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-838...@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 > > > _______________________________________________ cffi-devel mailing list cffi-devel@common-lisp.net http://lists.common-lisp.net/cgi-bin/mailman/listinfo/cffi-devel
