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Message-ID: <529588D8.3020006@skarnet.org>
Date: Wed, 27 Nov 2013 05:53:28 +0000
From: Laurent Bercot <ska-dietlibc@...rnet.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Add support for leap seconds in zoneinfo files
> __secs_to_tm and __tm_to_secs are not the right places for applying
> leap seconds, because they affect gmtime, which is specified strictly
> by POSIX to have a particular relationship with time_t...
I disagree that they are not the right place for it.
* the point of using TAI-10 instead of UTC is to have a linear system
clock, so leap seconds must not be applied to clock_gettime() and
friends.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/time.html and
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_15
are vague enough ("approximate the number of seconds that have elapsed since
the Epoch") for TAI-10 to be a compliant setup here. The numerical relationship
between number of seconds and broken-down time is not respected if there are
leap seconds, but:
* http://pubs.opengroup.org/onlinepubs/9699919799/functions/gmtime.html
makes it very clear that gmtime() should return UTC, and user applications
like "date" rely on it, so if there are leap seconds, they should definitely
be applied here;
* POSIX followers will use posix/ timezones with 0 leap seconds anyway,
so the numerical relationship will be respected.
To sum it up:
- leap seconds break POSIX anyway, but should break as little as possible
- leap second users only care about system clock time, not broken-down time
- so gmtime() should always return UTC, this is relied on by userland so
it's more important than the exact relationship between tm and secs
- so time() should ignore leap seconds but broken-down time should apply them
- so the right place to apply them is in __secs_to_tm and __tm_to_secs, the
conversion routines
- using a posix/ zone will make everything POSIX in any case.
--
Laurent
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