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<title>std::counting_semaphore<LeastMaxValue>::acquire</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::counting_semaphore<LeastMaxValue>::</span>acquire</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-thread.html">thread</a>‎ | <a href="cpp-thread-counting_semaphore.html">counting semaphore</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> acquire<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>若内部计数器大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 则尝试将它减少 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> ；否则阻塞直至它大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 且能成功减少内部计数器。</p>
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<h3><span class="mw-headline" id=".E5.89.8D.E6.9D.A1.E4.BB.B6">前条件</span></h3>
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<p>（无）</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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<p>（无）</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>可能抛出 <span class="t-lc"><a href="cpp-error-system_error.html">std::system_error</a></span> 。</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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<p>该示例可视化数个随机化线程在不多于 N 个活跃线程函数，而且其他可能在信号量上等待时的并发作业（ N 为信号量 <code><i>desired</i></code> 值）。</p>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <array></span>
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<span class="co2">#include <chrono></span>
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<span class="co2">#include <cstddef></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <mutex></span>
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<span class="co2">#include <random></span>
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<span class="co2">#include <semaphore></span>
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<span class="co2">#include <thread></span>
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<span class="co2">#include <vector></span>
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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> max_threads<span class="br0">{</span>10U<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 更改并查看效果</span>
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<span class="kw4">constexpr</span> <a href="cpp-types-ptrdiff_t.html"><span class="kw101">std::<span class="me2">ptrdiff_t</span></span></a> max_sema_threads<span class="br0">{</span><span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 对二元信号量为 {1}</span>
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std<span class="sy4">::</span><span class="me2">counting_semaphore</span> semaphore<span class="br0">{</span>max_sema_threads<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> time_tick<span class="br0">{</span>10ms<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">unsigned</span> rnd<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">static</span> <a href="cpp-numeric-random-uniform_int_distribution.html"><span class="kw1502">std::<span class="me2">uniform_int_distribution</span></span></a><span class="sy1"><</span><span class="kw4">unsigned</span><span class="sy1">></span> distribution<span class="br0">{</span>2U, 9U<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// [延迟]</span>
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<span class="kw4">static</span> <a href="cpp-numeric-random-random_device.html"><span class="kw1486">std::<span class="me2">random_device</span></span></a> engine<span class="sy4">;</span>
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<span class="kw4">static</span> <a href="cpp-numeric-random-mersenne_twister_engine.html"><span class="kw1489">std::<span class="me2">mt19937</span></span></a> noise<span class="br0">{</span>engine<span class="br0">(</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">return</span> distribution<span class="br0">(</span>noise<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">class</span> alignas<span class="br0">(</span> <span class="nu0">128</span> <span class="coMULTI">/*std::hardware_destructive_interference_size*/</span> <span class="br0">)</span> Guide <span class="br0">{</span>
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<span class="kw1">inline</span> <span class="kw4">static</span> <a href="cpp-thread-mutex.html"><span class="kw2154">std::<span class="me2">mutex</span></span></a> cout_mutex<span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">static</span> <a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span><a href="cpp-chrono-high_resolution_clock.html"><span class="kw1008">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span></span></a><span class="sy1">></span> started_at<span class="sy4">;</span>
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<span class="kw4">unsigned</span> delay<span class="br0">{</span>rnd<span class="br0">(</span><span class="br0">)</span><span class="br0">}</span>, occupy<span class="br0">{</span>rnd<span class="br0">(</span><span class="br0">)</span><span class="br0">}</span>, wait_on_sema<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw4">static</span> <span class="kw4">void</span> start_time<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> started_at <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> initial_delay<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</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>delay <span class="sy2">*</span> time_tick<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> occupy_sema<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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wait_on_sema <span class="sy1">=</span>
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<span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">unsigned</span><span class="sy1">></span><span class="br0">(</span><a href="cpp-chrono-duration-duration_cast.html"><span class="kw1000">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration_cast</span></span></a><span class="sy1"><</span><a href="cpp-chrono-duration.html"><span class="kw996">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span></span></a><span class="sy1">></span><span class="br0">(</span>
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std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> started_at <span class="sy2">-</span> delay <span class="sy2">*</span> time_tick<span class="br0">)</span>
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.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">/</span> time_tick.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<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>occupy <span class="sy2">*</span> time_tick<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> visualize<span class="br0">(</span><span class="kw4">unsigned</span> id, <span class="kw4">unsigned</span> x_scale <span class="sy1">=</span> <span class="nu0">2</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<span class="kw4">auto</span> cout_n <span class="sy1">=</span> <span class="br0">[</span><span class="sy1">=</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">auto</span> str, <span class="kw4">unsigned</span> n<span class="br0">)</span> <span class="br0">{</span>
