[cffi-devel] [cffi] FSBV (#2)

[Martin Simmons]

To answer Ryan's point, I think there should be a different function to get a
pointer to an element of an array, called something like mem-aptr.  It should
work for all element types, not just aggregates.

That allows mem-aref to have "value" semantics for all types.  For aggregates,
converting to a plist is one possible way to representing it.

-- 
Martin Simmons
LispWorks Ltd
http://www.lispworks.com/


>>>>> On Mon, 20 Feb 2012 12:12:44 -0500, Liam Healy said:
> 
> Ryan -
> 
> I am forwarding this to CFFI-devel and Martin directly (though I'm pretty
> sure he's subscribed to cffi-devel).  Can you please send future messages
> to cffi-devel?  I'm not sure many people read the github mailing list; this
> issue should get wider visibility.   Thanks.
> 
> Liam
> 
> On Mon, Feb 20, 2012 at 12:02 PM, Ryan Pavlik <
> reply+i-1614209-ba246666762196459413560690eb7d3a39c7c7ee-838019 at reply.github.com
> > wrote:
> 
> > I'm not sure what you mean.  It doesn't really matter that a pointer is
> > aggregate or not.  The goal is to get at the Nth member of an
> > array-of-something.  In the case of scalar C types, you're getting the
> > value; in the case of structs it's far more useful to get a pointer,
> > because you probably only want a single value out of the struct, or to
> > **put a value into the struct**.  (The `setf` form works for scalars in the
> > latter case, but not for "member-of-struct-of-index-N"... you most likely
> > want the pointer in these cases.)
> >
> > It also occurred to me after posting that there is no difference between
> > `(mem-aref ptr '(:pointer (:struct foo)) N)` and just simply `(mem-aref ptr
> > :pointer N)` ... both return a pointer value as if `ptr` is a `void
> > *ptr[k]`.
> >
> > A struct of more bytes works the same way:
> >
> > ```
> > (cffi:defcstruct my-struct
> >  (x :long)
> >  (y :long)
> >  (x :long)
> >  (t :long))
> >
> > ...
> >
> > Old-ref style:
> > ptr : #.(SB-SYS:INT-SAP #X7FFFEEFC7FB8)
> > aref: #.(SB-SYS:INT-SAP #X7FFFEEFC7FD8)
> > New-ref style:
> > ptr : #.(SB-SYS:INT-SAP #X7FFFEEFC7FB8)
> > aref: (T 0 Y 0 X 0)
> > New-ref with :pointer style:
> > ptr : #.(SB-SYS:INT-SAP #X7FFFEEFC7FB8)
> > aref: #.(SB-SYS:INT-SAP #X00000000)
> > ```
> >
> > Note that on my system, a pointer is 8 bytes, not 4.  This is why I
> > initially found the problem, when trying to access an array of points
> > defined by 2 short; each member is 4 bytes, and it was giving offsets to
> > `sizeof(void*)`.
> >
> > ---
> > Reply to this email directly or view it on GitHub:
> > https://github.com/cffi/cffi/pull/2#issuecomment-4057418
> >
>