The function int pthread_once(pthread_once_t *once_control, void (*init_routine) (void)); The first call to pthread_once() by any thread in a process, with a given 'once_control', shall call the 'init_routine' with no arguments. Subsequent calls of pthread_once() with the same once_control shall not call the 'init_routine'. The 'once_control' paramter shall determine whether the associated initialization routine has been called. On return from pthread_once(), 'init_routine' shall have completed. The pthread_once() function is not a cancelation point. But if 'init_routine' is a cancelation point and is canceled, the effect on 'once_control' shall be as if pthread_once() was never called. The constant PTHREAD_ONCE_INIT is defined in the pthread.h header. The behavior of pthread_once() is undefined if 'once_control' has automatic storage duration or is not initialized by PTHREAD_ONCE_INIT. Upon success, it returns a 0; Upon failure, it may return an error number: -[EINVAL] If either 'once_control' or 'init_routine' is invalid It shall not return [EINTR]