Re: [LAD] a *simple* ring buffer, comments pls?

On Friday, July 08, 2011 12:17:34 pm Fons Adriaensen wrote:
> On Fri, Jul 08, 2011 at 09:21:55AM -0400, Paul Davis
wrote:
> > the one wrinkle in this is that in theory a compiler
> > could so completely reorder instructions that even the
> > basic assumptions that make the single
> > reader/single-writer ringbuffer safe would break down.
>
> AFAIK nothing fatal can happen if the variables involved
> are declared volatile. A compiler is not allowed to
> omit, repeat, or re-order instructions involving them.

Take for instance jack_ringbuffer_read(), which has this
line:

  rb->read_ptr = (rb->read_ptr + n) & rb->size_mask;

There's a remote possibility that the compiler could
optimize this as:

  rb->read_ptr += n;
  rb->read_ptr &= rb->size_mask;

...and this would break the ringbuffer. I don't know if the
`volatile` keyword prevents this or not.

-gabriel
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Received on Sat Jul 9 00:15:01 2011