pthread_mutex_t initializer idiom

[Paolo Carlini]

Hi Benjamin,

>I'm still trying to unify some of the atomics and concurrency code, and
>remove duplicates.
>
>I see this idiom a lot:
>
>	  __gthread_mutex_t temp = __GTHREAD_MUTEX_INIT;
>	  _M_mutex = temp;
>
>Can somebody explain to me why we are doing this instead of:
>
>_M_mutex = __GTHREAD_MUTEX_INIT;
>
>?
>  
>
There is a rather old and funny comment at line 586 of rope:

    {
      // Do not copy a POSIX/gthr mutex once in use.  However, bits are 
bits.
      __gthread_mutex_t __tmp = __GTHREAD_MUTEX_INIT;
      _M_c_string_lock = __tmp;
    }

I think the idea is using __GTHREAD_MUTEX_INIT, which, in principle 
should be used only on static things, on a mutex that certainly doesn't 
exist yet, __tmp, then do a "bit by bit" copy to the mutex in the object 
(not yet in use) and hope for the best. In other terms, It seems to me, 
that is the best we can do to deal with two different sources of 
theoretical undefined behavior: 1- __GTHREAD_MUTEX_INIT on non-static 
things; 2- Copying (Posix) mutexes is undefined (in principle, also when 
not in use, I'm afraid...).

I would suggest showing this code (again) to someone really knowing well 
the internals of the various kinds of mutexes which GCC supports...

Paolo.