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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tuttimer5"></a><a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5 - Synchronising handlers
      in multithreaded programs</a>
</h3></div></div></div>
<p>
        This tutorial demonstrates the use of the <a class="link" href="../reference/strand.html" title="strand">strand</a>
        class template to synchronise callback handlers in a multithreaded program.
      </p>
<p>
        The previous four tutorials avoided the issue of handler synchronisation
        by calling the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        function from one thread only. As you already know, 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>.
        Consequently, calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        from only one thread ensures that callback handlers cannot run concurrently.
      </p>
<p>
        The single threaded approach is usually the best place to start when developing
        applications using asio. The downside is the limitations it places on programs,
        particularly servers, including:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Poor responsiveness when handlers can take a long time to complete.
          </li>
<li class="listitem">
            An inability to scale on multiprocessor systems.
          </li>
</ul></div>
<p>
        If you find yourself running into these limitations, an alternative approach
        is to have a pool of threads calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
        However, as this allows handlers to execute concurrently, we need a method
        of synchronisation when handlers might be accessing a shared, thread-unsafe
        resource.
      </p>
<pre class="programlisting">#include &lt;iostream&gt;
#include &lt;asio.hpp&gt;
#include &lt;boost/bind/bind.hpp&gt;
</pre>
<p>
        We start by defining a class called <code class="computeroutput">printer</code>, similar to the
        class in the previous tutorial. This class will extend the previous tutorial
        by running two timers in parallel.
      </p>
<pre class="programlisting">class printer
{
public:
</pre>
<p>
        In addition to initialising a pair of asio::steady_timer members, the constructor
        initialises the <code class="computeroutput">strand_</code> member, an object of type asio::strand&lt;asio::io_context::executor_type&gt;.
      </p>
<p>
        The <a class="link" href="../reference/strand.html" title="strand">strand</a> class template is
        an executor adapter that guarantees that, for those handlers that are dispatched
        through it, an executing handler will be allowed to complete before the next
        one is started. This is guaranteed irrespective of the number of threads
        that are calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
        Of course, the handlers may still execute concurrently with other handlers
        that were not dispatched through an <a class="link" href="../reference/strand.html" title="strand">strand</a>,
        or were dispatched through a different <a class="link" href="../reference/strand.html" title="strand">strand</a>
        object.
      </p>
<pre class="programlisting">  printer(asio::io_context&amp; io)
    : strand_(asio::make_strand(io)),
      timer1_(io, asio::chrono::seconds(1)),
      timer2_(io, asio::chrono::seconds(1)),
      count_(0)
  {
</pre>
<p>
        When initiating the asynchronous operations, each callback handler is "bound"
        to an asio::strand&lt;asio::io_context::executor_type&gt; object. The asio::bind_executor()
        function returns a new handler that automatically dispatches its contained
        handler through the <a class="link" href="../reference/strand.html" title="strand">strand</a> object.
        By binding the handlers to the same <a class="link" href="../reference/strand.html" title="strand">strand</a>,
        we are ensuring that they cannot execute concurrently.
      </p>
<pre class="programlisting">    timer1_.async_wait(asio::bind_executor(strand_,
          boost::bind(&amp;printer::print1, this)));

    timer2_.async_wait(asio::bind_executor(strand_,
          boost::bind(&amp;printer::print2, this)));
  }

  ~printer()
  {
    std::cout &lt;&lt; "Final count is " &lt;&lt; count_ &lt;&lt; std::endl;
  }
</pre>
<p>
        In a multithreaded program, the handlers for asynchronous operations should
        be synchronised if they access shared resources. In this tutorial, the shared
        resources used by the handlers (<code class="computeroutput">print1</code> and <code class="computeroutput">print2</code>)
        are <code class="computeroutput">std::cout</code> and the <code class="computeroutput">count_</code> data member.
      </p>
<pre class="programlisting">  void print1()
  {
    if (count_ &lt; 10)
    {
      std::cout &lt;&lt; "Timer 1: " &lt;&lt; count_ &lt;&lt; std::endl;
      ++count_;

      timer1_.expires_at(timer1_.expiry() + asio::chrono::seconds(1));

      timer1_.async_wait(asio::bind_executor(strand_,
            boost::bind(&amp;printer::print1, this)));
    }
  }

  void print2()
  {
    if (count_ &lt; 10)
    {
      std::cout &lt;&lt; "Timer 2: " &lt;&lt; count_ &lt;&lt; std::endl;
      ++count_;

      timer2_.expires_at(timer2_.expiry() + asio::chrono::seconds(1));

      timer2_.async_wait(asio::bind_executor(strand_,
            boost::bind(&amp;printer::print2, this)));
    }
  }

private:
  asio::strand&lt;asio::io_context::executor_type&gt; strand_;
  asio::steady_timer timer1_;
  asio::steady_timer timer2_;
  int count_;
};
</pre>
<p>
        The <code class="computeroutput">main</code> function now causes <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        to be called from two threads: the main thread and one additional thread.
        This is accomplished using an <a class="link" href="../reference/thread.html" title="thread">thread</a>
        object.
      </p>
<p>
        Just as it would with a call from a single thread, concurrent calls to <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a> will continue
        to execute while there is "work" left to do. The background thread
        will not exit until all asynchronous operations have completed.
      </p>
<pre class="programlisting">int main()
{
  asio::io_context io;
  printer p(io);
  asio::thread t(boost::bind(&amp;asio::io_context::run, &amp;io));
  io.run();
  t.join();

  return 0;
}
</pre>
<p>
        See the <a class="link" href="tuttimer5/src.html" title="Source listing for Timer.5">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="tuttimer4.html" title="Timer.4 - Using a member function as a handler">Timer.4 - Using a member
        function as a handler</a>
      </p>
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      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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