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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="boost_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 class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| 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 class="listitem"> |
| 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 class="listitem"> |
| 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="boost_asio.overview.core.strands.see_also"></a><h6> |
| <a name="id762589"></a> |
| <a class="link" href="strands.html#boost_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#boost_asio.examples.http_server_3">HTTP server 3 example</a>. |
| </p> |
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| <td align="left"></td> |
| <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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