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<html>
<head>
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<title>Timer.2 - Using a timer asynchronously</title>
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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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<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>
</h3></div></div></div>
<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">#include &lt;iostream&gt;
#include &lt;asio.hpp&gt;
</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">print</code> to be called when the asynchronous
wait finishes.
</p>
<pre class="programlisting">void print(const asio::error_code&amp; /*e*/)
{
std::cout &lt;&lt; "Hello, world!" &lt;&lt; std::endl;
}
int main()
{
asio::io_context io;
asio::steady_timer t(io, asio::chrono::seconds(5));
</pre>
<p>
Next, instead of doing a blocking wait as in tutorial Timer.1, we call the
<a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
function to perform an asynchronous wait. When calling this function we pass
the <code class="computeroutput">print</code> callback handler that was defined above.
</p>
<pre class="programlisting"> t.async_wait(&amp;print);
</pre>
<p>
Finally, we must call the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
member function on the io_context 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_context/run.html" title="io_context::run">io_context::run()</a>.
Therefore unless the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::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_context/run.html" title="io_context::run">io_context::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_context some work to do before
calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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>,
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
returned immediately.
</p>
<pre class="programlisting"> io.run();
return 0;
}
</pre>
<p>
See the <a class="link" href="tuttimer2/src.html" title="Source listing for Timer.2">full source listing</a>
</p>
<p>
Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
</p>
<p>
Previous: <a class="link" href="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a timer
synchronously</a>
</p>
<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>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2020 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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