The first call to
pthread_once() by any thread in a process, with a given
once_control, will call the
init_routine() with no arguments. Subsequent calls to
pthread_once() with the same
once_control will not call the
init_routine(). On return from
pthread_once(), it is guaranteed that
init_routine() has completed. The
once_control parameter is used to determine whether the associated initialization routine has been called.
The function
pthread_once() is not a cancellation point. However, if
init_routine() is a cancellation point and is cancelled, the effect on
once_control is as if
pthread_once() was never called.
The constant
PTHREAD_ONCE_INIT initializes the static once synchronization control structure
once_control to be used with
pthread_once(). The behavior of
pthread_once() is undefined if
once_control has automatic storage duration or is not initialized by
PTHREAD_ONCE_INIT.