Question

we use One-Time initialization for pthreads like this:

/* define a statically initialized pthread_once_t variable */
pthread_once_t once_var = PTHREAD_ONCE_INIT;

/* we call pthread_once function in threads */
int pthread_once(pthread_once_t *once_control, void (*init)(void));

when more than one thread is going to change the state of pthread_once_t variable, do we need a mutex for protecting it?

Answer 1

No, you don’t need a mutex for this. The pthread_once call (link here) is guaranteed to be executed once and once only, even if multiple threads try it at the same time.

It’s the once_var that’s protecting the call from being executed more than once. It will work as expected, provided that you:

• initialise the once_var to PTHREAD_ONCE_INIT; and
• ensure that once_var is not of automatic storage duration (eg, on the stack); and
• have all threads use the same once_var variable.