Using pthread_once() in libc

John Baldwin jhb at
Thu Nov 19 17:55:03 UTC 2009

On Thursday 19 November 2009 12:09:33 pm Daniel Eischen wrote:
> On Thu, 19 Nov 2009, John Baldwin wrote:
> > On Thursday 19 November 2009 11:48:54 am Daniel Eischen wrote:
> >> On Thu, 19 Nov 2009, John Baldwin wrote:
> >>
> >>> I would like to provide a pthread_once()-like facility in libc that library
> >>> bits can use to initialize data safely rather than trying to home-roll their
> >>> own variants (see the recent commit to stdtime in libc).  Ideally what I
> >>> would like to do is have libc use the "real" pthread_once() when libthr is
> >>> linked in and fall back to a simple stub without libthr linked in.  I know we
> >>> already do something like this for _spinlock() and friends.  My question is
> >>> what is the most correct way to do this?  Should libc grow a new _once()
> >>> symbol ala _spinlock() that is a weak symbol to a stub version and
> >>> pthread_once() in thr_once.c would override that, or should there be a
> >>> _pthread_once() in libc that is a stub in place of the current stub_zero?  I
> >>> noticed a comment in thr_spinlock.c saying the spinlock stuff is kept for
> >>> backwards compat.  Does this mean that for the future we would like to expose
> >>> pthread symbols directly in libc?  Meaning would we rather have libc export a
> >>> pthread_once() and that ideally libc would be using pthread_mutex_lock/unlock
> >>> instead of _spinlock/unlock?
> >>
> >> pthread_once() is already a stub in libc that gets overloaded with the
> >> real thing when libthr is linked.  See libc/gen/_pthread_stubs.c.
> >> Isn't that what you want or does it not serve your purpose?
> >
> > Hmm, the libc stub will never run the init routine.  I would like to do
> > something like this:
> Well, I suppose you could do that.  But what happens if libthr gets
> dlopen()'d and your once function needs to initialize a mutex or
> something that can only be properly done by a real threads library?
> Can we envision a scenario where that would be a problem?

Hmmm, so I guess __is_threaded is how the dlopen() case is handled now for
mutex lock/unlock as that avoids resolving the symbol until pthread_create()
has been invoked?  I guess then we could take an approach that works
something like this:

/* libc-internal function */
_once(pthread_once_t *once_control, void (*init_routine)(void))

	if (__is_threaded)
		return (_pthread_once(once_control, init_routine));

	return (_stub_once(once_control, init_routine));

It is probably still a good idea to have the stub_once() patch I think so
that pthread_once() DTRT in a single-threaded program.

John Baldwin

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