From n3220, 7.28.2.1, The call_once function

Synopsis

#include <threads.h>
void call_once(once_flag *flag, void (*func)(void));

Description

The call_once function uses the
once_flag pointed to by flag to ensure that func is called exactly
once, the first time the call_once function is called with that value
of flag. Completion of an effective call to the call_once function
synchronizes with all subsequent calls to the call_once function with
the same value of flag.

Returns

The call_once function returns no value.

From what it sounds like, is this supposed to be used for init()/config() functions that are only supposed to be called once? Or am I completely misunderstanding it and it is something pertinent to threads?

What is this useful for? What is flag, and how is it relevant here?