Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Marcin 'Qrczak' Kowalczyk
Date : Wed Feb 16 2005 10:21 am

gniccolai@yahoo.com (Giancarlo Niccolai) writes:

> Someone proposed to have a mutex to lock a static initializer
> constructor call (not pthread_once like Marcin only lately suggested,
> which brings other problems that I don't want to discuss now);

It is equivalent (modulo cancellation, i.e. the mutex should be
released when the initializer is cancelled).

> So, the thing here is not what happens in that COFU, but that there
> is someone saying (and from Mariani sayings, CURRENTLY putting in a
> compiler) THIS scheme:
>
> the_safe_initializer() {
>    acquire_mutex()
>    if (! cls.initialized)  //a classwide static or something the like
>       cls.construct() // a classwide constructor or something the like
>    release_mutex()
> }
>
> Now, we all know here that THIS may deadlock,

No, it can only deadlock with a program which already has undefined
behavior.

> as we don't know what cls.construct() is doing, and as someone may
> i.e. legitimaly do:
>
> acquire_recursive_mutex_used_in_cls_constructor()
> the_safe_initializer()
> cls.something = change()
> release_recursive_mutex_used_in_cls_constructor()

It may not reenter the_safe_initializer. Reentering a static local
initializer before its previous run has finished is forbidden by the
C++ standard.

You will not find a code which is broken by this change because it
cannot exist.

> Also, someone sustained that a scheme like the one in
> the_safe_initializer() is able to secure any single thread code
> against improper use of concurrency;

I don't believe anybody has claimed this.

-- 
   __("<         Marcin Kowalczyk
   \__/       qrczak@knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/

.