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| <a name="SYSTEM_005fCLOCK-_002d_002d_002d-Time-function"></a> |
| <h3 class="section">8.239 <code>SYSTEM_CLOCK</code> — Time function</h3> |
| <a name="index-SYSTEM_005fCLOCK"></a> |
| <a name="index-time_002c-clock-ticks-2"></a> |
| <a name="index-clock-ticks-2"></a> |
| |
| <dl compact="compact"> |
| <dt><em>Description</em>:</dt> |
| <dd><p>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 |
| monotonic clock, that clock is used and can, depending on the platform |
| clock implementation, provide up to nanosecond resolution. If a |
| monotonic clock is not available, the implementation falls back to a |
| realtime clock. |
| </p> |
| <p><var>COUNT_RATE</var> is system dependent and can vary depending on the |
| kind of the arguments. For <var>kind=4</var> arguments, <var>COUNT</var> |
| represents milliseconds, while for <var>kind=8</var> arguments, <var>COUNT</var> |
| typically represents micro- or nanoseconds depending on resolution of |
| the underlying platform clock. <var>COUNT_MAX</var> usually equals |
| <code>HUGE(COUNT_MAX)</code>. Note that the millisecond resolution of the |
| <var>kind=4</var> version implies that the <var>COUNT</var> will wrap around in |
| roughly 25 days. In order to avoid issues with the wrap around and for |
| more precise timing, please use the <var>kind=8</var> version. |
| </p> |
| <p>If there is no clock, or querying the clock fails, <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> |
| <p>When running on a platform using the GNU C library (glibc) version |
| 2.16 or older, 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. |
| </p> |
| <p>On the Windows platform, the version with <var>kind=4</var> arguments uses |
| the <code>GetTickCount</code> function, whereas the <var>kind=8</var> version |
| uses <code>QueryPerformanceCounter</code> and |
| <code>QueryPerformanceCounterFrequency</code>. For more information, and |
| potential caveats, please see the platform documentation. |
| </p> |
| </dd> |
| <dt><em>Standard</em>:</dt> |
| <dd><p>Fortran 95 and later |
| </p> |
| </dd> |
| <dt><em>Class</em>:</dt> |
| <dd><p>Subroutine |
| </p> |
| </dd> |
| <dt><em>Syntax</em>:</dt> |
| <dd><p><code>CALL SYSTEM_CLOCK([COUNT, COUNT_RATE, COUNT_MAX])</code> |
| </p> |
| </dd> |
| <dt><em>Arguments</em>:</dt> |
| <dd><table> |
| <tr><td width="15%"><var>COUNT</var></td><td width="70%">(Optional) shall be a scalar of type |
| <code>INTEGER</code> with <code>INTENT(OUT)</code>.</td></tr> |
| <tr><td width="15%"><var>COUNT_RATE</var></td><td width="70%">(Optional) shall be a scalar of type |
| <code>INTEGER</code> with <code>INTENT(OUT)</code>.</td></tr> |
| <tr><td width="15%"><var>COUNT_MAX</var></td><td width="70%">(Optional) shall be a scalar of type |
| <code>INTEGER</code> with <code>INTENT(OUT)</code>.</td></tr> |
| </table> |
| |
| </dd> |
| <dt><em>Example</em>:</dt> |
| <dd><div class="smallexample"> |
| <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></div> |
| |
| </dd> |
| <dt><em>See also</em>:</dt> |
| <dd><p><a href="DATE_005fAND_005fTIME.html#DATE_005fAND_005fTIME">DATE_AND_TIME</a>, <a href="CPU_005fTIME.html#CPU_005fTIME">CPU_TIME</a> |
| </p></dd> |
| </dl> |
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