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    10 <!DOCTYPE concept

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    12 <concept id="GUID-AAA2ABA4-5555-5E4B-AEFF-3DF61462B170" xml:lang="en"><title>Introduction

    13 to asynchronous programming</title><shortdesc>This document introduces the concept of asynchronous programming.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>When a program requests a service, this service can be performed either <i>synchronously</i> or <i>asynchronously</i>. </p>

    15 <p>A synchronous service is the normal pattern for function calls where the

    16 function returns and the service requested has either performed successfully

    17 or has failed and an error code returned.</p>

    18 <p>An asynchronous service is <i>requested</i> by a function call but completion

    19 occurs later. Completion is indicated by a <i>signal</i>. Between the issue

    20 of the request and the signal, the request is said to be <i>pending</i>. The

    21 requesting program can do other processing while the request is pending or

    22 it can issue a <i>wait</i>. The operating system wakes the program up when

    23 completion of any of its pending requests is signalled.</p>

    24 <p>A program may contain a number of independently-executing units, called

    25 threads. All programs have a primary thread, and may create other threads,

    26 possibly for providing or using asynchronous services.</p>

    27 <p>If all threads are in a wait state, for example, waiting for user input,

    28 Symbian platform powers down as much as possible of the machine’s electronics.

    29 This results in battery savings and, consequently, a considerable extension

    30 of useful battery life.</p>

    31 </conbody></concept>