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<table class="head">
<tr>
<td class="head-ltitle">CLOCKS(7)</td>
<td class="head-vol">Miscellaneous Information Manual</td>
<td class="head-rtitle">CLOCKS(7)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">clocks</code> — <span class="Nd">various
system timers</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="In">#include
<<a class="In">time.h</a>></code></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp"><code class="Dv">HZ</code> is not part of the application
interface in <span class="Ux">BSD</span>.</p>
<p class="Pp">There are many different real and virtual (timekeeping) clocks
with different frequencies:</p>
<ul class="Bl-bullet">
<li>The scheduling clock. This is a real clock with frequency that happens to
be 100. It is not available to applications.</li>
<li>The statistics clock. This is a real clock with frequency that happens to
be 128. It is not directly available to applications.</li>
<li>The clock reported by <a class="Xr">clock(3)</a>. This is a virtual clock
with a frequency that happens to be 128. Its actual frequency is given by
the macro <code class="Dv">CLOCKS_PER_SEC</code>. Note that
<code class="Dv">CLOCKS_PER_SEC</code> may be floating point. Do not use
<a class="Xr">clock(3)</a> in new programs under
<span class="Ux">FreeBSD</span>. It is feeble compared with
<a class="Xr">getrusage(2)</a>. It is provided for ANSI conformance. It is
implemented by calling <a class="Xr">getrusage(2)</a> and throwing away
information and resolution.</li>
<li id="sysconf">The clock reported by <a class="Xr">times(3)</a>. This is a
virtual clock with a frequency that happens to be 128. Its actual
frequency is given by the macro <code class="Dv">CLK_TCK</code>
(deprecated; do not use) and by
<a class="permalink" href="#sysconf"><code class="Fn">sysconf</code></a>(<var class="Fa">_SC_CLK_TCK</var>)
and by <a class="Xr">sysctl(3)</a>. Note that its frequency may be
different from <code class="Dv">CLOCKS_PER_SEC</code>. Do not use
<a class="Xr">times(3)</a> in new programs under
<span class="Ux">FreeBSD</span>. It is feeble compared with
<a class="Xr">gettimeofday(2)</a> together with
<a class="Xr">getrusage(2)</a>. It is provided for POSIX conformance. It
is implemented by calling <a class="Xr">gettimeofday(2)</a> and
<a class="Xr">getrusage(2)</a> and throwing away information and
resolution.</li>
<li>The profiling clock. This is a real clock with frequency 1024. It is used
mainly by <a class="Xr">moncontrol(3)</a> and <a class="Xr">gprof(1)</a>.
Applications should determine its actual frequency using
<a class="Xr">sysctl(3)</a> or by reading it from the header in the
profiling data file.</li>
<li>The mc146818a clock. This is a real clock with a nominal frequency of
32768. It is divided down to give the statistic clock and the profiling
clock. It is not available to applications.</li>
<li>The microseconds clock. This is a virtual clock with frequency 1000000. It
is used for most timekeeping in <span class="Ux">BSD</span> and is
exported to applications in <a class="Xr">getrusage(2)</a>,
<a class="Xr">gettimeofday(2)</a>, <a class="Xr">select(2)</a>,
<a class="Xr">getitimer(2)</a>, etc. This is the clock that should
normally be used by <span class="Ux">BSD</span> applications.</li>
<li>The i8254 clock. This is a real clock/timer with a nominal frequency of
1193182. It has three independent time counters to be used. It is divided
down to give the scheduling clock. It is not available to
applications.</li>
<li>The TSC clock (64-bit register) on fifth-generation or later x86 systems.
This is a real clock with a frequency that is equivalent to the number of
cycles per second of the CPU(s). Its frequency can be found using the
<var class="Va">machdep.tsc_freq</var> sysctl, if it is available. It is
used to interpolate between values of the scheduling clock.</li>
<li>The ACPI clock. This is a real clock/timer with a nominal frequency of
3579545. It is accessed via a 24 or 32 bit register. Unlike the TSC clock,
it maintains a constant tick rate even when the CPU sleeps or its clock
rate changes. It is not available to applications.</li>
</ul>
<p class="Pp">Summary: if <code class="Dv">HZ</code> is not 1000000 then the
application is probably using the wrong clock.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">gprof(1)</a>, <a class="Xr">clock_gettime(2)</a>,
<a class="Xr">getitimer(2)</a>, <a class="Xr">getrusage(2)</a>,
<a class="Xr">gettimeofday(2)</a>, <a class="Xr">select(2)</a>,
<a class="Xr">clock(3)</a>, <a class="Xr">moncontrol(3)</a>,
<a class="Xr">times(3)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">This manual page was written by <span class="An">Jörg
Wunsch</span> after a description posted by <span class="An">Bruce
Evans</span>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">January 18, 2008</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
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