On 2014-01-18 09:39 AM, Zefram wrote:
Joseph Gwinn wrote:
No. If your poke around into how time is used, you will discover that
what is stored in the cound of seconds since the Epoch. Broken-down
time is used only when there is a human to be humored.
Sure, scalar time_t values are used underneath, and I didn't say
otherwise. That's what time_t is for. The kernel even increments the
time_t clock, most of the time, as if it's a linear count of seconds,
which is how it behaves on the small scale outside the immediate
vicinity of leap seconds. But a kernel that knows about leap seconds
then introduces a discontinuity in the scalar value, somewhere near each
leap, to maintain the scalar<->UTC relationship.
Yes, two related issues -
1) There's no specification how the kernel should behave.
2) The POSIX API has its shortcomings, as you note, and these are
tangled with the kernel's behavior.
On Windows, in c/c++, the POSIX API is implemented as a sub-system which
in turn calls proprietary Windows time APIs, like ::GetSystemTime(),
::GetFileTime() and related. Divining exactly how those are behaving is
a challenge. Some parts of the POSIX API are just not implemented, and
some versions of MSVC c/c++ implement non-standard 64-bit versions.
Also, not all versions of POSIX are equally good. I've found smoking gun
bugs in some implementations of gmtime() and related.
Meantime, as you note in
http://www.fysh.org/~zefram/time/prog_on_time_scales.pdf, Disseminating
Leap Schedule to Machines, is a missing link in the whole UTC world is a
well defined and reliable method of obtaining the proper metadata (Leap
Seconds table, announce signals, etc). This needs to be addressed
somehow by somebody.
POSIX time is defined without reference to NTP,
Indeed. The two definitions are separate, but match in most of their
design features.
POSIX count "steps back" on Leap Second insertion. NTP count "freezes"
on Leap Second insertion. Either way there's an ambiguity, so that's one
design feature they share :-).
NTP *does* refer to POSIX - Figure 4: Interesting Historic NTP Dates
refers to "First day UNIX" and locates it 63072000 seconds before
1972-01-01T00:00:00Z (UTC). This helps solve one problem - when,
exactly, was the POSIX "the Epoch".
-Brooks
-zefram
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