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### 9.265 `SYSTEM_CLOCK` — Time function

Description:

  Determines the COUNT of a processor clock since an unspecified time in the past modulo COUNT_MAX, COUNT_RATE 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.

  COUNT_RATE is system dependent and can vary depending on the kind of the arguments. For kind=4 arguments (and smaller integer kinds), COUNT represents milliseconds, while for kind=8 arguments (and larger integer kinds), COUNT typically represents micro- or nanoseconds depending on resolution of the underlying platform clock. COUNT_MAX usually equals `HUGE(COUNT_MAX)`. Note that the millisecond resolution of the kind=4 version implies that the COUNT will wrap around in roughly 25 days. In order to avoid issues with the wrap around and for more precise timing, please use the kind=8 version.

  If there is no clock, or querying the clock fails, COUNT is set to `-HUGE(COUNT)`, and COUNT_RATE and COUNT_MAX are set to zero.

  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 rt library. This can be done explicitly by adding the `-lrt` flag when linking the application, but is also done implicitly when using OpenMP.

  On the Windows platform, the version with kind=4 arguments uses the `GetTickCount` function, whereas the kind=8 version uses `QueryPerformanceCounter` and `QueryPerformanceCounterFrequency`. For more information, and potential caveats, please see the platform documentation.

Standard:

  Fortran 90 and later

Class:

  Subroutine

Syntax:

  `CALL SYSTEM_CLOCK([COUNT, COUNT_RATE, COUNT_MAX])`

Arguments:

  COUNT | (Optional) shall be a scalar of type `INTEGER` with `INTENT(OUT)`.
  COUNT_RATE | (Optional) shall be a scalar of type `INTEGER` or `REAL`, with `INTENT(OUT)`.
  COUNT_MAX | (Optional) shall be a scalar of type `INTEGER` with `INTENT(OUT)`.

Example:

      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

See also:

  DATE_AND_TIME, CPU_TIME

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