[tz] WG14/C localtime_r draft is not reentrant

Paul Eggert eggert at cs.ucla.edu
Sun Aug 18 00:38:07 UTC 2019

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.

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. I'll cc. 
this to Jens Gustedt to give the draft's author a heads-up. Even if WG14 decides 
to stick with POSIXish rather than reentrant functions, the rationale document 
should explain why.

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