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Re: utimensat UTIME_NOW granularity bug

On Thu, Oct 08, 2009 at 08:27:42PM +0000, Eric Blake wrote:
>The gnulib test for utimensat found a bug in utimensat: it truncates time too 
>much, and has the result of making time appear to flow backwards.
>This sample code sequence shows the problem (when run at speed; of course 
>single stepping this introduces delays and masks the problem):
>close (creat ("file", 0600));
>stat ("file", &st1);
>utimensat (AT_FDCWD, "file", NULL, 0);
>stat ("file", &st1);
>(gdb) p st1.st_mtim 
>$3 = {tv_sec = 1255024956, tv_nsec = 526817600}
>(gdb) p st2.st_mtim
>$4 = {tv_sec = 1255024956, tv_nsec = 526817000}
>Windows expresses time as a fraction, rounded to a quanta with significant 
>digits out to a 10th of a microsecond (even though the quanta at which time 
>observably advances is much larger than that), but utimensat is currently 
>rounding to the nearest microsecond.  The result is that the timestamp set 
>during Windows events (such as close) is recorded with more resolution than 
>what happens for utimensat, such that consecutive instructions appear to run 
>out of order.
>I think we need to implement a companion to systime(), which returns the system 
>time without any truncation, so that the function clock_gettime(CLOCK_REALTIME) 
>can report time with resolution to the 10th of a microsecond rather than to 
>plain microseconds.  Then utimensat needs to use clock_gettime rather than 
>gettimeofday, so that it is not needlessly truncating the 10th of microsecond 
>resolution available from Windows.

Why not send these type of musings to the cygwin-developers list?  It really
is more appropriate for this type of discussion.


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