* Bardur Arantsson:

> I don't think the glibc/Linux localtime() man page explicitly states 
> this, but I expect that it returns a pointer to a *thread-local* 
> statically allocated struct tm... in which case there's no problem.

Thread-local storage is a recent innovation in the Linux camp.
Previously, developers seem to think that no efficient implementation
was possible.  That's why there all the *_r variants (such as
localtime_r).

I believe Solaris does something in the direction you suggest.