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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="atomic.usage_examples"></a><a class="link" href="usage_examples.html" title="Usage examples">Usage examples</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters">Reference
counting</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock">Spinlock</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton">Singleton with
double-checked locking pattern</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer">Wait-free
ring buffer</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue">Lock-free multi-producer
queue</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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
counting</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.implementation">Implementation</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.usage">Usage</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_reference_counters.discussion">Discussion</a></span></dt>
</dl></div>
<p>
The purpose of a <span class="emphasis"><em>reference counter</em></span> is to count the number
of pointers to an object. The object can be destroyed as soon as the reference
counter reaches zero.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<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>
<span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<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>
<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>
<span class="keyword">private</span><span class="special">:</span>
<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>
<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>
<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_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>
<span class="special">}</span>
<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>
<span class="special">{</span>
<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>
<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>
<span class="keyword">delete</span> <span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<p>
Increasing the reference counter can always be done with <code class="literal">memory_order_relaxed</code>:
New references to an object can only be formed from an existing reference,
and passing an existing reference from one thread to another must already
provide any required synchronization.
</p>
<p>
It is important to enforce any possible access to the object in one thread
(through an existing reference) to <span class="emphasis"><em>happen before</em></span> deleting
the object in a different thread. This is achieved by a "release"
operation after dropping a reference (any access to the object through
this reference must obviously happened before), and an "acquire"
operation before deleting the object.
</p>
<p>
It would be possible to use <code class="literal">memory_order_acq_rel</code> for
the <code class="literal">fetch_sub</code> operation, but this results in unneeded
"acquire" operations when the reference counter does not yet
reach zero and may impose a performance penalty.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.implementation">Implementation</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.usage">Usage</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_spinlock.discussion">Discussion</a></span></dt>
</dl></div>
<p>
The purpose of a <span class="emphasis"><em>spin lock</em></span> is to prevent multiple threads
from concurrently accessing a shared data structure. In contrast to a mutex,
threads will busy-wait and waste CPU cycles instead of yielding the CPU to
another thread. <span class="emphasis"><em>Do not use spinlocks unless you are certain that
you understand the consequences.</em></span>
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<span class="keyword">class</span> <span class="identifier">spinlock</span> <span class="special">{</span>
<span class="keyword">private</span><span class="special">:</span>
<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>
<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>
<span class="keyword">public</span><span class="special">:</span>
<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>
<span class="keyword">void</span> <span class="identifier">lock</span><span class="special">()</span>
<span class="special">{</span>
<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>
<span class="comment">/* busy-wait */</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">unlock</span><span class="special">()</span>
<span class="special">{</span>
<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>
<span class="special">}</span>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<pre class="programlisting"><span class="identifier">spinlock</span> <span class="identifier">s</span><span class="special">;</span>
<span class="identifier">s</span><span class="special">.</span><span class="identifier">lock</span><span class="special">();</span>
<span class="comment">// access data structure here</span>
<span class="identifier">s</span><span class="special">.</span><span class="identifier">unlock</span><span class="special">();</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<p>
The purpose of the spinlock is to make sure that one access to the shared
data structure always strictly "happens before" another. The
usage of acquire/release in lock/unlock is required and sufficient to guarantee
this ordering.
</p>
<p>
It would be correct to write the "lock" operation in the following
way:
</p>
<pre class="programlisting"><span class="identifier">lock</span><span class="special">()</span>
<span class="special">{</span>
<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>
<span class="comment">/* busy-wait */</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>
<span class="special">}</span>
</pre>
<p>
This "optimization" is however a) useless and b) may in fact
hurt: a) Since the thread will be busily spinning on a blocked spinlock,
it does not matter if it will waste the CPU cycles with just "exchange"
operations or with both useless "exchange" and "acquire"
operations. b) A tight "exchange" loop without any memory-synchronizing
instruction introduced through an "acquire" operation will on
some systems monopolize the memory subsystem and degrade the performance
of other system components.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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
double-checked locking pattern</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton.implementation">Implementation</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton.usage">Usage</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.singleton.discussion">Discussion</a></span></dt>
</dl></div>
<p>
The purpose of the <span class="emphasis"><em>Singleton with double-checked locking pattern</em></span>
is to ensure that at most one instance of a particular object is created.
If one instance has been created already, access to the existing object should
be as light-weight as possible.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<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>
<span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">static</span> <span class="identifier">X</span> <span class="special">*</span> <span class="identifier">instance</span><span class="special">()</span>
<span class="special">{</span>
<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>
<span class="keyword">if</span> <span class="special">(!</span><span class="identifier">tmp</span><span class="special">)</span> <span class="special">{</span>
<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>
<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>
<span class="keyword">if</span> <span class="special">(!</span><span class="identifier">tmp</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">tmp</span> <span class="special">=</span> <span class="keyword">new</span> <span class="identifier">X</span><span class="special">;</span>
<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>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="identifier">tmp</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<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>
<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>
<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">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>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<span class="comment">// dereference x</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<p>
The mutex makes sure that only one instance of the object is ever created.
