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From: "hristo at venev dot name" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114645] std::chrono::current_zone ignores $TZ on
 Linux
Date: Tue, 09 Apr 2024 09:41:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114645

--- Comment #9 from Hristo Venev <hristo at venev dot name> ---
I stumbled upon this comment in the library you linked:

https://github.com/HowardHinnant/date/blob/0e65940a7fbc4ed617a1ee111a60311e=
ccbead9a/include/date/tz.h#L35

That comment is wrong in its explanation of the mechanism used to determine=
 the
local time zone on Linux. However, it clearly shows that the intent is to m=
atch
the platform's "local time" as closely as reasonably possible.

The implementation also has some comments:

https://github.com/HowardHinnant/date/blob/0e65940a7fbc4ed617a1ee111a60311e=
ccbead9a/src/tz.cpp#L3936

The intent seems to be clear -- apply a lot of heuristics to try to match w=
hat
libc would do as closely as possible.

Even on Linux there are no guarantees whatsoever that it is possible to ext=
ract
a IANA time zone from /etc/localtime. In fact, the problem is exactly ident=
ical
to that with $TZ, if not worse -- $TZ is normally an IANA time zone name,
whereas /etc/localtime is a symlink (but sometimes a hardlink or a copy) of=
 a
file in some OS-specific directory  (sometimes, but not always,
/usr/share/zoneinfo) where the name of the file relative to the base direct=
ory
is a IANA time zone name.=