1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Usage examples</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../atomic.html" title="Chapter 6. Boost.Atomic"> 10<link rel="prev" href="interface.html" title="Programming interfaces"> 11<link rel="next" href="limitations.html" title="Limitations"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="interface.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../atomic.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="limitations.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="atomic.usage_examples"></a><a class="link" href="usage_examples.html" title="Usage examples">Usage examples</a> 29</h2></div></div></div> 30<div class="toc"><dl class="toc"> 31<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters">Reference 32 counting</a></span></dt> 33<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock">Spinlock</a></span></dt> 34<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton">Singleton with 35 double-checked locking pattern</a></span></dt> 36<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer">Wait-free 37 ring buffer</a></span></dt> 38<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue">Lock-free multi-producer 39 queue</a></span></dt> 40</dl></div> 41<div class="section"> 42<div class="titlepage"><div><div><h3 class="title"> 43<a name="boost_atomic.usage_examples.example_reference_counters"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters" title="Reference counting">Reference 44 counting</a> 45</h3></div></div></div> 46<div class="toc"><dl class="toc"> 47<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.implementation">Implementation</a></span></dt> 48<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.usage">Usage</a></span></dt> 49<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.discussion">Discussion</a></span></dt> 50</dl></div> 51<p> 52 The purpose of a <span class="emphasis"><em>reference counter</em></span> is to count the number 53 of pointers to an object. The object can be destroyed as soon as the reference 54 counter reaches zero. 55 </p> 56<div class="section"> 57<div class="titlepage"><div><div><h4 class="title"> 58<a name="boost_atomic.usage_examples.example_reference_counters.implementation"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.implementation" title="Implementation">Implementation</a> 59</h4></div></div></div> 60<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 61<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 62 63<span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{</span> 64<span class="keyword">public</span><span class="special">:</span> 65 <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span> <span class="identifier">pointer</span><span class="special">;</span> 66 <span class="identifier">X</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">refcount_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span> 67 68<span class="keyword">private</span><span class="special">:</span> 69 <span class="keyword">mutable</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">refcount_</span><span class="special">;</span> 70 <span class="keyword">friend</span> <span class="keyword">void</span> <span class="identifier">intrusive_ptr_add_ref</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">X</span> <span class="special">*</span> <span class="identifier">x</span><span class="special">)</span> 71 <span class="special">{</span> 72 <span class="identifier">x</span><span class="special">-></span><span class="identifier">refcount_</span><span class="special">.</span><span class="identifier">fetch_add</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_relaxed</span><span class="special">);</span> 73 <span class="special">}</span> 74 <span class="keyword">friend</span> <span class="keyword">void</span> <span class="identifier">intrusive_ptr_release</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">X</span> <span class="special">*</span> <span class="identifier">x</span><span class="special">)</span> 75 <span class="special">{</span> 76 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span><span class="special">-></span><span class="identifier">refcount_</span><span class="special">.</span><span class="identifier">fetch_sub</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_release</span><span class="special">)</span> <span class="special">==</span> <span class="number">1</span><span class="special">)</span> <span class="special">{</span> 77 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic_thread_fence</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_acquire</span><span class="special">);</span> 78 <span class="keyword">delete</span> <span class="identifier">x</span><span class="special">;</span> 79 <span class="special">}</span> 80 <span class="special">}</span> 81<span class="special">};</span> 82</pre> 83</div> 84<div class="section"> 85<div class="titlepage"><div><div><h4 class="title"> 86<a name="boost_atomic.usage_examples.example_reference_counters.usage"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.usage" title="Usage">Usage</a> 87</h4></div></div></div> 88<pre class="programlisting"><span class="identifier">X</span><span class="special">::</span><span class="identifier">pointer</span> <span class="identifier">x</span> <span class="special">=</span> <span class="keyword">new</span> <span class="identifier">X</span><span class="special">;</span> 89</pre> 90</div> 91<div class="section"> 92<div class="titlepage"><div><div><h4 class="title"> 93<a name="boost_atomic.usage_examples.example_reference_counters.discussion"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.discussion" title="Discussion">Discussion</a> 94</h4></div></div></div> 95<p> 96 Increasing the reference counter can always be done with <code class="literal">memory_order_relaxed</code>: 97 New references to an object can only be formed from an existing reference, 98 and passing an existing reference from one thread to another must already 99 provide any required synchronization. 