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<title>Timer.5 - Synchronising completion handlers in multithreaded programs</title>
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<a name="asio.tutorial.tuttimer5"></a><a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising completion handlers in multithreaded programs">Timer.5 - Synchronising completion
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 completion 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 completion 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 completion 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"><span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">bind</span><span class="special">/</span><span class="identifier">bind</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
We start by defining a class called <code class="computeroutput"><span class="identifier">printer</span></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"><span class="keyword">class</span> <span class="identifier">printer</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
</pre>
<p>
In addition to initialising a pair of asio::steady_timer members, the constructor
initialises the <code class="computeroutput"><span class="identifier">strand_</span></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"> <span class="identifier">printer</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span class="special">&amp;</span> <span class="identifier">io</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">strand_</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">make_strand</span><span class="special">(</span><span class="identifier">io</span><span class="special">)),</span>
<span class="identifier">timer1_</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">asio</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">1</span><span class="special">)),</span>
<span class="identifier">timer2_</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">asio</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">1</span><span class="special">)),</span>
<span class="identifier">count_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
<span class="special">{</span>
</pre>
<p>
When initiating the asynchronous operations, each completion 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"> <span class="identifier">timer1_</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">bind_executor</span><span class="special">(</span><span class="identifier">strand_</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print1</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)));</span>
<span class="identifier">timer2_</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">bind_executor</span><span class="special">(</span><span class="identifier">strand_</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print2</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)));</span>
<span class="special">}</span>
<span class="special">~</span><span class="identifier">printer</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Final count is "</span> <span class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</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"><span class="identifier">print1</span></code>
and <code class="computeroutput"><span class="identifier">print2</span></code>) are <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>
and the <code class="computeroutput"><span class="identifier">count_</span></code> data member.
</p>
<pre class="programlisting"> <span class="keyword">void</span> <span class="identifier">print1</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">count_</span> <span class="special">&lt;</span> <span class="number">10</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Timer 1: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>
<span class="identifier">timer1_</span><span class="special">.</span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">timer1_</span><span class="special">.</span><span class="identifier">expiry</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">asio</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">1</span><span class="special">));</span>
<span class="identifier">timer1_</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">bind_executor</span><span class="special">(</span><span class="identifier">strand_</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print1</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)));</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">print2</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">count_</span> <span class="special">&lt;</span> <span class="number">10</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Timer 2: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>
<span class="identifier">timer2_</span><span class="special">.</span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">timer2_</span><span class="special">.</span><span class="identifier">expiry</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">asio</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">1</span><span class="special">));</span>
<span class="identifier">timer2_</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">bind_executor</span><span class="special">(</span><span class="identifier">strand_</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print2</span><span class="special">,</span> <span class="keyword">this</span><span class="special">)));</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">strand</span><span class="special">&lt;</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">executor_type</span><span class="special">&gt;</span> <span class="identifier">strand_</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span class="identifier">timer1_</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span class="identifier">timer2_</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">count_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
The <code class="computeroutput"><span class="identifier">main</span></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"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io</span><span class="special">;</span>
<span class="identifier">printer</span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">io</span><span class="special">);</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">thread</span> <span class="identifier">t</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">io</span><span class="special">));</span>
<span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
<span class="identifier">t</span><span class="special">.</span><span class="identifier">join</span><span class="special">();</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
See the <a class="link" href="tuttimer5/src.html" title="Source listing for Timer.5">full source listing</a>
</p>
<p>
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</p>
<p>
Previous: <a class="link" href="tuttimer4.html" title="Timer.4 - Using a member function as a completion handler">Timer.4 - Using a member
function as a completion handler</a>
</p>
</div>
<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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