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<title>atomic_compare_exchange_strong</title>
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1" />
<meta name="keywords" content="atomic_compare_exchange_strong" />
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<body>
<div class="refentry">
<a id="id-1"></a>
<div class="titlepage"></div>
<div xmlns="" class="refnamediv">
<a xmlns="http://www.w3.org/1999/xhtml" id="atomic_compare_exchange"></a>
<h1>atomic_compare_exchange functions</h1>
<p>
atomic_compare_exchange functions.
</p>
</div>
<div class="refsynopsisdiv">
<h2></h2>
<div class="funcsynopsis">
<table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
<tr valign="bottom">
<td>
<code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
<a class="link" href="scalarDataTypes.html" target="pagedisplay">bool</a>
<strong class="fsfunc">atomic_compare_exchange_strong</strong>
(</code>
<td>volatile A  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*object</var>, </td>
</td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*expected</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">desired</var><code>)</code></td>
</tr>
</table>
</div>
<div class="funcsynopsis">
<table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
<tr valign="bottom">
<td>
<code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
<a class="link" href="scalarDataTypes.html" target="pagedisplay">bool</a>
<strong class="fsfunc">atomic_compare_exchange_strong_explicit</strong>
(</code>
<td>volatile A  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*object</var>, </td>
</td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*expected</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">desired</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">success</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">failure</var><code>)</code></td>
</tr>
</table>
</div>
<div class="funcsynopsis">
<table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
<tr valign="bottom">
<td>
<code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
<a class="link" href="scalarDataTypes.html" target="pagedisplay">bool</a>
<strong class="fsfunc">atomic_compare_exchange_strong_explicit</strong>
(</code>
<td>volatile A  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*object</var>, </td>
</td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*expected</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">desired</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">success</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">failure</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_scope.html" target="pagedisplay">memory_scope</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">scope</var><code>)</code></td>
</tr>
</table>
</div>
<div class="funcsynopsis">
<table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
<tr valign="bottom">
<td>
<code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
<a class="link" href="scalarDataTypes.html" target="pagedisplay">bool</a>
<strong class="fsfunc">atomic_compare_exchange_weak</strong>
(</code>
<td>volatile A  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*object</var>, </td>
</td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*expected</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">desired</var><code>)</code></td>
</tr>
</table>
</div>
<div class="funcsynopsis">
<table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
<tr valign="bottom">
<td>
<code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
<a class="link" href="scalarDataTypes.html" target="pagedisplay">bool</a>
<strong class="fsfunc">atomic_compare_exchange_weak_explicit</strong>
(</code>
<td>volatile A  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*object</var>, </td>
</td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*expected</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">desired</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">success</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">failure</var><code>)</code></td>
</tr>
</table>
</div>
<div class="funcsynopsis">
<table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
<tr valign="bottom">
<td>
<code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
<a class="link" href="scalarDataTypes.html" target="pagedisplay">bool</a>
<strong class="fsfunc">atomic_compare_exchange_weak_explicit</strong>
(</code>
<td>volatile A  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*object</var>, </td>
</td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*expected</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td>C  <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">desired</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">success</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_order.html" target="pagedisplay">memory_order</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">failure</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="memory_scope.html" target="pagedisplay">memory_scope</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">scope</var><code>)</code></td>
</tr>
</table>
</div>
</div>
<div class="refsect1">
<a id="parameters"></a>
<h2>Parameters</h2>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
<code class="varname">object</code>
</span>
</dt>
<dd></dd>
<dt>
<span class="term">
<code class="varname">expected</code>
</span>
</dt>
<dd></dd>
<dt>
<span class="term">
<code class="varname">desired</code>
</span>
</dt>
<dd></dd>
<dt>
<span class="term">
<code class="varname">success</code>
</span>
</dt>
<dd></dd>
<dt>
<span class="term">
<code class="varname">failure</code>
</span>
</dt>
<dd>
The <code class="varname">failure</code> argument shall not be
<code class="constant">memory_order_release</code> nor
<code class="constant">memory_order_acq_rel</code>.
</dd>
<dt>
<span class="term">
<code class="varname">scope</code>
</span>
</dt>
<dd></dd>
</dl>
</div>
</div>
<div class="refsect1">
<a id="description"></a>
<h2>Description</h2>
<p>
These functions can only be used with an object of any atomic integer type.
</p>
<p>
The <code class="varname">failure</code> argument shall be no
stronger than the <code class="varname">success</code>
argument. Atomically, compares the value pointed
to by <code class="varname">object</code> for equality with that in
<code class="varname">expected</code>, and if true, replaces the value pointed
to by <code class="varname">object</code> with <code class="varname">desired</code>, and if false,
updates the value in <code class="varname">expected</code>
with the value pointed to by <code class="varname">object</code>. Further, if the
comparison is true, memory is affected according
to the value of <code class="varname">success</code>, and if the
comparison is false, memory is affected according
to the value of <code class="varname">failure</code>. These
operations
are atomic read-modify-write operations
(as defined by section 5.1.2.4 of the C11 specification).