100 </p> 101<p> 102 It is important to enforce any possible access to the object in one thread 103 (through an existing reference) to <span class="emphasis"><em>happen before</em></span> deleting 104 the object in a different thread. This is achieved by a "release" 105 operation after dropping a reference (any access to the object through 106 this reference must obviously happened before), and an "acquire" 107 operation before deleting the object. 108 </p> 109<p> 110 It would be possible to use <code class="literal">memory_order_acq_rel</code> for 111 the <code class="literal">fetch_sub</code> operation, but this results in unneeded 112 "acquire" operations when the reference counter does not yet 113 reach zero and may impose a performance penalty. 114 </p> 115</div> 116</div> 117<div class="section"> 118<div class="titlepage"><div><div><h3 class="title"> 119<a name="boost_atomic.usage_examples.example_spinlock"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_spinlock" title="Spinlock">Spinlock</a> 120</h3></div></div></div> 121<div class="toc"><dl class="toc"> 122<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.implementation">Implementation</a></span></dt> 123<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.usage">Usage</a></span></dt> 124<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.discussion">Discussion</a></span></dt> 125</dl></div> 126<p> 127 The purpose of a <span class="emphasis"><em>spin lock</em></span> is to prevent multiple threads 128 from concurrently accessing a shared data structure. In contrast to a mutex, 129 threads will busy-wait and waste CPU cycles instead of yielding the CPU to 130 another thread. <span class="emphasis"><em>Do not use spinlocks unless you are certain that 131 you understand the consequences.</em></span> 132 </p> 133<div class="section"> 134<div class="titlepage"><div><div><h4 class="title"> 135<a name="boost_atomic.usage_examples.example_spinlock.implementation"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.implementation" title="Implementation">Implementation</a> 136</h4></div></div></div> 137<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 138 139<span class="keyword">class</span> <span class="identifier">spinlock</span> <span class="special">{</span> 140<span class="keyword">private</span><span class="special">:</span> 141 <span class="keyword">typedef</span> <span class="keyword">enum</span> <span class="special">{</span><span class="identifier">Locked</span><span class="special">,</span> <span class="identifier">Unlocked</span><span class="special">}</span> <span class="identifier">LockState</span><span class="special">;</span> 142 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">LockState</span><span class="special">></span> <span class="identifier">state_</span><span class="special">;</span> 143 144<span class="keyword">public</span><span class="special">:</span> 145 <span class="identifier">spinlock</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">state_</span><span class="special">(</span><span class="identifier">Unlocked</span><span class="special">)</span> <span class="special">{}</span> 146 147 <span class="keyword">void</span> <span class="identifier">lock</span><span class="special">()</span> 148 <span class="special">{</span> 149 <span class="keyword">while</span> <span class="special">(</span><span class="identifier">state_</span><span class="special">.</span><span class="identifier">exchange</span><span class="special">(</span><span class="identifier">Locked</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_acquire</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">Locked</span><span class="special">)</span> <span class="special">{</span> 150 <span class="comment">/* busy-wait */</span> 151 <span class="special">}</span> 152 <span class="special">}</span> 153 <span class="keyword">void</span> <span class="identifier">unlock</span><span class="special">()</span> 154 <span class="special">{</span> 155 <span class="identifier">state_</span><span class="special">.</span><span class="identifier">store</span><span class="special">(</span><span class="identifier">Unlocked</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_release</span><span class="special">);</span> 156 <span class="special">}</span> 157<span class="special">};</span> 158</pre> 159</div> 160<div class="section"> 161<div class="titlepage"><div><div><h4 class="title"> 162<a name="boost_atomic.usage_examples.example_spinlock.usage"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.usage" title="Usage">Usage</a> 163</h4></div></div></div> 164<pre class="programlisting"><span class="identifier">spinlock</span> <span class="identifier">s</span><span class="special">;</span> 165 166<span class="identifier">s</span><span class="special">.