<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Chapter 5. Boost.Atomic</title> <link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> <link rel="prev" href="array/ack.html" title="Acknowledgements"> <link rel="next" href="atomic/thread_coordination.html" title="Thread coordination using Boost.Atomic"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td> <td align="center"><a href="../../index.html">Home</a></td> <td align="center"><a href="../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="array/ack.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="atomic/thread_coordination.html"><img src="../../doc/src/images/next.png" alt="Next"></a> </div> <div class="chapter"> <div class="titlepage"><div> <div><h2 class="title"> <a name="atomic"></a>Chapter 5. Boost.Atomic</h2></div> <div><div class="author"><h3 class="author"> <span class="firstname">Helge</span> <span class="surname">Bahmann</span> </h3></div></div> <div><div class="author"><h3 class="author"> <span class="firstname">Andrey</span> <span class="surname">Semashev</span> </h3></div></div> <div><p class="copyright">Copyright © 2011 Helge Bahmann</p></div> <div><p class="copyright">Copyright © 2012 Tim Blechmann</p></div> <div><p class="copyright">Copyright © 2013 Andrey Semashev</p></div> <div><div class="legalnotice"> <a name="atomic.legal"></a><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>) </p> </div></div> </div></div> <div class="toc"> <p><b>Table of Contents</b></p> <dl class="toc"> <dt><span class="section"><a href="atomic.html#atomic.introduction">Introduction</a></span></dt> <dd><dl> <dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_presenting">Presenting Boost.Atomic</a></span></dt> <dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_purpose">Purpose</a></span></dt> </dl></dd> <dt><span class="section"><a href="atomic/thread_coordination.html">Thread coordination using Boost.Atomic</a></span></dt> <dd><dl> <dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.mutex">Enforcing <span class="emphasis"><em>happens-before</em></span> through mutual exclusion</a></span></dt> <dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.release_acquire"><span class="emphasis"><em>happens-before</em></span> through <code class="literal">release</code> and <code class="literal">acquire</code></a></span></dt> <dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.fences">Fences</a></span></dt> <dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.release_consume"><span class="emphasis"><em>happens-before</em></span> through <code class="literal">release</code> and <code class="literal">consume</code></a></span></dt> <dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.seq_cst">Sequential consistency</a></span></dt> </dl></dd> <dt><span class="section"><a href="atomic/interface.html">Programming interfaces</a></span></dt> <dd><dl> <dt><span class="section"><a href="atomic/interface.html#atomic.interface.configuration">Configuration and building</a></span></dt> <dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_memory_order">Memory order</a></span></dt> <dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_atomic_object">Atomic objects</a></span></dt> <dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_fences">Fences</a></span></dt> <dt><span class="section"><a href="atomic/interface.html#atomic.interface.feature_macros">Feature testing macros</a></span></dt> </dl></dd> <dt><span class="section"><a href="atomic/usage_examples.html">Usage examples</a></span></dt> <dd><dl> <dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters">Reference counting</a></span></dt> <dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.example_spinlock">Spinlock</a></span></dt> <dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.singleton">Singleton with double-checked locking pattern</a></span></dt> <dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.example_ringbuffer">Wait-free ring buffer</a></span></dt> <dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.mp_queue">Wait-free multi-producer queue</a></span></dt> </dl></dd> <dt><span class="section"><a href="atomic/limitations.html">Limitations</a></span></dt> <dt><span class="section"><a href="atomic/porting.html">Porting</a></span></dt> <dd><dl> <dt><span class="section"><a href="atomic/porting.html#atomic.porting.unit_tests">Unit tests</a></span></dt> <dt><span class="section"><a href="atomic/porting.html#atomic.porting.tested_compilers">Tested compilers</a></span></dt> <dt><span class="section"><a href="atomic/porting.html#atomic.porting.acknowledgements">Acknowledgements</a></span></dt> </dl></dd> </dl> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="atomic.introduction"></a><a class="link" href="atomic.html#atomic.introduction" title="Introduction">Introduction</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_presenting">Presenting Boost.Atomic</a></span></dt> <dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_purpose">Purpose</a></span></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="atomic.introduction.introduction_presenting"></a><a class="link" href="atomic.html#atomic.introduction.introduction_presenting" title="Presenting Boost.Atomic">Presenting Boost.Atomic</a> </h3></div></div></div> <p> <span class="bold"><strong>Boost.Atomic</strong></span> is a library that provides <code class="literal">atomic</code> data types and operations on these data types, as well as memory ordering constraints required for coordinating multiple threads through atomic variables. It implements the interface as defined by the C++11 standard, but makes this feature available for platforms lacking system/compiler support for this particular C++11 feature. </p> <p> Users of this library should already be familiar with concurrency in general, as well as elementary concepts such as "mutual exclusion". </p> <p> The implementation makes use of processor-specific instructions where possible (via inline assembler, platform libraries or compiler intrinsics), and falls back to "emulating" atomic operations through locking. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="atomic.introduction.introduction_purpose"></a><a class="link" href="atomic.html#atomic.introduction.introduction_purpose" title="Purpose">Purpose</a> </h3></div></div></div> <p> Operations on "ordinary" variables are not guaranteed to be atomic. This means that with <code class="literal">int n=0</code> initially, two threads concurrently executing </p> <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">function</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">n</span> <span class="special">++;</span> <span class="special">}</span> </pre> <p> might result in <code class="literal">n==1</code> instead of 2: Each thread will read the old value into a processor register, increment it and write the result back. Both threads may therefore write <code class="literal">1</code>, unaware that the other thread is doing likewise. </p> <p> Declaring <code class="literal">atomic<int> n=0</code> instead, the same operation on this variable will always result in <code class="literal">n==2</code> as each operation on this variable is <span class="emphasis"><em>atomic</em></span>: This means that each operation behaves as if it were strictly sequentialized with respect to the other. </p> <p> Atomic variables are useful for two purposes: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> as a means for coordinating multiple threads via custom coordination protocols </li> <li class="listitem"> as faster alternatives to "locked" access to simple variables </li> </ul></div> <p> Take a look at the <a class="link" href="atomic/usage_examples.html" title="Usage examples">examples</a> section for common patterns. </p> </div> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"><p><small>Last revised: December 14, 2015 at 09:17:07 GMT</small></p></td> <td align="right"><div class="copyright-footer"></div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="array/ack.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="atomic/thread_coordination.html"><img src="../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>