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<concept id="GUID-AAA2ABA4-5555-5E4B-AEFF-3DF61462B170" xml:lang="en"><title>Introduction
to asynchronous programming</title><shortdesc>This document introduces the concept of asynchronous programming.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>When a program requests a service, this service can be performed either <i>synchronously</i> or <i>asynchronously</i>. </p>
<p>A synchronous service is the normal pattern for function calls where the
function returns and the service requested has either performed successfully
or has failed and an error code returned.</p>
<p>An asynchronous service is <i>requested</i> by a function call but completion
occurs later. Completion is indicated by a <i>signal</i>. Between the issue
of the request and the signal, the request is said to be <i>pending</i>. The
requesting program can do other processing while the request is pending or
it can issue a <i>wait</i>. The operating system wakes the program up when
completion of any of its pending requests is signalled.</p>
<p>A program may contain a number of independently-executing units, called
threads. All programs have a primary thread, and may create other threads,
possibly for providing or using asynchronous services.</p>
<p>If all threads are in a wait state, for example, waiting for user input,
Symbian platform powers down as much as possible of the machine’s electronics.
This results in battery savings and, consequently, a considerable extension
of useful battery life.</p>
</conbody></concept>