148 lines
12 KiB
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148 lines
12 KiB
HTML
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<title>atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_atomic_atomic_compare_exchange skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-atomic.html">atomic</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<div>定义于头文件 <code><stdatomic.h></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi c source-c">_Bool atomic_compare_exchange_strong<span class="br0">(</span> <span class="kw4">volatile</span> A<span class="sy2">*</span> obj,<br />
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C<span class="sy2">*</span> expected, C desired <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
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<div><span class="mw-geshi c source-c">_Bool atomic_compare_exchange_weak<span class="br0">(</span> <span class="kw4">volatile</span> A <span class="sy2">*</span>obj,<br />
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C<span class="sy2">*</span> expected, C desired <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
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<div><span class="mw-geshi c source-c">_Bool atomic_compare_exchange_strong_explicit<span class="br0">(</span> <span class="kw4">volatile</span> A<span class="sy2">*</span> obj,<br /></span>
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<p> C<span class="sy2">*</span> expected, C desired,<br />
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<a href="c-atomic-memory_order.html"><span class="kw910">memory_order</span></a> succ,<br /></p>
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<a href="c-atomic-memory_order.html"><span class="kw910">memory_order</span></a> fail <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
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<div><span class="mw-geshi c source-c">_Bool atomic_compare_exchange_weak_explicit<span class="br0">(</span> <span class="kw4">volatile</span> A <span class="sy2">*</span>obj,<br /></span>
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<p> C<span class="sy2">*</span> expected, C desired,<br />
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<a href="c-atomic-memory_order.html"><span class="kw910">memory_order</span></a> succ,<br /></p>
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<a href="c-atomic-memory_order.html"><span class="kw910">memory_order</span></a> fail <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>原子地比较 <code>obj</code> 所指向对象的内存的内容与 <code>expected</code> 所指向的内存的内容。若它们相等，则以 <code>desired</code> 替换前者（进行读修改写操作）。否则，将 <code>obj</code> 所指向的实际内存内容加载到 <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>succ</code> 和 <code>fail</code> 。 (1-2) 版本默认使用 <span class="t-lc"><a href="c-atomic-memory_order.html">memory_order_seq_cst</a></span> 。</p>
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<p>函数的弱形式（ (2) 与 (4) ）允许虚假失败，即表现为如同 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span>obj <span class="sy3">!</span><span class="sy1">=</span> <span class="sy2">*</span>expected</span></span> ，即使它们相等。当比较并交换在循环中时，弱版本在某些平台上会生成更好的性能。在弱版本会要求循环而强版本不要求时，最好用强版本。</p>
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<p>这是为所有<a href="c-language-atomic.html">原子对象类型</a> <code>A</code> 定义的<a href="c-language-generic.html">泛型函数</a>。参数为指向 volatile 原子对象的指针，以接受非 volatile 与 <a href="c-language-volatile.html">volatile</a> （例如内存映射 I/O ）的原子对象，而 volatile 语义在应用此操作时保留。 <code>C</code> 为对应 <code>A</code> 的非原子类型。</p>
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<p>泛型函数名是宏或是声明有外部链接的标识符是未指定的。若为访问实际函数压制宏定义（例如像 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">(</span>atomic_compare_exchange<span class="br0">)</span><span class="br0">(</span>...<span class="br0">)</span></span></span> 这样加括号），或程序定义拥有泛型函数名的外部标识符，则行为未定义。</p>
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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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<tr class="t-par">
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<td>obj</td>
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<td>-</td>
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<td>指向要测试及修改的原子对象的指针</td>
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</tr>
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<tr class="t-par">
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<td>expected</td>
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<td>-</td>
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<td>指向期待在原子对象中找到的值的指针</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>要存储于原子对象的值，若它得到期待</td>
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</tr>
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<tr class="t-par">
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<td>succ</td>
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<td>-</td>
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<td>读修改写操作的内存同步顺序，若比较成功。容许所有值。</td>
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</tr>
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<tr class="t-par">
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<td>fail</td>
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<td>-</td>
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<td>加载操作的内存同步顺序，若比较失败。不能为 <span class="t-lc"><a href="c-atomic-memory_order.html">memory_order_release</a></span> 或 <span class="t-lc"><a href="c-atomic-memory_order.html">memory_order_acq_rel</a></span> 且不能指定强于 <code>succ</code> 的顺序</td>
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</table>
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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>比较结果：若 <code>*obj</code> 等于 <code>*expected</code> 则为 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw2">true</span></span></span> ，否则 <span class="t-c"><span class="mw-geshi c source-c"><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><code>atomic_compare_exchange_strong</code> 表现为如同原子地执行下列代码：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="kw1">if</span> <span class="br0">(</span><a href="c-string-byte-memcmp.html"><span class="kw570">memcmp</span></a><span class="br0">(</span>obj, expected, <span class="kw4">sizeof</span> <span class="sy2">*</span>obj<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="c-string-byte-memcpy.html"><span class="kw572">memcpy</span></a><span class="br0">(</span>obj, <span class="sy3">&</span>desired, <span class="kw4">sizeof</span> <span class="sy2">*</span>obj<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="kw2">true</span>
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<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
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<a href="c-string-byte-memcpy.html"><span class="kw572">memcpy</span></a><span class="br0">(</span>expected, obj, <span class="kw4">sizeof</span> <span class="sy2">*</span>obj<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.17.7.4 The atomic_compare_exchange generic functions (p: 283-284)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="c-atomic-atomic_exchange.html"><span class="t-lines"><span>atomic_exchange</span><span>atomic_exchange_explicit</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div>
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</div>
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</td>
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<td>与原子对象的值交换值<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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