Re: How To Add a Sequence Point?

[David Brown <david dot brown at hesbynett dot no>]

On 03/02/13 02:10, Jeffrey Walton wrote:
> Thanks Andrew.
>
> So, it looks like I don't understand sequence points. Please forgive
> my ignorance.
>
> What does C/C++ and GCC offer to ensure writes are complete before
> reads are performed on a value in a multi-threaded program?
>
> Jeff


You are probably looking for a "memory barrier", rather than a "sequence 
point".  But even that will not be enough if you have an SMP system - 
though it might let you get better results from your Googling.  If 
possible, your best bet is probably to use facilities provided by your 
OS - but that is very much outside the scope of this mailing list.

Anyway, the simplest memory barrier in gcc is :

	asm volatile("" ::: "memory");

This tells gcc that memory may be used in unexpected ways at this point 
in the code - so any outstanding writes are handled before this point, 
and no reads are started until after it.  But that only applies at the 
assembly code level - the compiler does not control things like caches, 
write buffers, read-ahead buffers, etc.

mvh.,

David


> On Sat, Feb 2, 2013 at 8:07 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> > On Sat, Feb 2, 2013 at 4:59 PM, Jeffrey Walton <noloader@gmail.com> wrote:
> > > Hi All,
> > >
> > > How do I add a sequence point in my C/C++ code?
> >
> > A semi-colon (end of the statement), a comma (but not as an argument
> > separator though) are both sequence points.
> >
> > Thanks,
> > Andrew Pinski
> >
> > > Googling brings up a lot of 'volatile' hits, but I believe that's an
> > > abuse of GCC's interpretation of volatile since I'm not working with
> > > memory mapped hardware or registers.