[ntp:questions] Re: Time set backward

David L. Mills mills at udel.edu
Fri Jan 6 22:51:38 UTC 2006


Thanks for the clarification. How do I discipline the frequency of the 


Garrett Wollman wrote:
> In article <dpmna9$6h8$1 at dewey.udel.edu>,
> David L. Mills <mills at udel.edu> wrote:
>>I don't understand how POSIX can implement a time of day function using 
> It does precisely the opposite: timers are implemented in terms of the
> TOD.
> Here's what the standard says:
> # Each implementation shall define a set of clocks that can be used as
> # timing bases for per-process timers. All implementations shall
> # support a clock_id of CLOCK_REALTIME. If the Monotonic Clock option
> # is supported, implementations shall support a clock_id of
> (timer_create, XSI6 page 1514, IEEE Std.1003.1-2001)
> # If the flag TIMER_ABSTIME is set in the argument flags,
> # timer_settime( ) shall behave as if the time until next expiration
> # is set to be equal to the difference between the absolute time
> # specified by the it_value member of value and the current value of
> # the clock associated with timerid. That is, the timer shall expire
> # when the clock reaches the value specified by the it_value member of
> # value.
> (timer_getoverrun [sic], XSI6 page 1518, IEEE Std.1003.1-2001)
> # If the value of the CLOCK_REALTIME clock is set via clock_settime( ),
> # the new value of the clock shall be used to determine the time of
> # expiration for absolute time services based upon the CLOCK_REALTIME
> # clock. This applies to the time at which armed absolute timers
> # expire. If the absolute time requested at the invocation of such a
> # time service is before the new value of the clock, the time service
> # shall expire immediately as if the clock had reached the requested
> # time normally.
> # Setting the value of the CLOCK_REALTIME clock via clock_settime( )
> # shall have no effect on threads that are blocked waiting for a
> # relative time service based upon this clock, including the
> # nanosleep( ) function; nor on the expiration of relative timers
> # based upon this clock.  Consequently, these time services shall
> # expire when the requested relative interval elapses, independently
> # of the new or old value of the clock.
> (clock_getres [sic], XSI6 page 195, IEEE Std.1003.1-2001)
> -GAWollman

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