Re: What does atomic_read actually do?

[Joseph Seigh]

On Sat, 18 Dec 2004 18:11:02 +0100, Paolo Ornati <ornati@xxxxxxxxxxxxx>  
wrote:

> On Sat, 18 Dec 2004 11:23:37 -0500
> "Joseph Seigh" <jseigh_02@xxxxxxxxxx> wrote:
>
> > It doesn't do anything that would actually guarantee that the fetch
> > from memory would be atomic as far as I can see, at least in the x86
> > version. The C standard has nothing to say about atomicity w.r.t.
> > multithreading or multiprocessing.  Is this a gcc compiler thing?  If
> > so, does gcc guarantee that it will fetch aligned ints with a single
> > instruction on all platforms or just x86?   And what's with volatile
> > since if the C standard implies nothing about multithreading then it
> > follows that volatile has no meaning with respect to multithreading
> > either?  Also a gcc thing?  Are volatile semantics well defined enough
> > that you can use it to make the compiler synchronize memory state as
> > far as it is concerned?
>
> http://www.gnu.org/software/libc/manual/html_node/Atomic-Data-Access.html#Atomic%20Data%20Access
>
> http://www.gnu.org/software/libc/manual/html_node/Atomic-Types.html#Atomic%20Types

sig_atomic_t is only meaningful with respect to unix signals and is  
meaningless with
respect to threads.  And sig_atomic_t isn't atomic_t anyway.  And it has  
to useful
size, it could be in int on some platforms but only a byte on others.   
Signals and
threading are difficult enough as separate subjects.  Combining them is  
insanely
difficult but that's another topic.

> http://gcc.gnu.org/onlinedocs/gcc/Volatiles.html

"The C standard leaves it implementation defined as to what constitutes a  
volatile access."
Plus, I'm not sure sequence points are defined in C, and IIRC, C++ can  
combine and/or
reorder volatile accesses between sequence points.

Joe Seigh

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