</span><span class="identifier">lock</span><span class="special">();</span> 167<span class="comment">// access data structure here</span> 168<span class="identifier">s</span><span class="special">.</span><span class="identifier">unlock</span><span class="special">();</span> 169</pre> 170</div> 171<div class="section"> 172<div class="titlepage"><div><div><h4 class="title"> 173<a name="boost_atomic.usage_examples.example_spinlock.discussion"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.discussion" title="Discussion">Discussion</a> 174</h4></div></div></div> 175<p> 176 The purpose of the spinlock is to make sure that one access to the shared 177 data structure always strictly "happens before" another. The 178 usage of acquire/release in lock/unlock is required and sufficient to guarantee 179 this ordering. 180 </p> 181<p> 182 It would be correct to write the "lock" operation in the following 183 way: 184 </p> 185<pre class="programlisting"><span class="identifier">lock</span><span class="special">()</span> 186<span class="special">{</span> 187 <span class="keyword">while</span> <span class="special">(</span><span class="identifier">state_</span><span class="special">.</span><span class="identifier">exchange</span><span class="special">(</span><span class="identifier">Locked</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_relaxed</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">Locked</span><span class="special">)</span> <span class="special">{</span> 188 <span class="comment">/* busy-wait */</span> 189 <span class="special">}</span> 190 <span class="identifier">atomic_thread_fence</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_acquire</span><span class="special">);</span> 191<span class="special">}</span> 192</pre> 193<p> 194 This "optimization" is however a) useless and b) may in fact 195 hurt: a) Since the thread will be busily spinning on a blocked spinlock, 196 it does not matter if it will waste the CPU cycles with just "exchange" 197 operations or with both useless "exchange" and "acquire" 198 operations. b) A tight "exchange" loop without any memory-synchronizing 199 instruction introduced through an "acquire" operation will on 200 some systems monopolize the memory subsystem and degrade the performance 201 of other system components. 202 </p> 203</div> 204</div> 205<div class="section"> 206<div class="titlepage"><div><div><h3 class="title"> 207<a name="boost_atomic.usage_examples.singleton"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.singleton" title="Singleton with double-checked locking pattern">Singleton with 208 double-checked locking pattern</a> 209</h3></div></div></div> 210<div class="toc"><dl class="toc"> 211<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton.implementation">Implementation</a></span></dt> 212<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton.usage">Usage</a></span></dt> 213<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton.discussion">Discussion</a></span></dt> 214</dl></div> 215<p> 216 The purpose of the <span class="emphasis"><em>Singleton with double-checked locking pattern</em></span> 217 is to ensure that at most one instance of a particular object is created. 218 If one instance has been created already, access to the existing object should 219 be as light-weight as possible. 220 </p> 221<div class="section"> 222<div class="titlepage"><div><div><h4 class="title"> 223<a name="boost_atomic.usage_examples.singleton.implementation"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.singleton.implementation" title="Implementation">Implementation</a> 224</h4></div></div></div> 225<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 226<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">thread</span><span class="special">/</span><span class="identifier">mutex</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 227 228<span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{</span> 229<span class="keyword">public</span><span class="special">:</span> 230 <span class="keyword">static</span> <span class="identifier">X</span> <span class="special">*</span> <span class="identifier">instance</span><span class="special">()</span> 231 <span class="special">{</span> 232 <span class="identifier">X</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">=</span> <span class="identifier">instance_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_consume</span><span class="special">);</span> 233 <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">tmp</span><span class="special">)</span> <span class="special">{</span> 234 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mutex</span><span class="special">::</span><span class="identifier">scoped_lock</span> <span class="identifier">guard</span><span class="special">(</span><span class="identifier">instantiation_mutex</span><span class="special">);</span> 235 <span class="identifier">tmp</span> <span class="special">=</span> <span class="identifier">instance_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_consume</span><span class="special">);</span> 236 <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">tmp</span><span class="special">)</span> <span class="special">{</span> 237 <span class="identifier">tmp</span> <span class="special">=</span> <span class="keyword">new</span> <span class="identifier">X</span><span class="special">;</span> 238 <span class="identifier">instance_</span><span class="special">.