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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.overview.timers"></a><a class="link" href="timers.html" title="Timers">Timers</a>
</h3></div></div></div>
<p>
Long running I/O operations will often have a deadline by which they must
have completed. These deadlines may be expressed as absolute times, but are
often calculated relative to the current time.
</p>
<p>
As a simple example, to perform a synchronous wait operation on a timer using
a relative time one may write:
</p>
<pre class="programlisting"><span class="identifier">io_context</span> <span class="identifier">i</span><span class="special">;</span>
<span class="special">...</span>
<span class="identifier">steady_timer</span> <span class="identifier">t</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">t</span><span class="special">.</span><span class="identifier">expires_after</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>
<span class="identifier">t</span><span class="special">.</span><span class="identifier">wait</span><span class="special">();</span>
</pre>
<p>
More commonly, a program will perform an asynchronous wait operation on a
timer:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span>
<span class="special">...</span>
<span class="identifier">io_context</span> <span class="identifier">i</span><span class="special">;</span>
<span class="special">...</span>
<span class="identifier">steady_timer</span> <span class="identifier">t</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">t</span><span class="special">.</span><span class="identifier">expires_after</span><span class="special">(</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">milliseconds</span><span class="special">(</span><span class="number">400</span><span class="special">));</span>
<span class="identifier">t</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">handler</span><span class="special">);</span>
<span class="special">...</span>
<span class="identifier">i</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
</pre>
<p>
The deadline associated with a timer may also be obtained as an absolute
time:
</p>
<pre class="programlisting"><span class="identifier">steady_timer</span><span class="special">::</span><span class="identifier">time_point</span> <span class="identifier">time_of_expiry</span> <span class="special">=</span> <span class="identifier">t</span><span class="special">.</span><span class="identifier">expiry</span><span class="special">();</span>
</pre>
<p>
which allows composition of timers:
</p>
<pre class="programlisting"><span class="identifier">steady_timer</span> <span class="identifier">t2</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">t2</span><span class="special">.</span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">t</span><span class="special">.</span><span class="identifier">expiry</span><span class="special">()</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">30</span><span class="special">));</span>
</pre>
<h5>
<a name="asio.overview.timers.h0"></a>
<span><a name="asio.overview.timers.see_also"></a></span><a class="link" href="timers.html#asio.overview.timers.see_also">See
Also</a>
</h5>
<p>
<a class="link" href="../reference/basic_waitable_timer.html" title="basic_waitable_timer">basic_waitable_timer</a>,
<a class="link" href="../reference/steady_timer.html" title="steady_timer">steady_timer</a>, <a class="link" href="../reference/system_timer.html" title="system_timer">system_timer</a>,
<a class="link" href="../reference/high_resolution_timer.html" title="high_resolution_timer">high_resolution_timer</a>,
<a class="link" href="../tutorial/tuttimer1.html" title="Timer.1 - Using a timer synchronously">timer tutorials</a>.
</p>
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<div class="copyright-footer">Copyright © 2003-2023 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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