123 lines
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123 lines
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<title>Timer.2 - Using a timer asynchronously</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_asio.tutorial.tuttimer2"></a><a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a timer
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asynchronously</a>
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</h3></div></div></div>
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<p>
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This tutorial program demonstrates how to use asio's asynchronous callback
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functionality by modifying the program from tutorial Timer.1 to perform an
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asynchronous wait on the timer.
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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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</pre>
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<p>
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Using asio's asynchronous functionality means having a callback function
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that will be called when an asynchronous operation completes. In this program
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we define a function called <code class="computeroutput">print</code> to be called when the asynchronous
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wait finishes.
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</p>
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<pre class="programlisting">void print(const boost::system::error_code& /*e*/)
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{
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std::cout << "Hello, world!" << std::endl;
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}
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int main()
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{
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boost::asio::io_context io;
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boost::asio::steady_timer t(io, boost::asio::chrono::seconds(5));
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</pre>
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<p>
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Next, instead of doing a blocking wait as in tutorial Timer.1, we call the
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<a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
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function to perform an asynchronous wait. When calling this function we pass
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the <code class="computeroutput">print</code> callback handler that was defined above.
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</p>
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<pre class="programlisting"> t.async_wait(&print);
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</pre>
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<p>
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Finally, we must call the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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member function on the io_context object.
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</p>
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<p>
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The asio library provides a guarantee that callback handlers will only be
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called from threads that are currently calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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Therefore unless the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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function is called the callback for the asynchronous wait completion will
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never be invoked.
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</p>
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<p>
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The <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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function will also continue to run while there is still "work"
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to do. In this example, the work is the asynchronous wait on the timer, so
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the call will not return until the timer has expired and the callback has
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completed.
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</p>
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<p>
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It is important to remember to give the io_context some work to do before
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calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
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For example, if we had omitted the above call to <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>,
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the io_context would not have had any work to do, and consequently <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a> would
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have returned immediately.
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</p>
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<pre class="programlisting"> io.run();
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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="tuttimer2/src.html" title="Source listing for Timer.2">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="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a
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timer synchronously</a>
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</p>
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<p>
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Next: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding arguments
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to a handler</a>
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</p>
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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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