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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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<title>std::atomic_flag</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_atomic_atomic_flag skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>atomic_flag</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-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><a href="cpp-header-atomic.html"><atomic></a></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span> atomic_flag<span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p><code>std::atomic_flag</code> 是原子布尔类型。不同于所有 <span class="t-lc"><a href="cpp-atomic-atomic.html">std::atomic</a></span> 的特化，它保证是免锁的。不同于 <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><span class="kw4">bool</span><span class="sy1">></span></span></span> ， <code>std::atomic_flag</code> 不提供加载或存储操作。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-atomic-atomic_flag-atomic_flag.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造 atomic_flag<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>赋值运算符<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-clear.html"><span class="t-lines"><span>clear</span></span></a></div>
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</div>
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</td>
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<td>原子地设置标志为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-test_and_set.html"><span class="t-lines"><span>test_and_set</span></span></a></div>
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</div>
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</td>
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<td>原子地设置标志为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 并获得其先前值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-test.html"><span class="t-lines"><span>test</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>原子地返回标志的值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-wait.html"><span class="t-lines"><span>wait</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>阻塞线程直至被提醒且原子值更改<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-notify_one.html"><span class="t-lines"><span>notify_one</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>提醒至少一个在原子对象上的等待中阻塞的线程<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag-notify_all.html"><span class="t-lines"><span>notify_all</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>提醒所有在原子对象上的等待中阻塞的线程<br />
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<span class="t-mark">(公开成员函数)</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>可于用户空间用 atomic_flag 实现自旋互斥</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 <thread></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <atomic></span>
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std<span class="sy4">::</span><span class="me2">atomic_flag</span> lock <span class="sy1">=</span> <a href="cpp-atomic-ATOMIC_FLAG_INIT.html"><span class="kw2090">ATOMIC_FLAG_INIT</span></a><span class="sy4">;</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> n<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> cnt <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> cnt <span class="sy1"><</span> <span class="nu0">100</span><span class="sy4">;</span> <span class="sy2">++</span>cnt<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">while</span> <span class="br0">(</span>lock.<span class="me1">test_and_set</span><span class="br0">(</span><a href="cpp-atomic-memory_order.html"><span class="kw2094">std::<span class="me2">memory_order_acquire</span></span></a><span class="br0">)</span><span class="br0">)</span> <span class="co1">// 获得锁</span>
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<span class="sy4">;</span> <span class="co1">// 自旋</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Output from thread "</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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lock.<span class="me1">clear</span><span class="br0">(</span><a href="cpp-atomic-memory_order.html"><span class="kw2095">std::<span class="me2">memory_order_release</span></span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 释放锁</span>
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<span class="br0">}</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="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-thread-thread.html"><span class="kw2145">std::<span class="me2">thread</span></span></a><span class="sy1">></span> v<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> <span class="br0">{</span>
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v.<span class="me1">emplace_back</span><span class="br0">(</span>f, n<span class="br0">)</span><span class="sy4">;</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">auto</span><span class="sy3">&</span> t <span class="sy4">:</span> v<span class="br0">)</span> <span class="br0">{</span>
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t.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>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">Output from thread 2
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Output from thread 6
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Output from thread 7
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...<exactly 1000 lines>...</pre></div>
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</div>
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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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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag_test_and_set.html"><span class="t-lines"><span>atomic_flag_test_and_set</span><span>atomic_flag_test_and_set_explicit</span></span></a></div>
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<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></div>
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</div>
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</td>
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<td>原子地设置标志为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 并返回其先前值<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag_clear.html"><span class="t-lines"><span>atomic_flag_clear</span><span>atomic_flag_clear_explicit</span></span></a></div>
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<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></div>
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</div>
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</td>
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<td>原子地设置标志值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span><br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag_wait.html"><span class="t-lines"><span>atomic_flag_wait</span><span>atomic_flag_wait_explicit</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>阻塞线程，直至被提醒且标志更改<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag_notify_one.html"><span class="t-lines"><span>atomic_flag_notify_one</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>提醒一个在 <code>atomic_flag_wait</code> 中阻塞的线程<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_flag_notify_all.html"><span class="t-lines"><span>atomic_flag_notify_all</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>提醒所有在 <code>atomic_flag_wait</code> 中阻塞的线程<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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</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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