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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.overview.timers"></a><a class="link" href="timers.html" title="Timers">Timers</a>
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<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">io_context i;
...
deadline_timer t(i);
t.expires_from_now(boost::posix_time::seconds(5));
t.wait();
</pre>
<p>
        More commonly, a program will perform an asynchronous wait operation on a
        timer:
      </p>
<pre class="programlisting">void handler(boost::system::error_code ec) { ... }
...
io_context i;
...
deadline_timer t(i);
t.expires_from_now(boost::posix_time::milliseconds(400));
t.async_wait(handler);
...
i.run();
</pre>
<p>
        The deadline associated with a timer may also be obtained as a relative time:
      </p>
<pre class="programlisting">boost::posix_time::time_duration time_until_expiry
  = t.expires_from_now();
</pre>
<p>
        or as an absolute time to allow composition of timers:
      </p>
<pre class="programlisting">deadline_timer t2(i);
t2.expires_at(t.expires_at() + boost::posix_time::seconds(30));
</pre>
<h5>
<a name="boost_asio.overview.timers.h0"></a>
        <span class="phrase"><a name="boost_asio.overview.timers.see_also"></a></span><a class="link" href="timers.html#boost_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="../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; 2003-2018 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
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