Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : David Schwartz
Date : Thu Feb 10 2005 04:51 pm


"Giancarlo Niccolai" <gniccolai@yahoo.com> wrote in message 
news:95e4efda.0502101630.6e6c144d@posting.google.com...
>> You have still not shown what will break under the synchronized
>> semantics. And this is because nothing will break that wasn't
>> already broken.
>
> Metacode:
>
> static int CountOfItems = 0;
> static the_reentrant_mutex;
>
> class Item {
>
> Item() {
>   the_reentrant_mutex.lock();
>   CountOfItems ++;
>   the_reentrant_mutex.unlock();
> }
> };
>
> // Change this with your favorite static initializer method.
> Item &cofu() {
>   static Item *the_item = 0;
>   if ( the_item == 0 ) {
>      the_item = new Item();  // compiler lock here.
>   }
>   return the_item;
> }
>
> void thread_func_1()
> {
>   Item *i = &cofu();
>   // do something
> }
>
> void thread_func_2()
> {
>   // this threads wants to create a new Item, but without
>   // changing the count.
>   the_reentrant_mutex.lock();
>   int old_count = CountOfItems;
>   Item *i = &cofu();
>   CountOfItems = old_count;
>   the_reentrant_mutex.unlock();
>   // use I.
> }

    This code is already broken. The '&cofu()' method may modify 
'*the_item', so must be called while holding some sort of lock. The 
'thread_func_1' function calls '&cofu' without holding any lock, so its 
interaction with 'thread_func_2' is undefined under POSIX.

    DS

.