Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : David Schwartz
Date : Wed Jul 06 2005 11:14 am


"Laurent Deniau" <Laurent.Deniau@cern.ch> wrote in message 
news:dagj50$2uj$1@sunnews.cern.ch...

> Right. But this is not what I say. I was talking about value of 
> initialized before and after lock. It is clear that if extern_init_code() 
> has some side effects, it must lock/unlock the ressource it deals with 
> otherwise you break the semantic. For example if initialized is a pointer 
> to a singleton instance and extern_init_code() is a factory calling 
> malloc, you do not need a mutex, but malloc will certainly use one 
> internally. Therefore the correct code is (assuming a different 
> translation unit):
>
> int i=0;
> pthread_mutex_t init_mutex;
>
> int extern_init_code(void)
> {
>   pthread_mutex_lock(&init_mutex);
>   i=1;
>   pthread_mutex_unlock(&init_mutex);
>
>   return 1;
> }

    You still have the same problem. This didn't fix the problem I was 
talking about. The exact same sequence still has the exact same problem.

    Perhaps you don't understand the semantics of POSIX mutexes. They do 
*nothing* unless you have one thread that holds a lock when it changes a 
variable and then another thread that holds that same lock when it reads it.

    DS

.