Manual Pages  — DUPLOCALE

#include <locale.h>

Duplicates an existing locale_t returning a new locale_t that refers to the same locale values but has an independent internal state. Various functions, such as mblen(3) require a persistent state. These functions formerly used static variables and calls to them from multiple threads had undefined behavior. They now use fields in the locale_t associated with the current thread by uselocale(3). These calls are therefore only thread safe on threads with a unique per-thread locale. The locale returned by this call must be freed with freelocale(3).

freelocale(3), localeconv(3), newlocale(3), querylocale(3), uselocale(3), xlocale(3)

Ideally, uselocale(3) should make a copy of the locale_t implicitly to ensure thread safety, and a copy of the global locale should be installed lazily on each thread. The FreeBSD implementation does not do this, for compatibility with Darwin.