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<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="atomic.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="atomic.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="atomic.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="atomic.html#Keywords"><span class="tocnumber">4</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="atomic.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="atomic.html#References"><span class="tocnumber">6</span> <span class="toctext">References</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/atomic&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>_Atomic</b></code> <code><b>(</b></code> <span class="t-spar">type-name</span> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>_Atomic</b></code> <span class="t-spar">type-name</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Use as a type specifier; this designates a new atomic type</div>
<div class="t-li1"><span class="t-li">2)</span> Use as a type qualifier; this designates the atomic version of <span class="t-spar">type-name</span>. In this role, it may be mixed with <a href="const.html" title="c/language/const">const</a>, <a href="volatile.html" title="c/language/volatile">volatile</a>, and <a href="restrict.html" title="c/language/restrict">restrict</a>), although unlike other qualifiers, the atomic version of <span class="t-spar">type-name</span> may have a different size, alignment, and object representation.</div>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">type-name</span>
</td>
<td> -
</td>
<td> any type other than array or function. For <span class="t-v">(1)</span>, <span class="t-spar">type-name</span> also cannot be atomic or cvr-qualified
</td></tr></table>
<p>The header <code><stdatomic.h></code> defines <a href="../atomic.html" title="c/atomic">37 convenience macros</a>, from <span class="t-c"><span class="mw-geshi c source-c"><a href="../atomic.html"><span class="kw959">atomic_bool</span></a></span></span> to <span class="t-c"><span class="mw-geshi c source-c"><a href="../atomic.html"><span class="kw995">atomic_uintmax_t</span></a></span></span>, which simplify the use of this keyword with built-in and library types.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">_Atomic <span class="kw4">const</span> <span class="kw4">int</span> <span class="sy2">*</span> p1<span class="sy4">;</span> <span class="co1">// p is a pointer to an atomic const int</span>
<span class="kw4">const</span> <a href="../atomic.html"><span class="kw965">atomic_int</span></a> <span class="sy2">*</span> p2<span class="sy4">;</span> <span class="co1">// same</span>
<span class="kw4">const</span> _Atomic<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy2">*</span> p3<span class="sy4">;</span> <span class="co1">// same</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/atomic&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Objects of atomic types are the only objects that are free from <a href="memory_model.html" title="c/language/memory model">data races</a>, that is, they may be modified by two threads concurrently or modified by one and read by another.
</p><p>Each atomic object has its own associated <i>modification order</i>, which is a total order of modifications made to that object. If, from some thread's point of view, modification <code>A</code> of some atomic M <a href="../atomic/memory_order.html" title="c/atomic/memory order">happens-before</a> modification <code>B</code> of the same atomic M, then in the modification order of M, A occurs before B.
</p><p>Note that although each atomic object has its own modification order, it is not a total order; different threads may observe modifications to different atomic objects in different orders.
</p><p>There are four coherences that are guaranteed for all atomic operations:
</p>
<ul><li> <b>write-write coherence</b>: If an operation A that modifies an atomic object M <i>happens-before</i> an operation B that modifies M, then A appears earlier than B in the modification order of M.
</li><li> <b>read-read coherence</b>: If a value computation A of an atomic object M happens before a value computation B of M, and A takes its value from a side effect X on M, then the value computed by B is either the value stored by X or is the value stored by a side effect Y on M, where Y appears later than X in the modification order of M.
</li><li> <b>read-write coherence</b>: If a value computation A of an atomic object M <i>happens-before</i> an operation B on M, then A takes its value from a side effect X on M, where X appears before B in the modification order of M.
</li><li> <b>write-read coherence</b>: If a side effect X on an atomic object M <i>happens-before</i> a value computation B of M, then the evaluation B takes its value from X or from a side effect Y that appears after X in the modification order of M.
</li></ul>
<p>Some atomic operations are also synchronization operations; they may have additional release semantics, acquire semantics, or sequentially-consistent semantics. See <span class="t-lc"><a href="../atomic/memory_order.html" title="c/atomic/memory order">memory_order</a></span>.
</p><p>Built-in <a href="operator_incdec.html" title="c/language/operator incdec">increment and decrement operators</a> and <a href="operator_assignment.html" title="c/language/operator assignment">compound assignment</a> are read-modify-write atomic operations with total sequentially consistent ordering (as if using <span class="t-lc"><a href="../atomic/memory_order.html" title="c/atomic/memory order">memory_order_seq_cst</a></span>). If less strict synchronization semantics are desired, the <a href="../atomic.html" title="c/atomic">standard library functions</a> may be used instead.
</p><p>Atomic properties are only meaningful for <a href="value_category.html" title="c/language/value category">lvalue expressions</a>. Lvalue-to-rvalue conversion (which models a memory read from an atomic location to a CPU register) strips atomicity along with other qualifiers.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: more, review interaction with memory_order and atomic library pages </td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/atomic&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>If the macro constant <code>__STDC_NO_ATOMICS__</code><span class="t-mark-rev t-since-c11">(C11)</span> is defined by the compiler, the keyword <span class="t-c"><span class="mw-geshi c source-c">_Atomic</span></span> as well as the header <code><stdatomic.h></code>, is not provided.
</p><p>Accessing a member of an atomic struct/union is undefined behavior.
</p><p>The library type <span class="t-c"><span class="mw-geshi c source-c"><a href="../program/sig_atomic_t.html"><span class="kw499">sig_atomic_t</span></a></span></span> does not provide inter-thread synchronization or memory ordering, only atomicity.
</p><p>The <a href="volatile.html" title="c/language/volatile">volatile</a> types do not provide inter-thread synchronization, memory ordering, or atomicity.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/atomic&action=edit&section=4" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keyword/_Atomic.html" title="c/keyword/ Atomic"><tt>_Atomic</tt></a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/atomic&action=edit&section=5" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include <stdio.h></span>
<span class="co2">#include <threads.h></span>
<span class="co2">#include <stdatomic.h></span>
 
<a href="../atomic.html"><span class="kw965">atomic_int</span></a> acnt<span class="sy4">;</span>
<span class="kw4">int</span> cnt<span class="sy4">;</span>
 
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span> thr_data<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">1000</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span>
<span class="sy2">++</span>cnt<span class="sy4">;</span>
<span class="sy2">++</span>acnt<span class="sy4">;</span>
<span class="co1">// for this example, relaxed memory order is sufficient, e.g.</span>
<span class="co1">// atomic_fetch_add_explicit(&acnt, 1, memory_order_relaxed);</span>
<span class="br0">}</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../thread.html"><span class="kw996">thrd_t</span></a> thr<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
<a href="../thread/thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>thr<span class="br0">[</span>n<span class="br0">]</span>, f, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
<a href="../thread/thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>thr<span class="br0">[</span>n<span class="br0">]</span>, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
 
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"The atomic counter is %u<span class="es1">\n</span>"</span>, acnt<span class="br0">)</span><span class="sy4">;</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"The non-atomic counter is %u<span class="es1">\n</span>"</span>, cnt<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">The atomic counter is 10000
The non-atomic counter is 8644</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/atomic&action=edit&section=6" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011):
</li></ul>
<dl><dd><ul><li> 6.7.2.4 Atomic type specifiers (p: 121)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 7.17 Atomics <stdatomic.h> (p: 273-286)
</li></ul>
</dd></dl>
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