8.264 SYSTEM_CLOCK — Time function


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.


Fortran 90 and later





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).
PROGRAM test_system_clock
  INTEGER :: count, count_rate, count_max
  CALL SYSTEM_CLOCK(count, count_rate, count_max)
  WRITE(*,*) count, count_rate, count_max
See also:

DATE_AND_TIME — Date and time subroutine,
CPU_TIME — CPU elapsed time in seconds