</span><span class="identifier">store</span><span class="special">(</span><span class="identifier">tmp</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_release</span><span class="special">);</span> 239 <span class="special">}</span> 240 <span class="special">}</span> 241 <span class="keyword">return</span> <span class="identifier">tmp</span><span class="special">;</span> 242 <span class="special">}</span> 243<span class="keyword">private</span><span class="special">:</span> 244 <span class="keyword">static</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">X</span> <span class="special">*></span> <span class="identifier">instance_</span><span class="special">;</span> 245 <span class="keyword">static</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mutex</span> <span class="identifier">instantiation_mutex</span><span class="special">;</span> 246<span class="special">};</span> 247 248<span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">X</span> <span class="special">*></span> <span class="identifier">X</span><span class="special">::</span><span class="identifier">instance_</span><span class="special">(</span><span class="number">0</span><span class="special">);</span> 249</pre> 250</div> 251<div class="section"> 252<div class="titlepage"><div><div><h4 class="title"> 253<a name="boost_atomic.usage_examples.singleton.usage"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.singleton.usage" title="Usage">Usage</a> 254</h4></div></div></div> 255<pre class="programlisting"><span class="identifier">X</span> <span class="special">*</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">X</span><span class="special">::</span><span class="identifier">instance</span><span class="special">();</span> 256<span class="comment">// dereference x</span> 257</pre> 258</div> 259<div class="section"> 260<div class="titlepage"><div><div><h4 class="title"> 261<a name="boost_atomic.usage_examples.singleton.discussion"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.singleton.discussion" title="Discussion">Discussion</a> 262</h4></div></div></div> 263<p> 264 The mutex makes sure that only one instance of the object is ever created. 265 The <code class="literal">instance</code> method must make sure that any dereference 266 of the object strictly "happens after" creating the instance 267 in another thread. The use of <code class="literal">memory_order_release</code> after 268 creating and initializing the object and <code class="literal">memory_order_consume</code> 269 before dereferencing the object provides this guarantee. 270 </p> 271<p> 272 It would be permissible to use <code class="literal">memory_order_acquire</code> 273 instead of <code class="literal">memory_order_consume</code>, but this provides a 274 stronger guarantee than is required since only operations depending on 275 the value of the pointer need to be ordered. 276 </p> 277</div> 278</div> 279<div class="section"> 280<div class="titlepage"><div><div><h3 class="title"> 281<a name="boost_atomic.usage_examples.example_ringbuffer"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer" title="Wait-free ring buffer">Wait-free 282 ring buffer</a> 283</h3></div></div></div> 284<div class="toc"><dl class="toc"> 285<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.implementation">Implementation</a></span></dt> 286<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.usage">Usage</a></span></dt> 287<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.discussion">Discussion</a></span></dt> 288</dl></div> 289<p> 290 A <span class="emphasis"><em>wait-free ring buffer</em></span> provides a mechanism for relaying 291 objects from one single "producer" thread to one single "consumer" 292 thread without any locks. The operations on this data structure are "wait-free" 293 which means that each operation finishes within a constant number of steps. 294 This makes this data structure suitable for use in hard real-time systems 295 or for communication with interrupt/signal handlers. 296 </p> 297<div class="section"> 298<div class="titlepage"><div><div><h4 class="title"> 299<a name="boost_atomic.usage_examples.example_ringbuffer.implementation"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.implementation" title="Implementation">Implementation</a> 300</h4></div></div></div> 301<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">atomic</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 302 303<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">size_t</span> <span class="identifier">Size</span><span class="special">></span> 304<span class="keyword">class</span> <span class="identifier">ringbuffer</span> <span class="special">{</span> 305<span class="keyword">public</span><span class="special">:</span> 306 <span class="identifier">ringbuffer</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">head_</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">tail_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span> 307 308 <span class="keyword">bool</span> <span class="identifier">push</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> <span class="identifier">value</span><span class="special">)</span> 309 <span class="special">{</span> 310 <span class="identifier">size_t</span> <span class="identifier">head</span> <span class="special">=</span> <span class="identifier">head_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_relaxed</span><span class="special">);</span> 311 <span class="identifier">size_t</span> <span class="identifier">next_head</span> <span class="special">=</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">head</span><span class="special">);</span> 312 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">next_head</span> <span class="special">==</span> <span class="identifier">tail_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_acquire</span><span class="special">))</span> 313 <span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span> 314 <span class="identifier">ring_</span><span class="special">[</span><span class="identifier">head</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">value</span><span class="special">;</span> 315 <span class="identifier">head_</span><span class="special">.