[tz] WG14/C localtime_r draft is not reentrant

[Jens Gustedt]

Hello Paul,

On Sat, 17 Aug 2019 17:38:07 -0700 Paul Eggert <eggert at cs.ucla.edu>
wrote:

> Brian Inglis wrote:
> 
> > WG14/C is looking at:
> > 
> > 	http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2402.pdf
> > 
> > which seems somewhat based on the POSIX standard.  
> 
> Thanks for the heads-up. I see that this WG14/C draft is calling
> localtime_r "reentrant", but it's not: in POSIX, localtime_r relies
> on global state to obtain the time zone, and behavior is undefined if
> one thread calls localtime_r while another thread is changing the
> time zone. Similarly for some of the other "reentrant" functions
> mentioned in that draft.

Right, I probably was much too optimistic :(

> If WG14 wants reentrant, it should standardize localtime_rz etc. a
> la tzcode and NetBSD. These functions avoid the races inherent to
> localtime_r etc.

No, I guess that MT-safe with the specific conditions for locale and
perhaps env should be enough. If I see this correctly, C itself has
currently no means to change the TZ nor even an environment variable
from inside the program run, so all we can do here is just to add some
warnings: if the implementation permits changes to global state
concerning calendar time or somehow changes the LC_TIME category,
there will be trouble in a multi-threaded program execution.

Seeing that, we should add some phrase to the C standard concerning
strftime, too.

> I'll cc. this to Jens Gustedt to give the draft's author a heads-up.

I appreciate, thanks.

> Even if WG14 decides to stick with POSIXish rather than reentrant
> functions, the rationale document should explain why.

Locale and the interaction with threads is a can of worms, so I'd
probably not open that.



Thanks
Jens

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