221 lines
19 KiB
HTML
221 lines
19 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::counting_semaphore, std::binary_semaphore</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_thread_counting_semaphore skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>counting_semaphore, <span style="font-size:0.7em; line-height:130%">std::</span>binary_semaphore</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-thread.html">thread</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><a href="cpp-header-semaphore.html"><semaphore></a></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><a href="cpp-types-ptrdiff_t.html"><span class="kw101">std::<span class="me2">ptrdiff_t</span></span></a> LeastMaxValue <span class="sy1">=</span> <span class="coMULTI">/* implementation-defined */</span><span class="sy1">></span><br />
|
|
<span class="kw1">class</span> counting_semaphore<span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(1)</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> binary_semaphore <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">counting_semaphore</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(2)</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="t-li1"><span class="t-li">1)</span> <code>counting_semaphore</code> 是一个轻量同步元件,能控制对共享资源的访问。不同于 <span class="t-lc"><a href="cpp-thread-mutex.html">std::mutex</a></span> 、 <code>counting_semaphore</code> 允许同一资源有多于一个同时访问,至少允许 <code>LeastMaxValue</code> 个同时的访问者若<code>LeastMaxValue</code> 为负则程序为谬构。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> <code>binary_semaphore</code> 是 <span class="t-lc">std::counting_semaphore</span> 的特化的别名,其 <code>LeastMaxValue</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> 。实现可能将 <code>binary_semaphore</code> 实现得比 <span class="t-lc">std::counting_semaphore</span> 的默认实现更高效。</div>
|
|
<p><code>counting_semaphore</code> 含有由构造函数初始化的内部计数器。由调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">acquire<span class="br0">(</span><span class="br0">)</span></span></span> 与相关方法减少此计数器,而它通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">release<span class="br0">(</span><span class="br0">)</span></span></span> 增加。计数器为零时, <span class="t-c"><span class="mw-geshi cpp source-cpp">acquire<span class="br0">(</span><span class="br0">)</span></span></span> 阻塞该计数器直至它增加,但 <span class="t-c"><span class="mw-geshi cpp source-cpp">try_acquire<span class="br0">(</span><span class="br0">)</span></span></span> 不阻塞; <span class="t-c"><span class="mw-geshi cpp source-cpp">try_acquire_for<span class="br0">(</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">try_acquire_until<span class="br0">(</span><span class="br0">)</span></span></span> 阻塞直至计数器增加或到达时限。</p>
|
|
<p>类似 <span class="t-lc"><a href="cpp-thread-condition_variable.html">std::condition_variable</a></span> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">wait<span class="br0">(</span><span class="br0">)</span></span></span> , <code>counting_semaphore</code> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">try_acquire<span class="br0">(</span><span class="br0">)</span></span></span> 可能虚假地失败。</p>
|
|
<p><code>std::counting_semaphore</code> 的特化非<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a> 、<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 、<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 、<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 或<a href="cpp-named_req-MoveAssignable.html"><span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span></a> 。</p>
|
|
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div t-dsc-member-nobold-div">
|
|
<div><a href="cpp-thread-counting_semaphore-counting_semaphore.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>构造 <code>counting_semaphore</code><br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div t-dsc-member-nobold-div">
|
|
<div><a href="https://zh.cppreference.com/w/cpp/thread/counting_semaphore/%7Ecounting_semaphore" title="cpp/thread/counting semaphore/~counting semaphore"><span class="t-lines"><span>(析构函数)</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>销毁 <code>counting_semaphore</code><br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><span class="t-lines"><span>operator=</span></span></div>
|
|
<div><span class="t-lines"><span><span class="t-cmark">[被删除]</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><code>counting_semaphore</code> 不可赋值<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="2">
|
|
<h5><span class="mw-headline" id=".E6.93.8D.E4.BD.9C">操作</span></h5>
|
|