</span><span class="identifier">store</span><span class="special">(</span><span class="identifier">next_head</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_release</span><span class="special">);</span> 316 <span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span> 317 <span class="special">}</span> 318 <span class="keyword">bool</span> <span class="identifier">pop</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&</span> <span class="identifier">value</span><span class="special">)</span> 319 <span class="special">{</span> 320 <span class="identifier">size_t</span> <span class="identifier">tail</span> <span class="special">=</span> <span class="identifier">tail_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_relaxed</span><span class="special">);</span> 321 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">tail</span> <span class="special">==</span> <span class="identifier">head_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_acquire</span><span class="special">))</span> 322 <span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span> 323 <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">ring_</span><span class="special">[</span><span class="identifier">tail</span><span class="special">];</span> 324 <span class="identifier">tail_</span><span class="special">.</span><span class="identifier">store</span><span class="special">(</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">tail</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_release</span><span class="special">);</span> 325 <span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span> 326 <span class="special">}</span> 327<span class="keyword">private</span><span class="special">:</span> 328 <span class="identifier">size_t</span> <span class="identifier">next</span><span class="special">(</span><span class="identifier">size_t</span> <span class="identifier">current</span><span class="special">)</span> 329 <span class="special">{</span> 330 <span class="keyword">return</span> <span class="special">(</span><span class="identifier">current</span> <span class="special">+</span> <span class="number">1</span><span class="special">)</span> <span class="special">%</span> <span class="identifier">Size</span><span class="special">;</span> 331 <span class="special">}</span> 332 <span class="identifier">T</span> <span class="identifier">ring_</span><span class="special">[</span><span class="identifier">Size</span><span class="special">];</span> 333 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">head_</span><span class="special">,</span> <span class="identifier">tail_</span><span class="special">;</span> 334<span class="special">};</span> 335</pre> 336</div> 337<div class="section"> 338<div class="titlepage"><div><div><h4 class="title"> 339<a name="boost_atomic.usage_examples.example_ringbuffer.usage"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.usage" title="Usage">Usage</a> 340</h4></div></div></div> 341<pre class="programlisting"><span class="identifier">ringbuffer</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="number">32</span><span class="special">></span> <span class="identifier">r</span><span class="special">;</span> 342 343<span class="comment">// try to insert an element</span> 344<span class="keyword">if</span> <span class="special">(</span><span class="identifier">r</span><span class="special">.</span><span class="identifier">push</span><span class="special">(</span><span class="number">42</span><span class="special">))</span> <span class="special">{</span> <span class="comment">/* succeeded */</span> <span class="special">}</span> 345<span class="keyword">else</span> <span class="special">{</span> <span class="comment">/* buffer full */</span> <span class="special">}</span> 346 347<span class="comment">// try to retrieve an element</span> 348<span class="keyword">int</span> <span class="identifier">value</span><span class="special">;</span> 349<span class="keyword">if</span> <span class="special">(</span><span class="identifier">r</span><span class="special">.</span><span class="identifier">pop</span><span class="special">(</span><span class="identifier">value</span><span class="special">))</span> <span class="special">{</span> <span class="comment">/* succeeded */</span> <span class="special">}</span> 350<span class="keyword">else</span> <span class="special">{</span> <span class="comment">/* buffer empty */</span> <span class="special">}</span> 351</pre> 352</div> 353<div class="section"> 354<div class="titlepage"><div><div><h4 class="title"> 355<a name="boost_atomic.usage_examples.example_ringbuffer.discussion"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.discussion" title="Discussion">Discussion</a> 356</h4></div></div></div> 357<p> 358 The implementation makes sure that the ring indices do not "lap-around" 359 each other to ensure that no elements are either lost or read twice. 