317 lines
28 KiB
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317 lines
28 KiB
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<title>std::atomic<T>::compare_exchange_weak, std::atomic<T>::compare_exchange_strong</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::atomic<T>::</span>compare_exchange_weak, <span style="font-size:0.7em; line-height:130%">std::atomic<T>::</span>compare_exchange_strong</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-atomic.html">atomic</a>‎ | <a href="cpp-atomic-atomic.html">atomic</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-atomic.html"><atomic></a></code></div>
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<td></td>
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<td rowspan="142">(1)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_weak<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> success,<br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> failure <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_weak<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> success,<br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> failure <span class="br0">)</span> <span class="kw4">volatile</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<td rowspan="142">(2)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_weak<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span><br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_weak<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span><br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a> <span class="br0">)</span> <span class="kw4">volatile</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<td rowspan="142">(3)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_strong<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> success,<br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> failure <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_strong<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> success,<br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> failure <span class="br0">)</span> <span class="kw4">volatile</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<td rowspan="142">(4)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_strong<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span><br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_strong<span class="br0">(</span> T<span class="sy3">&</span> expected, T desired,<br /></span>
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<p> <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span><br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a> <span class="br0">)</span> <span class="kw4">volatile</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<p>原子地比较 <code>*this</code> 和 <code>expected</code> 的<span class="t-rev-inl t-until-cxx20"><span><a href="cpp-language-object.html">对象表示</a></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><a href="cpp-language-object.html">值表示</a></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>，而若它们逐位相等，则以 <code>desired</code> 替换前者（进行读修改写操作）。否则，将 <code>*this</code> 中的实际值加载进 <code>expected</code> （进行加载操作）。</p>
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<p>读修改写和加载操作的内存模型分别为 <code>success</code> 和 <code>failure</code> 。在 (2) 和 (4) 版本中， <code>order</code> 用于读修改写操作和加载操作，除了若 <span class="t-c"><span class="mw-geshi cpp source-cpp">order <span class="sy1">==</span> <a href="cpp-atomic-memory_order.html"><span class="kw2097">std::<span class="me2">memory_order_acq_rel</span></span></a></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">order <span class="sy1">==</span> <a href="cpp-atomic-memory_order.html"><span class="kw2096">std::<span class="me2">memory_order_release</span></span></a></span></span> ，则加载操作分别使用 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acquire</a></span> 和 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_relaxed</a></span> 。</p>
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<td>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">is_always_lock_free</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 则 volatile 限定版本被弃用。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>expected</td>
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<td>-</td>
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<td>到期待在原子对象中找到的值的引用。若比较失败则被存储 <code>*this</code> 的实际值。</td>
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</tr>
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<tr class="t-par">
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<td>desired</td>
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<td>-</td>
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<td>若符合期待则存储于原子对象的值</td>
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<tr class="t-par">
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<td>success</td>
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<td>-</td>
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<td>若比较成功，则读修改写操作所用的内存同步顺序。容许所有值。</td>
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<tr class="t-par">
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<td>failure</td>
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<td>-</td>
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<td>若比较失败，则加载操作所用的内存同步顺序。不能为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_release</a></span> 或 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acq_rel</a></span> <span class="t-rev-inl t-until-cxx17"><span>，且不能指定强于 <code>success</code> 的顺序</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span></td>
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</tr>
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<tr class="t-par">
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<td>order</td>
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<td>-</td>
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<td>两个操作所用的内存同步顺序</td>
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</tr>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>若成功更改底层原子值则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>比较和复制是逐位的（类似 <span class="t-lc"><a href="cpp-string-byte-memcmp.html">std::memcmp</a></span> 和 <span class="t-lc"><a href="cpp-string-byte-memcpy.html">std::memcpy</a></span> ）；不使用构造函数、赋值运算符或比较运算符。</p>
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<p>允许函数的弱形式 (1-2) 虚假地失败，即表现如同 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this <span class="sy3">!</span><span class="sy1">=</span> expected</span></span> ，纵使它们相等。比较和交换在循环中时，弱版本在有的平台上会产出更好的性能。</p>
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<p>弱版本比较和交换会要求循环，而强版本不要求时，推荐用强版本，除非 <code>T</code> 的对象表示可包含<span class="t-rev-inl t-until-cxx20"><span>填充位、</span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span>陷阱位或为同一值提供多个对象表示（例如浮点 NaN ）。这些情况下，弱比较和交换典型地可用，因为它在一些稳定对象表示上快速收敛。</p>
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<p>若有参与 union 某些成员，但非其他成员的值表示的位，则比较和交换可能始终失败，因为这种填充位在不参与活跃成员的值表示时拥有不确定值。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<td>
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<p>忽略决不参与对象值表示的填充位。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<p>比较和交换操作通常用作无锁数据结构的基本建筑块</p>
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <atomic></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> node
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<span class="br0">{</span>
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T data<span class="sy4">;</span>
