173 lines
17 KiB
HTML
173 lines
17 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>原子类型</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_atomic skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">原子类型</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-language.html">language</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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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><code><b>_Atomic</b></code> <code><b>(</b></code> <span class="t-spar">类型名</span> <code><b>)</b></code></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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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>_Atomic</b></code> <span class="t-spar">类型名</span></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 class="t-li1"><span class="t-li">1)</span> 用作类型说明符；指代新的原子类型</div>
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<div class="t-li1"><span class="t-li">2)</span> 用作类型限定符；指代 <span class="t-spar">类型名</span> 的原子版本。在此作用中，它可以与 <a href="c-language-const.html"><code>const</code></a> 、 <a href="c-language-volatile.html"><code>volatile</code></a> 及 <a href="c-language-restrict.html"><code>restrict</code></a> 混合使用。尽管不同于其他限定符， <span class="t-spar">类型名</span> 的原子版本可能拥有不同的大小、对齐以及对象表示。</div>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">类型名</span></td>
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<td>-</td>
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<td>任何异于数组或函数的类型。对于 <span class="t-v">(1)</span> ， <span class="t-spar">t类型名</span> 亦不能为原子或 cvr 限定</td>
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<p>头文件 <code><stdatomic.h></code> 定义 <a href="c-atomic.html">37 个便利宏</a>，从 <span class="t-c"><span class="mw-geshi c source-c"><a href="c-atomic.html"><span class="kw959">atomic_bool</span></a></span></span> 到 <span class="t-c"><span class="mw-geshi c source-c"><a href="c-atomic.html"><span class="kw995">atomic_uintmax_t</span></a></span></span> ，它们简化此关键词和内建及库类型的一同使用。</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">_Atomic <span class="kw4">const</span> <span class="kw4">int</span> <span class="sy2">*</span> p1<span class="sy4">;</span> <span class="co1">// p 是指向 _Atomic const int 的指针</span>
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<span class="kw4">const</span> <a href="c-atomic.html"><span class="kw965">atomic_int</span></a> <span class="sy2">*</span> p2<span class="sy4">;</span> <span class="co1">// 同上</span>
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<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">// 同上</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>原子类型的对象是仅有的免除<a href="c-language-memory_model.html">数据竞争</a>的对象，即它们可以被两个线程共时修改，或先被一个修改再被另一个读取。.</p>
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<p>每个原子对象都拥有关联于其自身的<i>修改顺序</i>，即对该对象的完整修改顺序。若从某个线程的视角来看，对于某原子对象M的修改 <code>A</code> <a href="c-atomic-memory_order.html">发生先于</a>同一原子对象 M 的修改 <code>B</code> ，则在 M 的修改顺序中 <code>A</code> 的出现先于 <code>B</code> 。</p>
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<p>注意即使每个原子对象都有其自身的修改顺序，却无单独的全序；不同线程可能会观测到不同原子对象有相异的修改顺序。</p>
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<p>对于所有原子运算，保证有四种连贯：</p>
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<ul>
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<li><b>写写连贯</b>：若原子对象 M 的修改操作 A <i>先发生于</i> M 的修改操作 B ，则 M 的修改顺序中 A 出现早于 B 。</li>
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<li><b>读读连贯</b>：若原子对象 M 的值计算 A <i>先发生于</i> M 的值计算 B ，且从 M 上的副效应 X 求得 A 值，则 B 所计算得的值要么是 X 所存储的值，要么是 M 上的副效应 Y 所存储的值，其中 Y 在 M 的修改顺序中出现后于 X 。</li>
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<li><b>读写连贯</b>：若原子对象 M 的值计算 A <i>先发生于</i> M 上的操作 B ，则从 M 上的副效应 X 求得 A 值，这里 X 在 M 的修改顺序中出现先于 B 。</li>
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<li><b>写读连贯</b>：若在原子对象 M 上的副效应 X <i>先发生于</i> M 的值计算 B ，则求值 B 从 X，或从在 M 的修改顺序中出现后于 X 的副效应 Y 求得其值。</li>
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</ul>
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<p>一些原子运算亦是同步操作：它们可以拥有附加的释放语义、获取语义，或顺序一致语义。见 <span class="t-lc"><a href="c-atomic-memory_order.html">memory_order</a></span> 。</p>
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<p>内建的<a href="c-language-operator_incdec.html">自增减运算符</a>和<a href="c-language-operator_assignment.html">复合赋值运算符</a>是拥有完全序列一致顺序（如同用 <span class="t-lc"><a href="c-atomic-memory_order.html">memory_order_seq_cst</a></span> ）的读-修改-写操作。若想要较不严格的同步语义，则可以用<a href="c-atomic.html">标准库函数</a>替代。</p>
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<p>原子属性仅对<a href="c-language-value_category.html">左值表达式</a>有意义。左值到右值转换（模仿从原子区域到 CPU 寄存器的内存读取）会把原子性及其他限定符剥去。</p>
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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<tr>
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：更多和 <code>memory_order</code> 及原子库页面的综述互动</td>
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</tr>
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</table>
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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>若编译器定义了宏常量 <code>__STDC_NO_ATOMICS__</code> ，则不提供关键词 <span class="t-c"><span class="mw-geshi c source-c">_Atomic</span></span> 以及 <code><stdatomic.h></code> 。</p>
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<p>访问原子结构体/联合体的成员是未定义行为。</p>
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<p>库类型 <span class="t-c"><span class="mw-geshi c source-c"><a href="c-program-sig_atomic_t.html"><span class="kw499">sig_atomic_t</span></a></span></span> 不提供线程间同步或内存顺序，仅提供原子性。</p>
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<p><a href="c-language-volatile.html">volatile</a> 类型不提供线程间同步、内存顺序或原子性。</p>
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<p>推荐实现确保对于每个可能的类型 <code>T</code> ， C 中 <span class="t-c"><span class="mw-geshi c source-c">_Atomic<span class="br0">(</span>T<span class="br0">)</span></span></span> 的表示与 C++ 中 <span class="t-c"><span class="mw-geshi c source-c">std<span class="sy4">::</span><span class="me2">atomic</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 的相同。用于确保原子性与内存顺序的机制应该兼容。</p>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="c-keyword-_Atomic.html"><tt>_Atomic</tt></a></p>
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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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<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="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <threads.h></span>
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<span class="co2">#include <stdatomic.h></span>
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<a href="c-atomic.html"><span class="kw965">atomic_int</span></a> acnt<span class="sy4">;</span>
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<span class="kw4">int</span> cnt<span class="sy4">;</span>
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<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>
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<span class="br0">{</span>
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<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>
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<span class="sy2">++</span>cnt<span class="sy4">;</span>
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<span class="sy2">++</span>acnt<span class="sy4">;</span>
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<span class="co1">// 对于此例，宽松内存顺序是足够的，例如</span>
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<span class="co1">// atomic_fetch_add_explicit(&acnt, 1, memory_order_relaxed);</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-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>
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<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>
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<a href="c-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="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
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<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>
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<a href="c-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="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-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>
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<a href="c-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>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">The atomic counter is 10000
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The non-atomic counter is 8644</pre></div>
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</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-17">
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<ul>
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<li>C17 标准（ISO/IEC 9899:2018）：</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>6.7.2.4 Atomic type specifiers (p: 87)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>7.17 Atomics <stdatomic.h> (p: 200-209)</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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<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>6.7.2.4 Atomic type specifiers (p: 121)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>7.17 Atomics <stdatomic.h> (p: 273-286)</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"></table>
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<div class="visualClear"></div>
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