Date: Mon, 10 Aug 2009 11:49:13 -0700 From: Brian Somers <brian@FreeBSD.org> To: "Peter Steele" <psteele@webmail.maxiscale.com> Cc: freebsd-hackers@FreeBSD.org Subject: Re: How to signal a time zone change? Message-ID: <20090810114913.74c0cb42@dev.lan.Awfulhak.org> In-Reply-To: <B8A480488C0C6849826655761349EA431F7175@owa.webmail.maxiscale.com> References: <B8A480488C0C6849826655761349EA431F716A@owa.webmail.maxiscale.com> <4A7C9738.10103@elischer.org> <B8A480488C0C6849826655761349EA431F7171@owa.webmail.maxiscale.com> <eaec67500908071423v95b7214t30a48ad3d75c3cd@mail.gmail.com> <B8A480488C0C6849826655761349EA431F7175@owa.webmail.maxiscale.com>
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On Fri, 7 Aug 2009 15:08:16 -0700 "Peter Steele" <psteele@webmail.maxiscale.com> wrote:
> >What's the value of the TZ environment variable for the C apps? You may
> need to have them read the new value from somewhere, and then rerun
> tzset().
>
> The default value of the TZ environment variable is null. I just tried
> passing the explicitly time zone value to the C app and setting TZ to
> that value and that seemed to work. I would think that that I should be
> able to retrieve that value from /etc/localtime as the docs imply. Guess
> not. If I have to pass the time zone to the C app, then I guess that's
> what I'll do...
This doesn't work because of two bugs in localtime.c. The first is what
you're hitting where tzset() calls tzset_basic() which calls tzsetwall_basic()
which says:
if (lcl_is_set < 0) {
if (!rdlocked)
_RWLOCK_UNLOCK(&lcl_rwlock);
return;
}
If you were to have your own TZ setting and wanted to modify the
file referred to by that, you'd bump into this bug in tzset_basic():
if (lcl_is_set > 0 && strcmp(lcl_TZname, name) == 0) {
if (!rdlocked)
_RWLOCK_UNLOCK(&lcl_rwlock);
return;
}
Roughly translated, localtime.c goes out of its way to never re-read the
same zone file twice in a row. This is just a mistake.
As you discovered, altering TZ before calling tzset() is the best way to
make it work right now. If you really want to ensure that you're reading
/etc/localtime, this bit of hackery works too:
putenv("TZ=/dev/null");
tzset();
unsetenv("TZ");
tzset();
If you raise a PR and let me know the number, I'd be happy to fix this.
--
Brian Somers <brian@Awfulhak.org>
Don't _EVER_ lose your sense of humour ! <brian@FreeBSD.org>
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