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Using and Porting GNU Fortran

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10.2.1 Timer Wraparounds

Intrinsics that return values computed from system timers, whether elapsed (wall-clock) timers, process CPU timers, or other kinds of timers, are prone to experiencing wrap-around errors (or returning wrapped-around values from successive calls) due to insufficient ranges offered by the underlying system's timers.

Some of the symptoms of such behaviors include apparently negative time being computed for a duration, an extremely short amount of time being computed for a long duration, and an extremely long amount of time being computed for a short duration.

See the following for intrinsics known to have potential problems in these areas on at least some systems: 8.11.9.49 CPU_Time Intrinsic, 10.5.2.36 DTime Intrinsic (function), 8.11.9.91 DTime Intrinsic (subroutine), 8.11.9.97 ETime Intrinsic (function), 8.11.9.96 ETime Intrinsic (subroutine), 8.11.9.185 MClock Intrinsic, 8.11.9.186 MClock8 Intrinsic, 10.5.2.127 Secnds Intrinsic, 8.11.9.220 Second Intrinsic (function), 8.11.9.221 Second Intrinsic (subroutine), 8.11.9.242 System_Clock Intrinsic, 8.11.9.245 Time Intrinsic (UNIX), 10.5.2.134 Time Intrinsic (VXT), 8.11.9.246 Time8 Intrinsic.


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