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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: CPU_Time Intrinsic, DTime Intrinsic (function), DTime Intrinsic (subroutine), ETime Intrinsic (function), ETime Intrinsic (subroutine), MClock Intrinsic, MClock8 Intrinsic, Secnds Intrinsic, Second Intrinsic (function), Second Intrinsic (subroutine), System_Clock Intrinsic, Time Intrinsic (UNIX), Time Intrinsic (VXT), Time8 Intrinsic.

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