Re: [boost] [thread] Proposal for thread safe initialization of local static and global objects

From: Roland Schwarz (roland.schwarz_at_[hidden])
Date: 2006-10-06 06:46:10

Anthony Williams wrote:
> On Windows we don't need two calls to call_once, as my implementation allows
> passing an arbitrary functor, which can therefore have state (e.g. use bind to
> pass parameters).

Hmm, I remember there was a sound reason why Bill Kempf ommitted
parameters deliberately. Not sure about this, I will need to try
to find the respective discussion thread.

> Your code demonstrates how we could provide the same facility with pthreads,
> as an extension to call_once. Incidentally, whilst we're thinking about this,
> the POSIX spec says:

Indeed I have shamelessly stolen the idea from looking at Bill Kempfs
call_once implementation for pthreads. He is using this there also,
altough I think, superfluously.

> "The behavior of pthread_once() is undefined if once_control has automatic
> storage duration or is not initialized by PTHREAD_ONCE_INIT."
> 
> So we need to document this restriction on the use of boost::once_flag, unless
> you can find a way round it. This restriction doesn't apply on Windows, at
> least not with my implementation.

I suspect this is because you are relying on zero initialization, right?

Roland