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<h3 class="section">8.235 <code>SYSTEM_CLOCK</code> — Time function</h3>
<p><a name="index-SYSTEM_005fCLOCK-1054"></a><a name="index-time_002c-clock-ticks-1055"></a><a name="index-clock-ticks-1056"></a>
<dl>
<dt><em>Description</em>:<dd>Determines the <var>COUNT</var> of a processor clock since an unspecified
time in the past modulo <var>COUNT_MAX</var>, <var>COUNT_RATE</var> determines
the number of clock ticks per second. If the platform supports a high
resolution monotonic clock, that clock is used and can provide up to
nanosecond resolution. If a high resolution monotonic clock is not
available, the implementation falls back to a potentially lower
resolution realtime clock.
<p><var>COUNT_RATE</var> and <var>COUNT_MAX</var> vary depending on the kind of the
arguments. For <var>kind=8</var> arguments, <var>COUNT</var> represents
nanoseconds, and for <var>kind=4</var> arguments, <var>COUNT</var> represents
milliseconds. Other than the kind dependency, <var>COUNT_RATE</var> and
<var>COUNT_MAX</var> are constant, however the particular values are
specific to <samp><span class="command">gfortran</span></samp>.
<p>If there is no clock, <var>COUNT</var> is set to <code>-HUGE(COUNT)</code>, and
<var>COUNT_RATE</var> and <var>COUNT_MAX</var> are set to zero.
<p>When running on a platform using the GNU C library (glibc), or a
derivative thereof, the high resolution monotonic clock is available
only when linking with the <var>rt</var> library. This can be done
explicitly by adding the <code>-lrt</code> flag when linking the
application, but is also done implicitly when using OpenMP.
<br><dt><em>Standard</em>:<dd>Fortran 95 and later
<br><dt><em>Class</em>:<dd>Subroutine
<br><dt><em>Syntax</em>:<dd><code>CALL SYSTEM_CLOCK([COUNT, COUNT_RATE, COUNT_MAX])</code>
<br><dt><em>Arguments</em>:<dd>
<p><table summary=""><tr align="left"><td valign="top" width="15%"><var>COUNT</var> </td><td valign="top" width="70%">(Optional) shall be a scalar of type
<code>INTEGER</code> with <code>INTENT(OUT)</code>.
<br></td></tr><tr align="left"><td valign="top" width="15%"><var>COUNT_RATE</var> </td><td valign="top" width="70%">(Optional) shall be a scalar of type
<code>INTEGER</code> with <code>INTENT(OUT)</code>.
<br></td></tr><tr align="left"><td valign="top" width="15%"><var>COUNT_MAX</var> </td><td valign="top" width="70%">(Optional) shall be a scalar of type
<code>INTEGER</code> with <code>INTENT(OUT)</code>.
<br></td></tr></table>
<br><dt><em>Example</em>:<dd>
<pre class="smallexample"> PROGRAM test_system_clock
INTEGER :: count, count_rate, count_max
CALL SYSTEM_CLOCK(count, count_rate, count_max)
WRITE(*,*) count, count_rate, count_max
END PROGRAM
</pre>
<br><dt><em>See also</em>:<dd><a href="DATE_005fAND_005fTIME.html#DATE_005fAND_005fTIME">DATE_AND_TIME</a>, <a href="CPU_005fTIME.html#CPU_005fTIME">CPU_TIME</a>
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