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n <span class="sy2">*</span><span class="sy1">=</span> x_scale<span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span>n<span class="sy2">--</span> <span class="sy1">></span> <span class="nu0">0</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> str<span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-thread-lock_guard.html"><span class="kw2166">std::<span class="me2">lock_guard</span></span></a> lk<span class="br0">{</span>cout_mutex<span class="br0">}</span><span class="sy4">;</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">"#"</span> <span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span> <span class="sy1"><<</span> id <span class="sy1"><<</span> <span class="st0">" "</span><span class="sy4">;</span>
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cout_n<span class="br0">(</span><span class="st0">"░"</span>, delay<span class="br0">)</span><span class="sy4">;</span>
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cout_n<span class="br0">(</span><span class="st0">"▒"</span>, wait_on_sema<span class="br0">)</span><span class="sy4">;</span>
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cout_n<span class="br0">(</span><span class="st0">"█"</span>, occupy<span class="br0">)</span><span class="sy4">;</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">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">static</span> <span class="kw4">void</span> show_info<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>
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<span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>Threads: "</span> <span class="sy1"><<</span> max_threads <span class="sy1"><<</span> <span class="st0">", Throughput: "</span> <span class="sy1"><<</span> max_sema_threads
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<span class="sy1"><<</span> <span class="st0">" │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██ <span class="es1">\n</span>"</span>
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<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>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span>Guide, max_threads<span class="sy1">></span> guides<span class="sy4">;</span>
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<span class="kw4">void</span> workerThread<span class="br0">(</span><span class="kw4">unsigned</span> id<span class="br0">)</span> <span class="br0">{</span>
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guides<span class="br0">[</span>id<span class="br0">]</span>.<span class="me1">initial_delay</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 模拟获取旗标前的某些工作</span>
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semaphore.<span class="me1">acquire</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 等待直至空闲旗标槽可用</span>
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guides<span class="br0">[</span>id<span class="br0">]</span>.<span class="me1">occupy_sema</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 模拟获取旗标时的某些工作</span>
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semaphore.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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guides<span class="br0">[</span>id<span class="br0">]</span>.<span class="me1">visualize</span><span class="br0">(</span>id<span class="br0">)</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="br0">)</span> <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>std<span class="sy4">::</span><span class="me2">jthread</span><span class="sy1">></span> threads<span class="sy4">;</span>
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threads.<span class="me1">reserve</span><span class="br0">(</span>max_threads<span class="br0">)</span><span class="sy4">;</span>
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Guide<span class="sy4">::</span><span class="me2">show_info</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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Guide<span class="sy4">::</span><span class="me2">start_time</span><span class="br0">(</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">auto</span> id<span class="br0">{</span>0U<span class="br0">}</span><span class="sy4">;</span> id <span class="sy3">!</span><span class="sy1">=</span> max_threads<span class="sy4">;</span> <span class="sy2">++</span>id<span class="br0">)</span> <span class="br0">{</span>
|
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threads.<span class="me1">push_back</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">jthread</span><span class="br0">(</span>workerThread, id<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>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">Default case: max_threads{10U}, max_sema_threads{3}
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|
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Threads: 10, Throughput: 3 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
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# 1 ░░░░██████
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# 0 ░░░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████████████
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──────────────────────────────────────────────────────────────────────────────────────────────────────────────
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"Enough for everyone" case (no wait states!): max_threads{10U}, max_sema_threads{10}
|
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Threads: 10, Throughput: 10 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
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# 4 ░░░░██████
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# 5 ░░░░░░████
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# 9 ░░░░░░░░░░░░░░░░██████████
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──────────────────────────────────────────────────────────────────────────────────────────────────────────────
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Binary semaphore case: max_threads{10U}, max_sema_threads{1}
|
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|
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Threads: 10, Throughput: 1 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
|
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# 6 ░░░░████
|
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# 5 ░░░░▒▒▒▒████
|
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# 4 ░░░░░░░░░░▒▒██████████
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# 7 ░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒████████████████
|
||||
# 2 ░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████
|
||||
# 3 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████████████
|
||||
# 0 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████████
|
||||
# 1 ░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████
|
||||
# 8 ░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████
|
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# 9 ░░░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████████████</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
<div class="visualClear"></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user