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<div class="titlepage"><div><div><h3 class="title">
<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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<p>
This tutorial program demonstrates how to use asio's asynchronous callback
functionality by modifying the program from tutorial Timer.1 to perform an
asynchronous wait on the timer.
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">date_time</span><span class="special">/</span><span class="identifier">posix_time</span><span class="special">/</span><span class="identifier">posix_time</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
Using asio's asynchronous functionality means having a callback function
that will be called when an asynchronous operation completes. In this program
we define a function called <code class="computeroutput"><span class="identifier">print</span></code>
to be called when the asynchronous wait finishes.
</p>
<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>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Hello, world!\n"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span> <span class="identifier">io</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">deadline_timer</span> <span class="identifier">t</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">posix_time</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">5</span><span class="special">));</span>
</pre>
<p>
Next, instead of doing a blocking wait as in tutorial Timer.1, we call the
<a class="link" href="../reference/basic_deadline_timer/async_wait.html" title="basic_deadline_timer::async_wait">deadline_timer::async_wait()</a>
function to perform an asynchronous wait. When calling this function we pass
the <code class="computeroutput"><span class="identifier">print</span></code> callback handler
that was defined above.
</p>
<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>
</pre>
<p>
Finally, we must call the <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>
member function on the io_service object.
</p>
<p>
The asio library provides a guarantee that callback handlers will only be
called from threads that are currently calling <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>.
Therefore unless the <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>
function is called the callback for the asynchronous wait completion will
never be invoked.
</p>
<p>
The <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>
function will also continue to run while there is still "work"
to do. In this example, the work is the asynchronous wait on the timer, so
the call will not return until the timer has expired and the callback has
completed.
</p>
<p>
It is important to remember to give the io_service some work to do before
calling <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>.
For example, if we had omitted the above call to <a class="link" href="../reference/basic_deadline_timer/async_wait.html" title="basic_deadline_timer::async_wait">deadline_timer::async_wait()</a>,
the io_service would not have had any work to do, and consequently <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a> would have
returned immediately.
</p>
<pre class="programlisting"> <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
See the <a class="link" href="tuttimer2/src.html" title="Source listing for Timer.2">full source listing</a>
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Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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<p>
Previous: <a class="link" href="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a timer
synchronously</a>
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<p>
Next: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding arguments
to a handler</a>
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003 - 2010 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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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