The <code class="literal">instance</code> method must make sure that any dereference
of the object strictly "happens after" creating the instance
in another thread. The use of <code class="literal">memory_order_release</code> after
creating and initializing the object and <code class="literal">memory_order_consume</code>
before dereferencing the object provides this guarantee.
</p>
<p>
It would be permissible to use <code class="literal">memory_order_acquire</code>
instead of <code class="literal">memory_order_consume</code>, but this provides a
stronger guarantee than is required since only operations depending on
the value of the pointer need to be ordered.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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
ring buffer</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.implementation">Implementation</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.usage">Usage</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.example_ringbuffer.discussion">Discussion</a></span></dt>
</dl></div>
<p>
A <span class="emphasis"><em>wait-free ring buffer</em></span> provides a mechanism for relaying
objects from one single "producer" thread to one single "consumer"
thread without any locks. The operations on this data structure are "wait-free"
which means that each operation finishes within a constant number of steps.
This makes this data structure suitable for use in hard real-time systems
or for communication with interrupt/signal handlers.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<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>
<span class="keyword">class</span> <span class="identifier">ringbuffer</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<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>
<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>
<span class="special">{</span>
<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>
<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>
<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>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<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>
<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>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="special">{</span>
<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>
<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>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<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>
<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>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<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>
<span class="special">{</span>
<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>
<span class="special">}</span>
<span class="identifier">T</span> <span class="identifier">ring_</span><span class="special">[</span><span class="identifier">Size</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">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>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<span class="comment">// try to insert an element</span>
<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>
<span class="keyword">else</span> <span class="special">{</span> <span class="comment">/* buffer full */</span> <span class="special">}</span>
<span class="comment">// try to retrieve an element</span>
<span class="keyword">int</span> <span class="identifier">value</span><span class="special">;</span>
<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>
<span class="keyword">else</span> <span class="special">{</span> <span class="comment">/* buffer empty */</span> <span class="special">}</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<p>
The implementation makes sure that the ring indices do not "lap-around"
each other to ensure that no elements are either lost or read twice.
</p>
<p>
Furthermore it must guarantee that read-access to a particular object in
<code class="literal">pop</code> "happens after" it has been written in
<code class="literal">push</code>. This is achieved by writing <code class="literal">head_ </code>
with "release" and reading it with "acquire". Conversely
the implementation also ensures that read access to a particular ring element
"happens before" before rewriting this element with a new value
by accessing <code class="literal">tail_</code> with appropriate ordering constraints.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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
queue</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue.implementation">Implementation</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue.usage">Usage</a></span></dt>
<dt><span class="section"><a href="usage_examples.html#boost_atomic.usage_examples.mp_queue.discussion">Discussion</a></span></dt>
</dl></div>
<p>
The purpose of the <span class="emphasis"><em>lock-free multi-producer queue</em></span> is
to allow an arbitrary number of producers to enqueue objects which are retrieved
and processed in FIFO order by a single consumer.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<span class="keyword">class</span> <span class="identifier">lockfree_queue</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">struct</span> <span class="identifier">node</span> <span class="special">{</span>
<span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span>
<span class="identifier">node</span> <span class="special">*</span> <span class="identifier">next</span><span class="special">;</span>
<span class="special">};</span>
<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>
<span class="special">{</span>
<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>
<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>
<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>
<span class="keyword">do</span> <span class="special">{</span>
<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>
<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>
<span class="special">}</span>
<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>
<span class="special">{</span>
<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>
<span class="keyword">while</span><span class="special">(</span><span class="identifier">last</span><span class="special">)</span> <span class="special">{</span>
<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>
<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>
<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>
<span class="identifier">first</span> <span class="special">=</span> <span class="identifier">tmp</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="identifier">first</span><span class="special">;</span>
<span class="special">}</span>
<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>
<span class="comment">// alternative interface if ordering is of no importance</span>
<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>
<span class="special">{</span>
<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>
<span class="special">}</span>
<span class="keyword">private</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">node</span> <span class="special">*></span> <span class="identifier">head_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<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>
<span class="comment">// insert elements</span>
<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>
<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>
<span class="comment">// pop elements</span>
<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>
<span class="keyword">while</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
<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>
<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>
<span class="comment">// process tmp->data, probably delete it afterwards</span>
<span class="keyword">delete</span> <span class="identifier">tmp</span><span class="special">;</span>
<span class="special">}</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></div></div></div>
<p>
The implementation guarantees that all objects enqueued are processed in
the order they were enqueued by building a singly-linked list of object
in reverse processing order. The queue is atomically emptied by the consumer
(in an operation that is not only lock-free but wait-free) and brought
into correct order.
</p>
<p>
It must be guaranteed that any access to an object to be enqueued by the
producer "happens before" any access by the consumer. This is
assured by inserting objects into the list with <span class="emphasis"><em>release</em></span>
and dequeuing them with <span class="emphasis"><em>consume</em></span> memory order. It is
not necessary to use <span class="emphasis"><em>acquire</em></span> memory order in <code class="literal">waitfree_queue::pop_all</code>
because all operations involved depend on the value of the atomic pointer
through dereference.
</p>
</div>
</div>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2011 Helge Bahmann<br>Copyright © 2012 Tim Blechmann<br>Copyright © 2013, 2017, 2018, 2020 Andrey
Semashev<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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