[tz] Getting current tzdb version in use

[Howard Hinnant]

On Jul 19, 2018, at 7:29 PM, Paul Eggert <eggert at cs.ucla.edu> wrote:
> 
> Howard Hinnant wrote:
>> vendors can easily add version files to their compiled binary databases (as Apple already does for macOS, iOS and watchOS).
> 
> How does that work, exactly? I don't have easy access to any of those systems. Is it documented anywhere?

Deborah Goldsmith should chime in if I get anything wrong…

I’m not aware of any documentation (I haven’t looked).  And I’m ignorant of the Apple deployment process.

On the most recent update to macOS, the file:

/private/var/db/timezone/tz/2018c.1.0/zoneinfo/+VERSION

contains:

2018c

And that’s good enough for a std::lib vendor on macOS to read and fill out std::chrono::get_tzdb().version.

There also exists a generic (version-free) symlink path to the same file:

/usr/share/zoneinfo/+VERSION

(which is what my prototype uses when compiled in the mode of using the zic-compiled binary database).

At this time, Apple does not do the following:  But it is possible that in the future, Apple could choose to update the zoneinfo database to a new directory, and atomically update the /usr/share/zoneinfo/ symlink, _without_ requiring a system reboot.  This would enable long-running programs to query the OS, detect the version change of the tzdb, and update their internal ram copies of the tzdb, without the program having to go through a shutdown/restart sequence.  The C++20 API is anticipating (but not requiring) that capability.

Howard

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