Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : David Schwartz
Date : Wed Jul 06 2005 05:55 am


"Laurent Deniau" <Laurent.Deniau@cern.ch> wrote in message 
news:dag5mv$2lf$1@sunnews.cern.ch...

> There is a simple, portable, processor independent, compiler independent 
> idiom which consists of delegating the work to an extern function, a 
> function with an implementation in a different translation unit.

    Err, what?! If it's not guaranteed to work by the relevent standard, 
it's not portable.

> void function(void)
> {
>     if (!initialized) {
>        pthread_mutex_lock(&mutex);
>        if (!initialized)
>           initialized = extern_init_code();
>        pthread_mutex_unlock(&mutex);
>     }
> }

> The compiler is not allowed to assign initialized *before* 
> extern_init_code() returns.

    Not allowed by what?

> The processor will not be able to do the same since this is a true jump 
> with a return value. The assignment could be delayed but not after the 
> unlock, so it is not a problem.

    Why can't the assignment be delayed until after the unlock? And what 
guarantees that a CPU that never acquired the mutex sees the initialization 
itself?

    DS

.