[Boost-users] thread safety of the mutex ctor

From: James E Taylor (james_at_[hidden])
Date: 2005-08-17 10:19:01

I hope this isn't a stupid question, but....

Is the boost mutex constructor thread-safe?

I am writing a singleton template and trying to avoid the static initialisation/destruction order fiasco.

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?

I've tried to use boost::call_once, which would give sufficient guarantees but it's quite inflexible as nothing can be passed to or from the function being called.

I know that integral types (like a pthread_t) of static storage are initialised before main() and before constructors of statically allocated objects are called, which means there would be no problem using a pthread_t above.  However, constructors of objects of static storage are called at a time when there could be multiple threads running.

Am I worrying about nothing?  Or is there a problem here?

Thanks,
James

-- 
James E Taylor
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