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Subject: Re: New kernel interface for sys_tz and timewarp?
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        Theodore Ts'o <tytso@mit.edu>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Alistair Francis <alistair.francis@wdc.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        Karel Zak <kzak@redhat.com>,
        Lennart Poettering <lennart@poettering.net>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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From:   Paul Eggert <eggert@cs.ucla.edu>
Organization: UCLA Computer Science Department
Message-ID: <ecf2742a-6cab-cc00-16ab-589fad07b8db@cs.ucla.edu>
Date:   Tue, 13 Aug 2019 10:49:03 -0700
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Linus Torvalds wrote:
> I assume/think that glibc uses (a) environment
> variables and (b) a filesystem-set default (per-user file with a
> system-wide default? I don't know what people do).
glibc relies on the TZ environment variable, with a system-wide default 
specified in /etc/localtime or suchlike (there is no per-user default). glibc 
ignores the kernel's 'struct timezone' settings for of this, as 'struct 
timezone' is obsolete/vestigial and doesn't contain enough info to do proper 
conversions anyway.

I've been thinking of adding NetBSD's localtime_rz etc. functions to glibc. 
These functions let user programs specify the time zone for each conversion 
between time_t and local time, and simplify and/or speed up applications dealing 
with many requests coming from different time zones. These functions also ignore 
'struct timezone'.

There's no need to put any of this stuff into the kernel.