Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : David Schwartz
Date : Fri Jul 08 2005 06:02 am


"Laurent Deniau" <Laurent.Deniau@cern.ch> wrote in message 
news:dalle2$nlu$1@sunnews.cern.ch...

> The as-if rule is an implementation-defined problem. The as-if-safe rule 
> becomes an as-if-unsafe rule because of some changes in the context of 
> execution which is not considered by the implementation (e.g. switching 
> off the computer). Unless specified, this is leading to an undefined 
> behavior. Imagine that the implementation of volatile includes memory 
> barrier or some cache synchronisation mechanism (nothing in the C says to 
> not do so), it would still be conform with and without thread. But at the 
> price of hidden penalties that C programmers do not want to hear about.

    Since no standards compliant C code can tell the difference, the C 
standard cannot require the barriers. Some other standard (for example, a 
threading standard) might specify that volatile provides ordering guarantees 
across threads, in which case failure to provide barriers might lead to 
violating that standard on that platform.

    DS

.