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<title>Timer.5 - Synchronising handlers in multithreaded programs</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_asio.tutorial.tuttimer5"></a><a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5 - Synchronising
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handlers in multithreaded programs</a>
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</h3></div></div></div>
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<p>
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This tutorial demonstrates the use of the <a class="link" href="../reference/strand.html" title="strand">strand</a>
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class template to synchronise callback handlers in a multithreaded program.
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</p>
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<p>
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The previous four tutorials avoided the issue of handler synchronisation
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by calling the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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function from one thread only. As you already know, the asio library provides
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a guarantee that callback handlers will only be called from threads that
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are currently calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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Consequently, calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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from only one thread ensures that callback handlers cannot run concurrently.
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</p>
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<p>
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The single threaded approach is usually the best place to start when developing
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applications using asio. The downside is the limitations it places on programs,
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particularly servers, including:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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Poor responsiveness when handlers can take a long time to complete.
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</li>
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<li class="listitem">
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An inability to scale on multiprocessor systems.
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</li>
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</ul></div>
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<p>
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If you find yourself running into these limitations, an alternative approach
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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>.
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However, as this allows handlers to execute concurrently, we need a method
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of synchronisation when handlers might be accessing a shared, thread-unsafe
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resource.
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</p>
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<pre class="programlisting">#include <iostream>
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#include <boost/asio.hpp>
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#include <boost/thread/thread.hpp>
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#include <boost/bind/bind.hpp>
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</pre>
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<p>
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We start by defining a class called <code class="computeroutput">printer</code>, similar to the
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class in the previous tutorial. This class will extend the previous tutorial
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by running two timers in parallel.
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</p>
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<pre class="programlisting">class printer
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{
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public:
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</pre>
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<p>
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In addition to initialising a pair of boost::asio::steady_timer members,
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the constructor initialises the <code class="computeroutput">strand_</code> member, an object of
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type boost::asio::strand<boost::asio::io_context::executor_type>.
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</p>
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<p>
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The <a class="link" href="../reference/strand.html" title="strand">strand</a> class template
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is an executor adapter that guarantees that, for those handlers that are
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dispatched through it, an executing handler will be allowed to complete before
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the next one is started. This is guaranteed irrespective of the number of
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threads that are calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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Of course, the handlers may still execute concurrently with other handlers
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that were not dispatched through an <a class="link" href="../reference/strand.html" title="strand">strand</a>,
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or were dispatched through a different <a class="link" href="../reference/strand.html" title="strand">strand</a>
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object.
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</p>
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<pre class="programlisting"> printer(boost::asio::io_context& io)
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: strand_(boost::asio::make_strand(io)),
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timer1_(io, boost::asio::chrono::seconds(1)),
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timer2_(io, boost::asio::chrono::seconds(1)),
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count_(0)
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{
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</pre>
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<p>
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When initiating the asynchronous operations, each callback handler is "bound"
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to an boost::asio::strand<boost::asio::io_context::executor_type> object.
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The boost::asio::bind_executor() function returns a new handler that automatically
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dispatches its contained handler through the <a class="link" href="../reference/strand.html" title="strand">strand</a>
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object. By binding the handlers to the same <a class="link" href="../reference/strand.html" title="strand">strand</a>,
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we are ensuring that they cannot execute concurrently.
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</p>
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<pre class="programlisting"> timer1_.async_wait(boost::asio::bind_executor(strand_,
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boost::bind(&printer::print1, this)));
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timer2_.async_wait(boost::asio::bind_executor(strand_,
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boost::bind(&printer::print2, this)));
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}
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~printer()
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{
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std::cout << "Final count is " << count_ << std::endl;
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}
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</pre>
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<p>
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In a multithreaded program, the handlers for asynchronous operations should
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be synchronised if they access shared resources. In this tutorial, the shared
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resources used by the handlers (<code class="computeroutput">print1</code> and <code class="computeroutput">print2</code>)
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are <code class="computeroutput">std::cout</code> and the <code class="computeroutput">count_</code> data member.
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</p>
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<pre class="programlisting"> void print1()
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{
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if (count_ < 10)
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{
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std::cout << "Timer 1: " << count_ << std::endl;
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++count_;
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timer1_.expires_at(timer1_.expiry() + boost::asio::chrono::seconds(1));
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timer1_.async_wait(boost::asio::bind_executor(strand_,
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boost::bind(&printer::print1, this)));
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}
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}
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void print2()
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{
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if (count_ < 10)
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{
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std::cout << "Timer 2: " << count_ << std::endl;
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++count_;
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timer2_.expires_at(timer2_.expiry() + boost::asio::chrono::seconds(1));
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timer2_.async_wait(boost::asio::bind_executor(strand_,
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boost::bind(&printer::print2, this)));
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}
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}
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private:
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boost::asio::strand<boost::asio::io_context::executor_type> strand_;
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boost::asio::steady_timer timer1_;
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boost::asio::steady_timer timer2_;
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int count_;
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};
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</pre>
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<p>
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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>
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to be called from two threads: the main thread and one additional thread.
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This is accomplished using an boost::thread object.
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</p>
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<p>
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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
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continue to execute while there is "work" left to do. The background
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thread will not exit until all asynchronous operations have completed.
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</p>
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<pre class="programlisting">int main()
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{
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boost::asio::io_context io;
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printer p(io);
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boost::thread t(boost::bind(&boost::asio::io_context::run, &io));
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io.run();
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t.join();
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return 0;
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}
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</pre>
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<p>
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See the <a class="link" href="tuttimer5/src.html" title="Source listing for Timer.5">full source listing</a>
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</p>
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<p>
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Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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</p>
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<p>
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Previous: <a class="link" href="tuttimer4.html" title="Timer.4 - Using a member function as a handler">Timer.4 - Using a
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member function as a handler</a>
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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