Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : Laurent Deniau
Date : Fri Jul 08 2005 01:15 pm

David Schwartz wrote:
> "Laurent Deniau" <Laurent.Deniau@cern.ch> wrote in message 
> news:daldon$6if$1@sunnews.cern.ch...
> 
> 
>>>CPUs are allowed to do *ANYTHING* as long as it meets the standard.
> 
> 
>>If find funny to think that cpus meet software standards or OS standards. 
>>A cpu is not designed for C neither for POSIX. C and POSIX try to support 
>>in their norm most of the hardwares through an abstract machine.

I should have added "and minimum requirement".

>     No, the C implementation is designed to force the CPU to comply with the 
> standard, and no more.

And if the CPU is not able to do it? C implementation does not *force* 
anything, it *uses* it to obtain the expected behavior. If the CPU does 
not provide minimum requirement to make a conformant compiler, the C is 
either not standard or not implemented, full stop.

>>CPUs are allowed to do anything as long as it does not change the 
>>observable behavior conforming to the assembly instructions semantics. 
>>Nothing to do with any software standards.
> 
>     He means once the C implementation is complete. At that point, the CPU 

If this implementation is conform, this is already a huge step.

> can do whatever it wants, however if it violates the standard, then either 
> the CPU or the C implementation is broken. He is assuming everything else 

If the CPU is broken, this has nothing to do with C and the problem will 
probably be reflected in the behavior of other languages.

> but the CPU has been designed or arranged to conform to the standard.
> 
> 
>>Compilers and OS are allowed to do anything as long as it does not change 
>>the observable behavior conforming to the standards.
> 
> 
>     Just be careful here. By "observable", it means observable as defined by 
> the standard!

C does not define observable behavior. I don't know about POSIX which I 
know much less.

But I would say that behavior as defined by the C would be enough: 
external appearance or action.

>     For example, the C standard cannot use the word "observable" to mean a 

C does not use it at all. So no speculation.

ld.

.