Re: [Boost-users] thread safety of the mutex ctor

From: t. scott urban (scottu_at_[hidden])
Date: 2005-08-17 17:24:37

On Thu, 2005-08-18 at 06:22 +1000, Christopher Hunt wrote:
> On 18/08/2005, at 1:19 AM, boost-users-request_at_[hidden] wrote:
> 
> > I envisage two threads accessing a function like this concurrently:
> >
> > template<typename T>
> > boost::shared_ptr<T>
> > my_func()
> > {
> >     static boost::mutex m;
> >     boost::mutex::scoped_lock l(m);
> >
> >     static boost::shared_ptr<T> p(new T);
> >     return p;
> > }
> >
> > and my worry is that both threads could attempt to do the static 
> > initialisation of m concurrently.  Is there protection against this?  
> > Is it even possible to protect against this?
> The scope of m is actually outside of your function given that it is 
> static. Thus it will be initialised outside of your function generally 
> before any other threads get to start up.

The scope of m is in the function, not global, but it has static
duration.  Unlike a global, m will only be constructed if my_func() is
ever called in your program.

That should set of warning bells in you brain.

I don't believe there are any guarantees about the thread safety of the
initialization of function scope statics.  This applies not just to the
mutex class in question but any type.  A particular compiler might make
this usage thread safe - or maybe only guarantee initialization of basic
types - but I don't know if it's covered by the pthread spec - and
that's not helpful anyway to boost users, in general.  And of course,
the C++ standard is as usual silent on the matter.

You can make the mutex global - anonymous namespace or whatever.  Since
global initializations are single threaded, you trade the threading
problem for the loss of on-demand resource usage.

Another way to deal with this kind of problem is to use a modified
singleton pattern, but to make that thread safe, you need another mutex
(unless you want to risk DCL), so seems kind of pointless for this
instance.

-- 
t. scott urban <scottu_at_[hidden]>