### 10.10 `__tz_lock`, `__tz_unlock`—lock time zone global variables ¶

Synopsis

    #include "local.h"
    void __tz_lock (void);
    void __tz_unlock (void);

Description
The `tzset` facility functions call these functions when they need to ensure the values of global variables. The version of these routines supplied in the library use the lock API defined in sys/lock.h. If multiple threads of execution can call the time functions and give up scheduling in the middle, then you you need to define your own versions of these functions in order to safely lock the time zone variables during a call. If you do not, the results of `localtime`, `mktime`, `ctime`, and `strftime` are undefined.

The lock `__tz_lock` may not be called recursively; that is, a call `__tz_lock` will always lock all subsequent `__tz_lock` calls until the corresponding `__tz_unlock` call on the same thread is made.