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Re: localtime on native Windows


From: Bruno Haible
Subject: Re: localtime on native Windows
Date: Sun, 18 Feb 2024 03:38:28 +0100

Paul Eggert wrote:
> > The first question is how to include tzdata in gnulib.
> >    - AFAICS, the main data file (without comments) is tzdata.zi and is about
> >      100 KB large. It can be upgraded simply by copying the newest tzdata.zi
> >      from a newer tzdata distribution. Including such a file in gnulib would
> >      be OK (re copyright, number of files, total size), right?
> 
> No problem with copyright; the file is in the public domain.
> 
> >   - Whereas including all files from /usr/share/zoneinfo is probably not
> >     acceptable (> 1300 files, ca. 6 MB total size).
> 
> That can be automatically generated from tzdata.zi, by using zic, the 
> source code of which is also in the public domain.

Thanks for the confirmation.

My current plan, in order to cover the two goals
  - localtime_r on native Windows,
  - nstrftime, c_nstrftime, parse-datetime, which all take a timezone_t
    argument,
is to write four pieces of code:
  1) code that represents the data for a single time zone in memory,
     (based on glibc's tzfile.c),
  2) code that reads a binary time zone file, for use on Unix,
     (based on glibc's tzfile.c), returning it in the format 1).
  3) code that reads a tzdata.zi, for use on Windows,
     (based on glibc's zic.c and tzfile.c), returning it in the format 1).
  4) an implementation of localtime_r for a given time zone
     (based on glibc's tzfile.c:__tzfile_compute, tzset.c:__tz_convert, and
     localtime_r).

This should fulfil Gnulib's needs (code under LGPLv2+, small number
of source files, small number and size of data files to include).

Note that this will have to wait for a few weeks. I have more urgent work on
gettext to do first.

Note also: The above approach will need locking, in order to protect caches
(in 1) the data for a single time zone, in 3) the contents of tzdata.zi).

Bruno






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