Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : Joe Seigh
Date : Fri Jul 08 2005 09:06 am

Alexander Terekhov wrote:
> Laurent Deniau wrote:
> [...]
> 
>>Right. Comming back to the example, returning from a function is a
>>sequence point. 
> 
> 
> "The C standard says that it must look that way from the point of 
> view of the abstract machine defined by the standard. But the as-if 
> rule says that things can be arbitrarily reordered as long as nobody 
> can tell. Since the standard says nothing about threading, it does 
> not concern itself with statement reorderings that are as-if-safe 
> under one thread of control but as-if-unsafe under multiple threads 
> of control." 
> 

The problems is threading is talked about as if it has well defined
behavior outside of C.  It doesn't.  "threading" and Posix pthreads
aren't the same thing.  And even Posix pthreads has no formal definition.
You can't say what it actually does.  I don't know what Posix compliance
entails but one thing it doesn't is proving the C compiler in question
correctly implements a certain behavior since that certain behavior has
no definition.  So when you talk about things like out of order memory
access, that's one form of threading as we understand it.  Since Posix
, Ansi C, and hardware designers don't talk to one another, it's possible
for a new "thread" model / memory model / hardware architecture that
breaks not only heretofore correct DCL implementations but Posix pthreads
as well.  You'd have to port the DCL and Posix pthreads to the new
architecture, assuming it's possible.  So given that both can break and
somebody has to do the porting, I'd say both DCL and Posix are equally
portable except there's probably more headcount available to do the porting
for Posix pthreads.

AKA future proofing and no, you can't future proof anything.  That's why
C doesn't work for things it never anticipated and Posix pthreads won't
work for things it never anticipated.

-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.