</p>
<p>
NOTE: The effect of the compare-and-exchange operations is:
</p>
<p>
</p>
<div class="literallayout">
<p>
<code class="code">if (memcmp(object, expected, sizeof(*object) == 0)<br />
     memcpy(object, &amp;desired, sizeof(*object));<br />
else<br />
     memcpy(expected, object, sizeof(*object));<br />
</code>
</p>
</div>
<p>
</p>
<p>
The weak compare-and-exchange operations may fail
spuriously. That is, even when the
contents of memory referred to by
<code class="varname">expected</code> and
<code class="varname">object</code> are equal,
it may return zero and
store back to <code class="varname">expected</code>
the same memory contents that were originally there.
This spurious failure enables implementation of
compare-and-exchange on a broader class of
machines, e.g. load-locked store-conditional machines.
</p>
<p>
These generic functions return the result of the comparison.
</p>
<p>
In these operation definitions:
</p>
<p>
</p>
<div class="itemizedlist">
<ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
An <code class="code">A</code> refers to one of the atomic types.
</li>
<li class="listitem">
A <code class="code">C</code> refers to its corresponding non-atomic type.
</li>
<li class="listitem">
An <code class="code">M</code> refers to the type of the other
argument for arithmetic operations. For atomic
integer types, <code class="code">M</code> is <code class="code">C</code>.
</li>
<li class="listitem">
The functions not ending in explicit have the same semantics as the corresponding
explicit function with <code class="constant">memory_order_seq_cst</code>
for the <code class="code">memory_order</code> argument.
</li>
<li class="listitem">
The functions that do not have <code class="code">memory_scope</code>
argument have the same semantics as
the corresponding functions with the <code class="code">memory_scope</code> argument set to
<code class="constant">memory_scope_device</code>.
</li>
</ul>
</div>
<p>
</p>
<p>
NOTE: With fine-grained system SVM, sharing
happens at the granularity of individual loads
and stores anywhere in host memory. Memory
consistency is always guaranteed at
synchronization points, but to obtain finer
control over consistency, the OpenCL atomics
functions may be used to ensure that the updates
to individual data values made by one unit of
execution are visible to other execution units.
In particular, when a host thread needs fine
control over the consistency of memory that is
shared with one or more OpenCL devices, it must
use atomic and fence operations that are
compatible with the C11 atomic operations.
</p>
<p>
We can't require C11 atomics since host programs can
be implemented in other programming languages and
versions of C or C++, but we do require that the host
programs use atomics and that those atomics be compatible
with those in C11.
</p>
<h4><a id="id-1.6.12"></a>Restrictions</h4>
<p>
All operations on atomic types must be performed using the built-in atomic
functions. C11 and C++11 support operators on atomic types. OpenCL C does not
support operators with atomic types. Using atomic types with operators should result in a
compilation error.
</p>
<p>
The <span class="type">atomic_bool</span>, <span class="type">atomic_char</span>,
<span class="type">atomic_uchar</span>, <span class="type">atomic_short</span>,
<span class="type">atomic_ushort</span>, <span class="type">atomic_intmax_t</span>
and <span class="type">atomic_uintmax_t</span> types are not
supported by OpenCL C.
</p>
<p>
OpenCL C requires that the built-in atomic functions on
atomic types are lock-free.
</p>
<p>
The <code class="code">_Atomic</code> type specifier and <code class="code">_Atomic</code>
type qualifier are not supported by OpenCL C.
</p>
<p>
The behavior of atomic operations where pointer
arguments to the atomic functions refers
to an atomic type in the private address space is undefined.
</p>
</div>
<div class="refsect1">
<a id="specification"></a>
<h2>Specification</h2>
<p>
<img src="pdficon_small1.gif" />
<a href="https://www.khronos.org/registry/cl/specs/opencl-2.1-openclc.pdf#page=108" target="OpenCL Spec">OpenCL Specification</a>
</p>
</div>
<div class="refsect1">
<a id="seealso"></a>
<h2>Also see</h2>
<p>
<a class="citerefentry" href="atomicFunctions.html"><span class="citerefentry"><span class="refentrytitle">Atomic Functions</span></span></a>
</p>
</div>
<div xmlns="" class="refsect3" lang="en" xml:lang="en"><a xmlns="http://www.w3.org/1999/xhtml" id="Copyright"></a><h4 xmlns="http://www.w3.org/1999/xhtml"></h4><img xmlns="http://www.w3.org/1999/xhtml" src="KhronosLogo.jpg" /><p xmlns="http://www.w3.org/1999/xhtml"></p>Copyright © 2007-2015 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and/or associated documentation files (the
"Materials"), to deal in the Materials without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Materials, and to
permit persons to whom the Materials are furnished to do so, subject to
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