360 </p> 361<p> 362 Furthermore it must guarantee that read-access to a particular object in 363 <code class="literal">pop</code> "happens after" it has been written in 364 <code class="literal">push</code>. This is achieved by writing <code class="literal">head_ </code> 365 with "release" and reading it with "acquire". Conversely 366 the implementation also ensures that read access to a particular ring element 367 "happens before" before rewriting this element with a new value 368 by accessing <code class="literal">tail_</code> with appropriate ordering constraints. 369 </p> 370</div> 371</div> 372<div class="section"> 373<div class="titlepage"><div><div><h3 class="title"> 374<a name="boost_atomic.usage_examples.mp_queue"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.mp_queue" title="Lock-free multi-producer queue">Lock-free multi-producer 375 queue</a> 376</h3></div></div></div> 377<div class="toc"><dl class="toc"> 378<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue.implementation">Implementation</a></span></dt> 379<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue.usage">Usage</a></span></dt> 380<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue.discussion">Discussion</a></span></dt> 381</dl></div> 382<p> 383 The purpose of the <span class="emphasis"><em>lock-free multi-producer queue</em></span> is 384 to allow an arbitrary number of producers to enqueue objects which are retrieved 385 and processed in FIFO order by a single consumer. 386 </p> 387<div class="section"> 388<div class="titlepage"><div><div><h4 class="title"> 389<a name="boost_atomic.usage_examples.mp_queue.implementation"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.mp_queue.implementation" title="Implementation">Implementation</a> 390</h4></div></div></div> 391<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 392<span class="keyword">class</span> <span class="identifier">lockfree_queue</span> <span class="special">{</span> 393<span class="keyword">public</span><span class="special">:</span> 394 <span class="keyword">struct</span> <span class="identifier">node</span> <span class="special">{</span> 395 <span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span> 396 <span class="identifier">node</span> <span class="special">*</span> <span class="identifier">next</span><span class="special">;</span> 397 <span class="special">};</span> 398 <span class="keyword">void</span> <span class="identifier">push</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span><span class="identifier">data</span><span class="special">)</span> 399 <span class="special">{</span> 400 <span class="identifier">node</span> <span class="special">*</span> <span class="identifier">n</span> <span class="special">=</span> <span class="keyword">new</span> <span class="identifier">node</span><span class="special">;</span> 401 <span class="identifier">n</span><span class="special">-></span><span class="identifier">data</span> <span class="special">=</span> <span class="identifier">data</span><span class="special">;</span> 402 <span class="identifier">node</span> <span class="special">*</span> <span class="identifier">stale_head</span> <span class="special">=</span> <span class="identifier">head_</span><span class="special">.</span><span class="identifier">load</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_relaxed</span><span class="special">);</span> 403 <span class="keyword">do</span> <span class="special">{</span> 404 <span class="identifier">n</span><span class="special">-></span><span class="identifier">next</span> <span class="special">=</span> <span class="identifier">stale_head</span><span class="special">;</span> 405 <span class="special">}</span> <span class="keyword">while</span> <span class="special">(!</span><span class="identifier">head_</span><span class="special">.</span><span class="identifier">compare_exchange_weak</span><span class="special">(</span><span class="identifier">stale_head</span><span class="special">,</span> <span class="identifier">n</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_release</span><span class="special">));</span> 406 <span class="special">}</span> 407 408 <span class="identifier">node</span> <span class="special">*</span> <span class="identifier">pop_all</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> 409 <span class="special">{</span> 410 <span class="identifier">T</span> <span class="special">*</span> <span class="identifier">last</span> <span class="special">=</span> <span class="identifier">pop_all_reverse</span><span class="special">(),</span> <span class="special">*</span> <span class="identifier">first</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 411 <span class="keyword">while</span><span class="special">(</span><span class="identifier">last</span><span class="special">)</span> <span class="special">{</span> 412 <span class="identifier">T</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">=</span> <span class="identifier">last</span><span class="special">;</span> 413 <span class="identifier">last</span> <span class="special">=</span> <span class="identifier">last</span><span