2-A494-ABD8392CE603"><p>the <i>thread request
+semaphore</i>, the means by which a service provider signals to a requester
+that it has completed a request.</p> </li>
+</ul>
+<p>High-level asynchronous service handling encompasses</p>
+<ul>
+<li id="GUID-C8ECE905-78DC-5F32-9386-D2948C71CD79"><p>the <keyword>active
+scheduler</keyword>, which encapsulates the wait loop which is at the heart
+of all programs using multiple asynchronous services.</p> </li>
+<li id="GUID-552B8268-9CDA-5D8C-BA61-B6715F83E777"><p><keyword>active objects</keyword>,
+which encapsulate an asynchronous service provider, its request and cancel
+functions, and the function which handles completion of requests.</p> </li>
+</ul>
+</conbody></concept>
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