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node<span class="sy2">*</span> next<span class="sy4">;</span>
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node<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> data<span class="br0">)</span> <span class="sy4">:</span> data<span class="br0">(</span>data<span class="br0">)</span>, next<span class="br0">(</span>nullptr<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw1">class</span> stack
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<span class="br0">{</span>
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<a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>node<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span><span class="sy1">></span> head<span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw4">void</span> push<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> data<span class="br0">)</span>
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<span class="br0">{</span>
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node<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> new_node <span class="sy1">=</span> new node<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>data<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 放 head 的当前值到 new_node->next 中</span>
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new_node<span class="sy2">-</span><span class="sy1">></span>next <span class="sy1">=</span> head.<span class="me1">load</span><span class="br0">(</span><a href="cpp-atomic-memory_order.html"><span class="kw2093">std::<span class="me2">memory_order_relaxed</span></span></a><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 现在令 new_node 为新的 head ，但若 head 不再是</span>
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<span class="co1">// 存储于 new_node->next 的值（某些其他线程必须在刚才插入结点）</span>
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<span class="co1">// 则放新的 head 到 new_node->next 中并再尝试</span>
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<span class="kw1">while</span><span class="br0">(</span><span class="sy3">!</span>head.<span class="me1">compare_exchange_weak</span><span class="br0">(</span>new_node<span class="sy2">-</span><span class="sy1">></span>next, new_node,
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<a href="cpp-atomic-memory_order.html"><span class="kw2096">std::<span class="me2">memory_order_release</span></span></a>,
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<a href="cpp-atomic-memory_order.html"><span class="kw2093">std::<span class="me2">memory_order_relaxed</span></span></a><span class="br0">)</span><span class="br0">)</span>
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<span class="sy4">;</span> <span class="co1">// 循环体为空</span>
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<span class="co1">// 注意：上述使用至少在这些版本不是线程安全的</span>
|
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<span class="co1">// 先于 4.8.3 的 GCC （漏洞 60272 ），先于 2014-05-05 的 clang （漏洞 18899 ）</span>
|
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<span class="co1">// 先于 2014-03-17 的 MSVC （漏洞 819819 ）。下面是变通方法：</span>
|
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<span class="co1">// node<T>* old_head = head.load(std::memory_order_relaxed);</span>
|
|
<span class="co1">// do {</span>
|
|
<span class="co1">// new_node->next = old_head;</span>
|
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<span class="co1">// } while(!head.compare_exchange_weak(old_head, new_node,</span>
|
|
<span class="co1">// std::memory_order_release,</span>
|
|
<span class="co1">// std::memory_order_relaxed));</span>
|
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<span class="br0">}</span>
|
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<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
stack<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> s<span class="sy4">;</span>
|
|
s.<span class="me1">push</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
|
|
s.<span class="me1">push</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
|
|
s.<span class="me1">push</span><span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
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</div>
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</div>
|
|
<div class="t-example">
|
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<p>演示 compare_exchange_strong 如何要么更改原子对象的值，要么将变量用于比较。</p>
|
|
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
|
|
<tr>
|
|
<td class="mbox-empty-cell"></td>
|
|
<td class="mbox-text" style="">本节未完成<br />
|
|
原因：强 CAS 的更实际用法会更好，例如在 Concurrency in Action 使用它的地方</td>
|
|
</tr>
|
|
</table>
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <atomic></span>
|
|
<span class="co2">#include <iostream></span>
|
|
|
|
<a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> ai<span class="sy4">;</span>
|
|
|
|
<span class="kw4">int</span> tst_val<span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> new_val<span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
|
|
<span class="kw4">bool</span> exchanged<span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">void</span> valsout<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"ai= "</span> <span class="sy1"><<</span> ai
|
|
<span class="sy1"><<</span> <span class="st0">" tst_val= "</span> <span class="sy1"><<</span> tst_val
|
|
<span class="sy1"><<</span> <span class="st0">" new_val= "</span> <span class="sy1"><<</span> new_val
|
|
<span class="sy1"><<</span> <span class="st0">" exchanged= "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a> <span class="sy1"><<</span> exchanged
|
|
<span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
ai<span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span>
|
|
valsout<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// tst_val != ai ==> tst_val 被修改</span>
|
|
exchanged<span class="sy1">=</span> ai.<span class="me1">compare_exchange_strong</span><span class="br0">(</span> tst_val, new_val <span class="br0">)</span><span class="sy4">;</span>
|
|
valsout<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// tst_val == ai ==> ai 被修改</span>
|
|
exchanged<span class="sy1">=</span> ai.<span class="me1">compare_exchange_strong</span><span class="br0">(</span> tst_val, new_val <span class="br0">)</span><span class="sy4">;</span>
|
|
valsout<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">ai= 3 tst_val= 4 new_val= 5 exchanged= false
|
|
ai= 3 tst_val= 3 new_val= 5 exchanged= false
|
|
ai= 5 tst_val= 3 new_val= 5 exchanged= true</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-atomic-atomic_compare_exchange.html"><span class="t-lines"><span>atomic_compare_exchange_weak</span><span>atomic_compare_exchange_weak_explicit</span><span>atomic_compare_exchange_strong</span><span>atomic_compare_exchange_strong_explicit</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>原子地比较原子对象和非原子实参的值，若相等则进行 atomic_exchange，若不相等则进行 atomic_load<br />
|
|
<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
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<div class="visualClear"></div>
|
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</div>
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</div>
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</div>
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</body>
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</html>
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