[PATCH] POSIX monotonic clock

Václav Haisman v.haisman@sh.cvut.cz
Tue Aug 3 07:32:00 GMT 2010

On Mon, 02 Aug 2010 15:49:08 -0500, "Yaakov (Cygwin/X)" wrote:
> Here is an attempt to implement POSIX.1-2004+ Monotonic Clock:
> In summary, I took hires_us and changed the resolution to nanoseconds. I
> dropped systime() because the only place hires_us was being used is in
> strace.cc which ignored it, and WRT POSIX monotonic clocks the absolute
> value of the clock is meaningless.  Since systime() has only 100ns
> precision, using it would either force a loss in resolution or (if
> multiplied by 100 to get ns) an early overflow.  I also switched from
> ENOSYS to EINVAL, as POSIX.1-2004 and 2008 dropped references to the
> former (as noted in Change History).
> Patches for newlib, winsup/cygwin and winsup/doc attached.
> I have also attached an STC for the new functionality.  FWIW, on my
> machine, QueryPerformanceFrequency() returns just over 2.9 million,
> resulting in a clock_getres(CLOCK_MONOTONIC) of 340ns.
> I would appreciate a careful review of this patch, both from the Cygwin
Is it really ok to use QueryPerformanceCounter() to implement this? Quote
from <http://msdn.microsoft.com/en-us/library/ms644904%28VS.85%29.aspx>:

"On a multiprocessor computer, it should not matter which processor is
called. However, you can get different results on different processors due
to bugs in the basic input/output system (BIOS) or the hardware abstraction
layer (HAL). To specify processor affinity for a thread, use the
SetThreadAffinityMask function."

This looks like you could get monotonic clock going backwards.


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