24 Feb
2012
24 Feb
'12
12:34 p.m.
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@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
> >
>