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<tr class="t-nv-h1"><td colspan="5"> Types</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="tm.html" title="c/chrono/tm"><span class="t-lines"><span>tm</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="time_t.html" title="c/chrono/time t"><span class="t-lines"><span>time_t</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="clock_t.html" title="c/chrono/clock t"><span class="t-lines"><span>clock_t</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="timespec.html" title="c/chrono/timespec"><span class="t-lines"><span>timespec</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div></div></td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:c/chrono/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>Defined in header <code><time.h></code>
</div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi c source-c"><a href="clock_t.html"><span class="kw524">clock_t</span></a> clock<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns the approximate processor time used by the process since the beginning of an implementation-defined era related to the program's execution. To convert result value to seconds, divide it by <span class="t-lc"><a href="CLOCKS_PER_SEC.html" title="c/chrono/CLOCKS PER SEC">CLOCKS_PER_SEC</a></span>.
</p><p>Only the difference between two values returned by different calls to <span class="t-lc"><strong class="selflink">clock</strong></span> is meaningful, as the beginning of the <span class="t-lc"><strong class="selflink">clock</strong></span> era does not have to coincide with the start of the program. <span class="t-lc"><strong class="selflink">clock</strong></span> time may advance faster or slower than the wall clock, depending on the execution resources given to the program by the operating system. For example, if the CPU is shared by other processes, <span class="t-lc"><strong class="selflink">clock</strong></span> time may advance slower than wall clock. On the other hand, if the current process is multithreaded and more than one execution core is available, <span class="t-lc"><strong class="selflink">clock</strong></span> time may advance faster than wall clock.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="clock.html#Parameters"><span class="tocnumber">1</span> <span class="toctext">Parameters</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="clock.html#Return_value"><span class="tocnumber">2</span> <span class="toctext">Return value</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="clock.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="clock.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="clock.html#References"><span class="tocnumber">5</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="clock.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/chrono/clock&action=edit&section=1" title="Edit section: Parameters">edit</a>]</span> <span class="mw-headline" id="Parameters">Parameters</span></h3>
<p>(none)
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/chrono/clock&action=edit&section=2" title="Edit section: Return value">edit</a>]</span> <span class="mw-headline" id="Return_value">Return value</span></h3>
<p>Processor time used by the program so far or <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">(</span><a href="clock_t.html"><span class="kw524">clock_t</span></a><span class="br0">)</span><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span></span></span> if that information is unavailable.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/chrono/clock&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>On POSIX-compatible systems, <code><a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_getres.html">clock_gettime</a></code> with clock id <span class="t-c"><span class="mw-geshi c source-c">CLOCK_PROCESS_CPUTIME_ID</span></span> offers better resolution.
</p><p>The value returned by <code>clock()</code> may wrap around on some implementations. For example, on a machine with 32-bit <span class="t-lc"><a href="clock_t.html" title="c/chrono/clock t">clock_t</a></span>, it wraps after 2147 seconds or 36 minutes.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/chrono/clock&action=edit&section=4" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><p> This example demonstrates the difference between clock() time and real time.
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include <stdio.h></span>
<span class="co2">#include <time.h></span>
<span class="co2">#include <stdlib.h></span>
<span class="co2">#include <threads.h> // pthread.h in POSIX</span>
 
<span class="co1">// the function f() does some time-consuming work</span>
<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> <span class="co1">// return void* in POSIX</span>
<span class="br0">{</span>
<span class="kw4">volatile</span> <span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<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>
<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>
d <span class="sy2">+</span><span class="sy1">=</span> d<span class="sy2">*</span>n<span class="sy2">*</span>m<span class="sy4">;</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">struct</span> timespec ts1, tw1<span class="sy4">;</span> <span class="co1">// both C11 and POSIX</span>
clock_gettime<span class="br0">(</span>CLOCK_PROCESS_CPUTIME_ID, <span class="sy3">&</span>ts1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// POSIX</span>
