Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : David Hopwood
Date : Thu Jul 07 2005 02:59 am

Laurent Deniau wrote:
> David Hopwood wrote:
> 
>> To a first approximation, anything that can go wrong with this idiom in
>> Java can also go wrong in C. While it is possible to solve the problems
>> by (nonportable, processor and compiler-specific) use of memory barriers,
>> it really isn't worth the hassle to do this in an application.
>> It might be worth it in a language implementation if you really know
>> what you're doing.
> 
> 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.
> 
> 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. The processor will not be able to do the 
> same since this is a true jump with a return value.

You're mistaken; processors can reorder memory accesses across jumps.

-- 
David Hopwood <david.nospam.hopwood@blueyonder.co.uk>

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