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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.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_service</span> <span class="identifier">i</span><span class="special">;</span>
<span class="special">...</span>
<span class="identifier">deadline_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_from_now</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>
<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_service</span> <span class="identifier">i</span><span class="special">;</span>
<span class="special">...</span>
<span class="identifier">deadline_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_from_now</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">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 a relative time:
      </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">posix_time</span><span class="special">::</span><span class="identifier">time_duration</span> <span class="identifier">time_until_expiry</span>
  <span class="special">=</span> <span class="identifier">t</span><span class="special">.</span><span class="identifier">expires_from_now</span><span class="special">();</span>
</pre>
<p>
        or as an absolute time to allow composition of timers:
      </p>
<pre class="programlisting"><span class="identifier">deadline_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">expires_at</span><span class="special">()</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">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_deadline_timer.html" title="basic_deadline_timer">basic_deadline_timer</a>,
        <a class="link" href="../reference/deadline_timer.html" title="deadline_timer">deadline_timer</a>, <a class="link" href="../reference/deadline_timer_service.html" title="deadline_timer_service">deadline_timer_service</a>,
        <a class="link" href="../tutorial/tuttimer1.html" title="Timer.1 - Using a timer synchronously">timer tutorials</a>.
      </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2014 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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