114 lines
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114 lines
8.3 KiB
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Timer.2 - Using a timer asynchronously</title>
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<link rel="stylesheet" href="../../boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
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<link rel="home" href="../../index.html" title="Asio">
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<link rel="up" href="../tutorial.html" title="Tutorial">
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<link rel="prev" href="tuttimer1/src.html" title="Source listing for Timer.1">
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<link rel="next" href="tuttimer2/src.html" title="Source listing for Timer.2">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td></tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="tuttimer1/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="tuttimer2/src.html"><img src="../../next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="asio.tutorial.tuttimer2"></a><a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a timer asynchronously</a>
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</h3></div></div></div>
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<p>
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This tutorial program demonstrates how to use asio's asynchronous functionality
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by modifying the program from tutorial Timer.1 to perform an asynchronous
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wait on the timer.
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</p>
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<pre class="programlisting"><span class="special">#</span><span class="identifier">include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="special">#</span><span class="identifier">include</span> <span class="special"><</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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</pre>
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<p>
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Using asio's asynchronous functionality means supplying a <a class="link" href="../overview/model/completion_tokens.html" title="Completion Tokens">completion
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token</a>, which determines how the result will be delivered to a <span class="bold"><strong>completion handler</strong></span> when an <a class="link" href="../overview/model/async_ops.html" title="Asynchronous Operations">asynchronous
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operation</a> completes. In this program we define a function called
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<code class="computeroutput"><span class="identifier">print</span></code> to be called when the
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asynchronous wait finishes.
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="comment">/*e*/</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Hello, world!"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io</span><span class="special">;</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span class="identifier">t</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">5</span><span class="special">));</span>
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</pre>
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<p>
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Next, instead of doing a blocking wait as in tutorial Timer.1, we call the
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<a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
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function to perform an asynchronous wait. When calling this function we pass
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the <code class="computeroutput"><span class="identifier">print</span></code> function that was
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defined above.
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</p>
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<pre class="programlisting"> <span class="identifier">t</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(&</span><span class="identifier">print</span><span class="special">);</span>
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</pre>
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<p>
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Finally, we must call the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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member function on the io_context object.
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</p>
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<p>
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The asio library provides a guarantee that completion handlers will only
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be called from threads that are currently calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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Therefore unless the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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function is called the completion handler for the asynchronous wait completion
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will never be invoked.
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</p>
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<p>
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The <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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function will also continue to run while there is still "work"
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to do. In this example, the work is the asynchronous wait on the timer, so
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the call will not return until the timer has expired and the completion handler
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has returned.
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</p>
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<p>
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It is important to remember to give the io_context some work to do before
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calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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For example, if we had omitted the above call to <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>,
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the io_context would not have had any work to do, and consequently <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a> would have
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returned immediately.
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</p>
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<pre class="programlisting"> <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
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<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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See the <a class="link" href="tuttimer2/src.html" title="Source listing for Timer.2">full source listing</a>
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</p>
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<p>
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Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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</p>
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<p>
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Previous: <a class="link" href="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a timer
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synchronously</a>
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</p>
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<p>
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Next: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a completion handler">Timer.3 - Binding arguments
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to a completion handler</a>
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="tuttimer1/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="tuttimer2/src.html"><img src="../../next.png" alt="Next"></a>
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</div>
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</body>
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</html>
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