class="special">-></span><span class="identifier">next</span><span class="special">;</span> 414 <span class="identifier">tmp</span><span class="special">-></span><span class="identifier">next</span> <span class="special">=</span> <span class="identifier">first</span><span class="special">;</span> 415 <span class="identifier">first</span> <span class="special">=</span> <span class="identifier">tmp</span><span class="special">;</span> 416 <span class="special">}</span> 417 <span class="keyword">return</span> <span class="identifier">first</span><span class="special">;</span> 418 <span class="special">}</span> 419 420 <span class="identifier">lockfree_queue</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">head_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span> 421 422 <span class="comment">// alternative interface if ordering is of no importance</span> 423 <span class="identifier">node</span> <span class="special">*</span> <span class="identifier">pop_all_reverse</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> 424 <span class="special">{</span> 425 <span class="keyword">return</span> <span class="identifier">head_</span><span class="special">.</span><span class="identifier">exchange</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">memory_order_consume</span><span class="special">);</span> 426 <span class="special">}</span> 427<span class="keyword">private</span><span class="special">:</span> 428 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">node</span> <span class="special">*></span> <span class="identifier">head_</span><span class="special">;</span> 429<span class="special">};</span> 430</pre> 431</div> 432<div class="section"> 433<div class="titlepage"><div><div><h4 class="title"> 434<a name="boost_atomic.usage_examples.mp_queue.usage"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.mp_queue.usage" title="Usage">Usage</a> 435</h4></div></div></div> 436<pre class="programlisting"><span class="identifier">lockfree_queue</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">q</span><span class="special">;</span> 437 438<span class="comment">// insert elements</span> 439<span class="identifier">q</span><span class="special">.</span><span class="identifier">push</span><span class="special">(</span><span class="number">42</span><span class="special">);</span> 440<span class="identifier">q</span><span class="special">.</span><span class="identifier">push</span><span class="special">(</span><span class="number">2</span><span class="special">);</span> 441 442<span class="comment">// pop elements</span> 443<span class="identifier">lockfree_queue</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">node</span> <span class="special">*</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">q</span><span class="special">.</span><span class="identifier">pop_all</span><span class="special">()</span> 444<span class="keyword">while</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> 445 <span class="identifier">X</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">;</span> 446 <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">-></span><span class="identifier">next</span><span class="special">;</span> 447 <span class="comment">// process tmp->data, probably delete it afterwards</span> 448 <span class="keyword">delete</span> <span class="identifier">tmp</span><span class="special">;</span> 449<span class="special">}</span> 450</pre> 451</div> 452<div class="section"> 453<div class="titlepage"><div><div><h4 class="title"> 454<a name="boost_atomic.usage_examples.mp_queue.discussion"></a><a class="link" href="usage_examples.html#boost_atomic.usage_examples.mp_queue.discussion" title="Discussion">Discussion</a> 455</h4></div></div></div> 456<p> 457 The implementation guarantees that all objects enqueued are processed in 458 the order they were enqueued by building a singly-linked list of object 459 in reverse processing order. The queue is atomically emptied by the consumer 460 (in an operation that is not only lock-free but wait-free) and brought 461 into correct order. 462 </p> 463<p> 464 It must be guaranteed that any access to an object to be enqueued by the 465 producer "happens before" any access by the consumer. This is 466 assured by inserting objects into the list with <span class="emphasis"><em>release</em></span> 467 and dequeuing them with <span class="emphasis"><em>consume</em></span> memory order. It is 468 not necessary to use <span class="emphasis"><em>acquire</em></span> memory order in <code class="literal">waitfree_queue::pop_all</code> 469 because all operations involved depend on the value of the atomic pointer 470 through dereference. 471 </p> 472</div> 473</div> 474</div> 475<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 476<td align="left"></td> 477<td align="right"><div class="copyright-footer">Copyright © 2011 Helge Bahmann<br>Copyright © 2012 Tim Blechmann<br>Copyright © 2013, 2017, 2018, 2020 Andrey 478 Semashev<p> 479 Distributed under the Boost Software License, Version 1.0. (See accompanying 480 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>) 481 </p> 482</div></td> 483</tr></table> 484<hr> 485<div class="spirit-nav"> 486<a accesskey="p" href="interface.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../atomic.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="limitations.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 487</div> 488</body> 489</html> 490
