[Bug time/23859] tzname, daylight are modified by localtime() in unexpected way

[eggert at cs dot ucla.edu]

https://sourceware.org/bugzilla/show_bug.cgi?id=23859

eggert at cs dot ucla.edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |eggert at cs dot ucla.edu
         Resolution|---                         |INVALID

--- Comment #4 from eggert at cs dot ucla.edu ---
(In reply to Andreas Schwab from comment #3)
> POSIX really only specifies the behaviour of POSIX style
> values for TZ, which cannot change over time.

This is correct. Programs that want time zone abbreviations, UT offsets, and
DST indications even in the presence of non-POSIX TZ settings should use the
tm_zone, tm_gmtoff, and tm_isdst members of struct tm. These members are
thread-safe whereas tzname, timezone, and daylight are not.

Unfortunately tm_zone and tm_gmtoff are not standardized by POSIX. But for
strictly portable POSIX code there should be no problem, since such code
shouldn't use non-POSIX TZ settings, and programs like Alexey's work fine when
they stick to POSIX TZ settings.

For semi-portable code that runs on POSIX platforms but does the right thing
even with non-POSIX TZ settings, Alexey can use strftime "%Z" for time zone
abbreviations, subtract gmtime_r from localtime_r for UT offsets, and use
tm_isdst to approximate the 'daylight' flag (the actual meaning of the
'daylight' flag is pretty much useless in this more-general context).
Admittedly this is a hassle; if this is a problem for Alexey then I suggest
lobbying POSIX to get the standard fixed to specify tm_zone and tm_gmtoff.

Closing the bug report as this is not a bug.

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