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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.overview.core.strands"></a><a class="link" href="strands.html" title="Strands: Use Threads Without Explicit Locking">Strands: Use Threads Without
Explicit Locking</a>
</h4></div></div></div>
<p>
A strand is defined as a strictly sequential invocation of event handlers
(i.e. no concurrent invocation). Use of strands allows execution of code
in a multithreaded program without the need for explicit locking (e.g.
using mutexes).
</p>
<p>
Strands may be either implicit or explicit, as illustrated by the following
alternative approaches:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
Calling io_service::run() from only one thread means all event handlers
execute in an implicit strand, due to the io_service's guarantee that
handlers are only invoked from inside run().
</li>
<li>
Where there is a single chain of asynchronous operations associated
with a connection (e.g. in a half duplex protocol implementation like
HTTP) there is no possibility of concurrent execution of the handlers.
This is an implicit strand.
</li>
<li>
An explicit strand is an instance of <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">strand</span></code>.
All event handler function objects need to be wrapped using <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">strand</span><span class="special">::</span><span class="identifier">wrap</span><span class="special">()</span></code>
or otherwise posted/dispatched through the <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">strand</span></code>
object.
</li>
</ul></div>
<p>
In the case of composed asynchronous operations, such as <code class="computeroutput"><span class="identifier">async_read</span><span class="special">()</span></code>
or <code class="computeroutput"><span class="identifier">async_read_until</span><span class="special">()</span></code>,
if a completion handler goes through a strand, then all intermediate handlers
should also go through the same strand. This is needed to ensure thread
safe access for any objects that are shared between the caller and the
composed operation (in the case of <code class="computeroutput"><span class="identifier">async_read</span><span class="special">()</span></code> it's the socket, which the caller can
close() to cancel the operation). This is done by having hook functions
for all intermediate handlers which forward the calls to the customisable
hook associated with the final handler:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_handler</span>
<span class="special">{</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">asio_handler_invoke</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">my_handler</span><span class="special">*)</span>
<span class="special">{</span>
<span class="comment">// Do custom invocation here.
</span> <span class="comment">// Default implementation calls f();
</span><span class="special">}</span>
</pre>
<p>
The <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">strand</span><span class="special">::</span><span class="identifier">wrap</span><span class="special">()</span></code>
function creates a new completion handler that defines <code class="computeroutput"><span class="identifier">asio_handler_invoke</span></code>
so that the function object is executed through the strand.
</p>
<a name="asio.overview.core.strands.see_also"></a><h6>
<a name="id621161"></a>
<a class="link" href="strands.html#asio.overview.core.strands.see_also">See Also</a>
</h6>
<p>
<a class="link" href="../../reference/io_service__strand.html" title="io_service::strand">io_service::strand</a>,
<a class="link" href="../../tutorial/tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">tutorial Timer.5</a>, <a class="link" href="../../examples.html#asio.examples.http_server_3">HTTP server 3 example</a>.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003 - 2010 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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