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<title>Stackful Coroutines</title>
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td></tr></table>
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<div class="spirit-nav">
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<a accesskey="p" href="coroutine.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../composition.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="futures.html"><img src="../../../next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="asio.overview.composition.spawn"></a><a class="link" href="spawn.html" title="Stackful Coroutines">Stackful Coroutines</a>
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</h4></div></div></div>
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<p>
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The <a class="link" href="../../reference/spawn.html" title="spawn"><code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code></a> function is a high-level wrapper
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for running stackful coroutines. It is based on the Boost.Coroutine library.
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The <code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code>
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function enables programs to implement asynchronous logic in a synchronous
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manner, as shown in the following example:
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</p>
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<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">spawn</span><span class="special">(</span><span class="identifier">my_strand</span><span class="special">,</span> <span class="identifier">do_echo</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">detached</span><span class="special">);</span>
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<span class="comment">// ...</span>
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<span class="keyword">void</span> <span class="identifier">do_echo</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">yield_context</span> <span class="identifier">yield</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">try</span>
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<span class="special">{</span>
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<span class="keyword">char</span> <span class="identifier">data</span><span class="special">[</span><span class="number">128</span><span class="special">];</span>
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<span class="keyword">for</span> <span class="special">(;;)</span>
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<span class="special">{</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">length</span> <span class="special">=</span>
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<span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">);</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_write</span><span class="special">(</span><span class="identifier">my_socket</span><span class="special">,</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">length</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="special">}</span>
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<span class="keyword">catch</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="comment">// ...</span>
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<span class="special">}</span>
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<span class="special">}</span>
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</pre>
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<p>
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The first argument to <code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code> may be an executor or execution context.
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This argument determines the context in which the coroutine is permitted
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to execute. For example, a server's per-client object may consist of multiple
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coroutines; they should all run on the same <code class="computeroutput"><span class="identifier">strand</span></code>
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so that no explicit synchronisation is required.
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</p>
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<p>
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The second argument is a function object with signature:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">coroutine</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">yield_context</span> <span class="identifier">yield</span><span class="special">);</span>
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</pre>
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<p>
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that specifies the code to be run as part of the coroutine. The parameter
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<code class="computeroutput"><span class="identifier">yield</span></code> may be passed to
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an asynchronous operation in place of the completion handler, as in:
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</p>
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<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">length</span> <span class="special">=</span>
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<span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">);</span>
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</pre>
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<p>
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This starts the asynchronous operation and suspends the coroutine. The
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coroutine will be resumed automatically when the asynchronous operation
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completes.
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</p>
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<p>
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Where an asynchronous operation's handler signature has the form:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">result_type</span> <span class="identifier">result</span><span class="special">);</span>
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</pre>
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<p>
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the initiating function returns the result_type. In the <code class="computeroutput"><span class="identifier">async_read_some</span></code>
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example above, this is <code class="computeroutput"><span class="identifier">size_t</span></code>.
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If the asynchronous operation fails, the <code class="computeroutput"><span class="identifier">error_code</span></code>
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is converted into a <code class="computeroutput"><span class="identifier">system_error</span></code>
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exception and thrown.
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</p>
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<p>
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Where a handler signature has the form:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">);</span>
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</pre>
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<p>
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the initiating function returns <code class="computeroutput"><span class="keyword">void</span></code>.
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As above, an error is passed back to the coroutine as a <code class="computeroutput"><span class="identifier">system_error</span></code>
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exception.
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</p>
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<p>
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To collect the <code class="computeroutput"><span class="identifier">error_code</span></code>
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from an operation, rather than have it throw an exception, associate the
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output variable with the <code class="computeroutput"><span class="identifier">yield_context</span></code>
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as follows:
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</p>
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<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">length</span> <span class="special">=</span>
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<span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">[</span><span class="identifier">ec</span><span class="special">]);</span>
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</pre>
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<p>
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<span class="bold"><strong>Note:</strong></span> if <code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code> is used with a specified executor of
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type <code class="computeroutput"><span class="identifier">Executor</span></code>, the function
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object signature is actually:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">coroutine</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">basic_yield_context</span><span class="special"><</span><span class="identifier">Executor</span><span class="special">></span> <span class="identifier">yield</span><span class="special">);</span>
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</pre>
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<h6>
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<a name="asio.overview.composition.spawn.h0"></a>
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<span><a name="asio.overview.composition.spawn.see_also"></a></span><a class="link" href="spawn.html#asio.overview.composition.spawn.see_also">See
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Also</a>
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</h6>
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<p>
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<a class="link" href="../../reference/spawn.html" title="spawn">spawn</a>, <a class="link" href="../../reference/yield_context.html" title="yield_context">yield_context</a>,
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<a class="link" href="../../reference/basic_yield_context.html" title="basic_yield_context">basic_yield_context</a>,
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<a class="link" href="../../examples/cpp03_examples.html#asio.examples.cpp03_examples.spawn">Spawn example (C++03)</a>,
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<a class="link" href="../../examples/cpp11_examples.html#asio.examples.cpp11_examples.spawn">Spawn example (C++11)</a>,
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<a class="link" href="coroutine.html" title="Stackless Coroutines">Stackless Coroutines</a>.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher 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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</p>
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</div>
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<hr>
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