</td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-counting_semaphore-release.html"><span class="t-lines"><span>release</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>增加内部计数器并除阻获取者<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-counting_semaphore-acquire.html"><span class="t-lines"><span>acquire</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>减少内部计数器或阻塞到直至能如此<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-counting_semaphore-try_acquire.html"><span class="t-lines"><span>try_acquire</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>尝试减少内部计数器而不阻塞<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-counting_semaphore-try_acquire_for.html"><span class="t-lines"><span>try_acquire_for</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>尝试减少内部计数器,至多阻塞一段时长<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-counting_semaphore-try_acquire_until.html"><span class="t-lines"><span>try_acquire_until</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>尝试减少内部计数器,阻塞直至一个时间点<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="2">
|
|
<h5><span class="mw-headline" id=".E5.B8.B8.E9.87.8F">常量</span></h5>
|
|
</td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-counting_semaphore-max.html"><span class="t-lines"><span>max</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>返回内部计数器的最大可能值<br />
|
|
<span class="t-mark">(公开静态成员函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>如其名所示, <code>LeastMaxValue</code> 是<i>最小</i>的最大值,而非<i>实际</i>最大值。从而 <span class="t-c"><span class="mw-geshi cpp source-cpp">max<span class="br0">(</span><span class="br0">)</span></span></span> 能产生大于 <code>LeastMaxValue</code> 的值。</p>
|
|
<p>不同于 <span class="t-lc"><a href="cpp-thread-mutex.html">std::mutex</a></span> , <code>counting_semaphore</code> 不捆绑到执行线程——能在不同于释放信号量的线程获取该信号量。能同时进行 <code>counting_semaphore</code> 上的所有操作而无需联系到任何特定的执行线程,除了不能同时执行,但能在一个不同的线程上执行析构函数。</p>
|
|
<p>信号量亦常用于发信/提醒而非互斥,通过初始化该信号量为 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 从而阻塞尝试 <span class="t-c"><span class="mw-geshi cpp source-cpp">acquire<span class="br0">(</span><span class="br0">)</span></span></span> 的接收者,直至提醒者通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">release<span class="br0">(</span>n<span class="br0">)</span></span></span> “发信”。在此方面可把信号量当作 <span class="t-lc"><a href="cpp-thread-condition_variable.html">std::condition_variable</a></span> 的替用品,通常它有更好的性能。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<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 <iostream></span>
|
|
<span class="co2">#include <thread></span>
|
|
<span class="co2">#include <chrono></span>
|
|
<span class="co2">#include <semaphore></span>
|
|
<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 全局二元信号量实例</span>
|
|
<span class="co1">// 设置对象计数为零</span>
|
|
<span class="co1">// 对象在未被发信状态</span>
|
|
std<span class="sy4">::</span><span class="me2">binary_semaphore</span> smphSignal<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">void</span> ThreadProc<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// 通过尝试减少信号量的计数等待来自主程序的信号</span>
|
|
smphSignal.<span class="me1">acquire</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 此调用阻塞直至信号量的计数被从主程序增加</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">"[thread] Got the signal"</span> <span class="sy1"><<</span> <a href="cpp-io-manip-endl.html"><span class="kw1791">std::<span class="me2">endl</span></span></a><span class="sy4">;</span> <span class="co1">// 回应消息</span>
|
|
|
|
<span class="co1">// 等待 3 秒以模仿某种线程正在进行的工作</span>
|
|
<a href="cpp-thread-sleep_for.html"><span class="kw2149">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span>3s<span class="br0">)</span><span class="sy4">;</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">"[thread] Send the signal<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 消息</span>
|
|
|
|
<span class="co1">// 对主程序回复发信</span>
|
|
smphSignal.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</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>
|
|
<span class="co1">// 创建某个背景工作线程,它将长期存在</span>
|
|
std<span class="sy4">::</span><span class="me2">jthread</span> thrWorker<span class="br0">(</span>ThreadProc<span class="br0">)</span><span class="sy4">;</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">"[main] Send the signal<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 消息</span>
|
|
|
|
<span class="co1">// 通过增加信号量的计数对工作线程发信以开始工作</span>
|
|
smphSignal.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// release() 后随 acquire() 可以阻止工作线程获取信号量,所以添加延迟:</span>
|
|
<a href="cpp-thread-sleep_for.html"><span class="kw2149">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span>50ms<span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 通过试图减少信号量的计数等待直至工作线程完成工作</span>
|
|
smphSignal.<span class="me1">acquire</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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">"[main] Got the signal<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 回应消息</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">[main] Send the signal
|
|
[thread] Got the signal
|
|
[thread] Send the signal
|
|
[main] Got the signal</pre></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|