clock_gettime<span class="br0">(</span>CLOCK_MONOTONIC, <span class="sy3">&</span>tw1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// POSIX; use timespec_get in C11</span>
<a href="clock_t.html"><span class="kw524">clock_t</span></a> t1 <span class="sy1">=</span> clock<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<a href="../thread.html"><span class="kw995">thrd_t</span></a> thr1, thr2<span class="sy4">;</span> <span class="co1">// C11; use pthread_t in POSIX</span>
<a href="../thread/thrd_create.html"><span class="kw996">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>thr1, f, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C11; use pthread_create in POSIX</span>
<a href="../thread/thrd_create.html"><span class="kw996">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>thr2, f, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="../thread/thrd_join.html"><span class="kw1003">thrd_join</span></a><span class="br0">(</span>thr1, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C11; use pthread_join in POSIX</span>
<a href="../thread/thrd_join.html"><span class="kw1003">thrd_join</span></a><span class="br0">(</span>thr2, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> timespec ts2, tw2<span class="sy4">;</span>
clock_gettime<span class="br0">(</span>CLOCK_PROCESS_CPUTIME_ID, <span class="sy3">&</span>ts2<span class="br0">)</span><span class="sy4">;</span>
clock_gettime<span class="br0">(</span>CLOCK_MONOTONIC, <span class="sy3">&</span>tw2<span class="br0">)</span><span class="sy4">;</span>
<a href="clock_t.html"><span class="kw524">clock_t</span></a> t2 <span class="sy1">=</span> clock<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">double</span> dur <span class="sy1">=</span> <span class="nu16">1000.0</span><span class="sy2">*</span><span class="br0">(</span>t2<span class="sy2">-</span>t1<span class="br0">)</span><span class="sy2">/</span><a href="CLOCKS_PER_SEC.html"><span class="kw521">CLOCKS_PER_SEC</span></a><span class="sy4">;</span>
<span class="kw4">double</span> posix_dur <span class="sy1">=</span> <span class="nu16">1000.0</span><span class="sy2">*</span>ts2.<span class="me1">tv_sec</span> <span class="sy2">+</span> <span class="nu19">1e-6</span><span class="sy2">*</span>ts2.<span class="me1">tv_nsec</span>
<span class="sy2">-</span> <span class="br0">(</span><span class="nu16">1000.0</span><span class="sy2">*</span>ts1.<span class="me1">tv_sec</span> <span class="sy2">+</span> <span class="nu19">1e-6</span><span class="sy2">*</span>ts1.<span class="me1">tv_nsec</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">double</span> posix_wall <span class="sy1">=</span> <span class="nu16">1000.0</span><span class="sy2">*</span>tw2.<span class="me1">tv_sec</span> <span class="sy2">+</span> <span class="nu19">1e-6</span><span class="sy2">*</span>tw2.<span class="me1">tv_nsec</span>
<span class="sy2">-</span> <span class="br0">(</span><span class="nu16">1000.0</span><span class="sy2">*</span>tw1.<span class="me1">tv_sec</span> <span class="sy2">+</span> <span class="nu19">1e-6</span><span class="sy2">*</span>tw1.<span class="me1">tv_nsec</span><span class="br0">)</span><span class="sy4">;</span>
 
<a href="../io/fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"CPU time used (per clock(): %.2f ms<span class="es1">\n</span>"</span>, dur<span class="br0">)</span><span class="sy4">;</span>
<a href="../io/fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"CPU time used (per clock_gettime()): %.2f ms<span class="es1">\n</span>"</span>, posix_dur<span class="br0">)</span><span class="sy4">;</span>
<a href="../io/fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"Wall time passed: %.2f ms<span class="es1">\n</span>"</span>, posix_wall<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">CPU time used (per clock(): 1580.00 ms
CPU time used (per clock_gettime()): 1582.76 ms
Wall time passed: 792.13 ms</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/chrono/clock&action=edit&section=5" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011):
</li></ul>
<dl><dd><ul><li> 7.27.2.1 The clock function (p: 389)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 7.23.2.1 The clock function (p: 339)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990):
</li></ul>
<dl><dd><ul><li> 4.12.2.1 The clock function
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/chrono/clock&action=edit&section=6" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="ctime.html" title="c/chrono/ctime"> <span class="t-lines"><span>ctime</span><span>ctime_s</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div></div>
</td>
<td> converts a <code>time_t</code> object to a textual representation <br /> <span class="t-mark">(function)</span> <span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:c/chrono/dsc_ctime&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="time.html" title="c/chrono/time"> <span class="t-lines"><span>time</span></span></a></div></div>
</td>
<td> returns the current calendar time of the system as time since epoch <br /> <span class="t-mark">(function)</span> <span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:c/chrono/dsc_time&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../cpp/chrono/c/clock.html" title="cpp/chrono/c/clock">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>clock</span></span></div>
</td></tr>
</table></div>
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