126 lines
12 KiB
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126 lines
12 KiB
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<title>std::clock</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_chrono_c_clock 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>clock</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-chrono.html">chrono</a>‎ | <a href="cpp-chrono-c.html">c</a></span></div>
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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-ctime.html"><ctime></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-chrono-c-clock_t.html"><span class="kw1024">std::<span class="me2">clock_t</span></span></a> clock<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>返回进程从关联到程序执行的实现定义时期开始，所用的粗略处理器时间。为转换结果为秒，可将它除以 <span class="t-lc"><a href="cpp-chrono-c-CLOCKS_PER_SEC.html">CLOCKS_PER_SEC</a></span> 。</p>
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<p>只有二次不同的 <code>std::clock</code> 调用所返回的值之差才有意义，因为 <code>std::clock</code> 时期的开始不必与程序的起始一致。 <code>std::clock</code> 可能前进快于或慢于现实时间，取决于操作系统给予程序的执行资源。例如，若与其他进程共享 CPU ，则 <code>std::clock</code> 时间的前进可能慢于现实时间。另一方面，若当前进程为多线程，且多于一个执行核心可用，则 <code>std::clock</code> 时间的前进可能快于现实时间。</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=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>程序迄今为止所用的处理器时间。若该信息不可用，或若其值无法表示，则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-chrono-c-clock_t.html"><span class="kw1024">std::<span class="me2">clock_t</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span></span></span> 。</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>不抛出。</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>在兼容 POSIX 的系统上，带时钟 id <span class="t-c"><span class="mw-geshi cpp source-cpp">CLOCK_PROCESS_CPUTIME_ID</span></span> 的 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_getres.html"><code>clock_gettime</code></a> 提供更高的解析度。</p>
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<p><code>clock()</code> 返回的值会在一些不遵从的实现上回卷。例如在某种实现上，若 <span class="t-lc"><a href="cpp-chrono-c-clock_t.html">std::clock_t</a></span> 为有符号 32 位整数而 <span class="t-lc"><a href="cpp-chrono-c-CLOCKS_PER_SEC.html">CLOCKS_PER_SEC</a></span> 为 1000000 ，则它将在约 2147 秒（约 36 分）后回卷。</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>此示例演示 <code>clock()</code> 时间与真实时间的差异</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 <iostream></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <chrono></span>
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<span class="co2">#include <ctime></span>
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<span class="co2">#include <thread></span>
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<span class="co1">// 函数 f() 做一些耗时工作</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">volatile</span> <span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu0">0</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">10000</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> m<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> m<span class="sy1"><</span><span class="nu0">10000</span><span class="sy4">;</span> <span class="sy2">++</span>m<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">double</span> diff <span class="sy1">=</span> d<span class="sy2">*</span>n<span class="sy2">*</span>m<span class="sy4">;</span>
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d <span class="sy1">=</span> diff <span class="sy2">+</span> d<span class="sy4">;</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-chrono-c-clock_t.html"><span class="kw1024">std::<span class="me2">clock_t</span></span></a> c_start <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">clock</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> t_start <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>
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<a href="cpp-thread-thread.html"><span class="kw2145">std::<span class="me2">thread</span></span></a> t1<span class="br0">(</span>f<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-thread-thread.html"><span class="kw2145">std::<span class="me2">thread</span></span></a> t2<span class="br0">(</span>f<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 在二个线程上调用 f()</span>
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t1.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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t2.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-chrono-c-clock_t.html"><span class="kw1024">std::<span class="me2">clock_t</span></span></a> c_end <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">clock</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> t_end <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>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1783">std::<span class="me2">fixed</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setprecision.html"><span class="kw1795">std::<span class="me2">setprecision</span></span></a><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"CPU time used: "</span>
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<span class="sy1"><<</span> <span class="nu16">1000.0</span> <span class="sy2">*</span> <span class="br0">(</span>c_end <span class="sy2">-</span> c_start<span class="br0">)</span> <span class="sy2">/</span> <a href="cpp-chrono-c-CLOCKS_PER_SEC.html"><span class="kw1021">CLOCKS_PER_SEC</span></a> <span class="sy1"><<</span> <span class="st0">" ms<span class="es1">\n</span>"</span>
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<span class="sy1"><<</span> <span class="st0">"Wall clock time passed: "</span>
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<span class="sy1"><<</span> <a href="cpp-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span><span class="kw4">double</span>, <a href="cpp-numeric-ratio-ratio.html"><span class="kw1527">std::<span class="me2">milli</span></span></a><span class="sy1">></span><span class="br0">(</span>t_end<span class="sy2">-</span>t_start<span class="br0">)</span>.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" ms<span class="es1">\n</span>"</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">CPU time used: 1590.00 ms
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Wall clock time passed: 808.23 ms</pre></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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<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-chrono-c-ctime.html"><span class="t-lines"><span>ctime</span></span></a></div>
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</div>
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</td>
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<td>转换 <code>time_t</code> 对象为文本表示<br />
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<span class="t-mark">(函数)</span></td>
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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-chrono-c-time.html"><span class="t-lines"><span>time</span></span></a></div>
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</div>
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</td>
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<td>返回自纪元起计的系统当前时间<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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</body>
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