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

[Martin Simmons]

This looks wrong to me -- a pointer is not an aggregate type.  Also, I think
your test-new-ptr-ref and test-generic-ptr-ref tests are bogus because the
type in mem-aref doesn't match the type of ptr.

Have you tried it with a struct containing 20 bytes?  Testing it with an array
of two 4 byte structs leads to confusion about the size of a pointer v.s. the
size of one struct v.s. the size of the array.

In fact, I don't understand what mem-aref is supposed to do for an aggregate
type.  Making it return a pointer to the first byte of the aggregate is
inconsistent with how it treats other types and IMHO leads to the current
confusion.

__Martin



>>>>> On Sat, 18 Feb 2012 12:53:17 -0500, Liam Healy said:
> 
> The new syntax is as follows.  If you want the structure itself, use
> (:struct foo).  If you want a pointer to it, use (:pointer (:struct
> foo)).   The latter should be identical to the old style bare struct
> specification, except for the annoying deprecation warning, of course.
> 
> The issues I can identify, with their resolutions:
> 
> 1) Using mem-aref with the (:pointer (:struct ...)) spec gives the wrong
> pointer.
> 
> I have fixed an error which should now return the correct pointer for an
> offset of 0.  For an offset of 1, it returns the base pointer +8 bytes,
> which is not what the old style gives (+ 4 bytes), but it seems to me
> correct, as I understand the index to refer to the number of whole
> structures.   Pull ee90bfd517 and try.
> 
> 2) The plist form representing the structure is not desirable.
> 
> You can have any CL representation of the structure you like; you need to
> define a method cffi:translate-from-foreign for your type class.  You are
> getting the default, a plist translation, because no such method is
> defined.   See for example how I translate complex
> numbers<http://repo.or.cz/w/antik.git/blob/1ee407c69525b84b441f8cf7b48ac590e78bd635:/foreign-array/complex-types.lisp#l50>to
> CL complex numbers.  You can even return a pointer if you want, but
> this
> probably isn't the specification to use if you want the pointer.
> 
> 
> On Mon, Feb 13, 2012 at 10:22 AM, Ryan Pavlik <
> reply+i-1614209-ba246666762196459413560690eb7d3a39c7c7ee-838019 at reply.github.com
> > wrote:
> 
> > I've pulled the latest and it appears the semantics have changed for
> > mem-aref, but there is still no way to get the old behavior.  Here is a
> > complete example, though it doesn't use the test system definitions because
> > actual foreign calls and definitions aren't really the problem:
> >
> > ```
> > (asdf:load-system :cffi)
> >
> > (cffi:defcstruct my-struct
> >  (x :short)
> >  (y :short))
> >
> > (defun test-old-ref ()
> >  (declare (notinline cffi:mem-ref cffi:mem-aref))
> >  (cffi:with-foreign-object (ptr '(:struct my-struct) 2)
> >    (format t "~&Old-ref style:~%ptr : ~A~%aref: ~A~%"
> >            ptr (cffi:mem-aref ptr 'my-struct 1))))
> >
> > (defun test-new-ref ()
> >  (cffi:with-foreign-object (ptr '(:struct my-struct) 2)
> >    (format t "~&New-ref style:~%ptr : ~A~%aref: ~A~%"
> >           ptr
> >           (cffi:mem-aref ptr '(:struct my-struct) 1))))
> >
> > (defun test-new-ptr-ref ()
> >  (cffi:with-foreign-object (ptr '(:struct my-struct) 2)
> >    (format t "~&New-ref with :pointer style:~%ptr : ~A~%aref: ~A~%"
> >           ptr
> >           (cffi:mem-aref ptr '(:pointer (:struct my-struct)) 1))))
> >
> > (progn
> >  (test-old-ref)
> >  (test-new-ref)
> >  (test-new-ptr-ref))
> > ```
> >
> > The output I get, sans style-warnings about bare structs:
> >
> > ```
> > Old-ref style:
> > ptr : #.(SB-SYS:INT-SAP #X7FFFEEFCFFF0)
> > aref: #.(SB-SYS:INT-SAP #X7FFFEEFCFFF4)
> > New-ref style:
> > ptr : #.(SB-SYS:INT-SAP #X7FFFEEFCFFF0)
> > aref: (Y 0 X 0)
> > New-ref with :pointer style:
> > ptr : #.(SB-SYS:INT-SAP #X7FFFEEFCFFF0)
> > aref: #.(SB-SYS:INT-SAP #X00000000)
> > ```
> >
> > Note that in the first example, with the original semantics, if you
> > mem-aref a pointer to an array of `my-struct`, you get a pointer to the
> > array element.  In the new style, with `(:struct my-struct)`, you get the
> > values parsed into a list, which is not particularly useful; it conses, and
> > you almost certainly have to re-parse a possibly long list for a single
> > element.  In the new style with `:pointer`, it appears to dereference the
> > Nth element in an *array of pointers to my-struct*, which is not at all
> > what we want.
> >
> > The latter differs from the behavior before I updated, which seemed to
> > return a *pointer* to the Nth element in an array-of-pointers.  None of the
> > above are like the old behavior.
> >
> > ---
> > Reply to this email directly or view it on GitHub:
> > https://github.com/cffi/cffi/pull/2#issuecomment-3941718
> >
>