[cffi-devel] Structure pointer with new structure syntax: mem-aptr

[Liam Healy]

I think Martin is right, so I have implemented mem-aptr and updated the
manual.  Here are Ryan's examples:

    (cffi:defcstruct rp-struct
     (x :short)
     (y :short))

    (defun test-old-ref (&optional (count 0))
     (declare (notinline cffi:mem-ref cffi:mem-aref))
     (cffi:with-foreign-object (ptr '(:struct rp-struct) 2)
       (format t "~&Old-ref style:~%ptr : ~A~%aref: ~A~%"
               ptr (cffi:mem-aref ptr 'rp-struct count))))

    (defun test-new-ref (&optional (count 0))
     (cffi:with-foreign-object (ptr '(:struct rp-struct) 2)
       (format t "~&New-ref style:~%ptr : ~A~%aref: ~A~%"
              ptr
              (cffi:mem-aptr ptr '(:struct rp-struct) count))))

    CFFI(14): (test-old-ref)
    STYLE-WARNING:
       bare references to struct types are deprecated. Please use (:STRUCT
RP-STRUCT) instead.
    STYLE-WARNING:
       bare references to struct types are deprecated. Please use (:STRUCT
RP-STRUCT) instead.
    STYLE-WARNING:
       bare references to struct types are deprecated. Please use (:POINTER
(:STRUCT RP-STRUCT)) instead.

    Old-ref style:
    ptr : #.(SB-SYS:INT-SAP #X7FFFF6E97FF0)
    aref: #.(SB-SYS:INT-SAP #X7FFFF6E97FF0)
    NIL
    CFFI(15): (test-new-ref)

    New-ref style:
    ptr : #.(SB-SYS:INT-SAP #X7FFFF6E97FF0)
    aref: #.(SB-SYS:INT-SAP #X7FFFF6E97FF0)
    NIL
    CFFI(16): (test-old-ref 1)
    STYLE-WARNING:
       bare references to struct types are deprecated. Please use (:STRUCT
RP-STRUCT) instead.
    STYLE-WARNING:
       bare references to struct types are deprecated. Please use (:STRUCT
RP-STRUCT) instead.
    STYLE-WARNING:
       bare references to struct types are deprecated. Please use (:POINTER
(:STRUCT RP-STRUCT)) instead.

    Old-ref style:
    ptr : #.(SB-SYS:INT-SAP #X7FFFF6E97FF0)
    aref: #.(SB-SYS:INT-SAP #X7FFFF6E97FF4)
    NIL
    CFFI(17): (test-new-ref 1)

    New-ref style:
    ptr : #.(SB-SYS:INT-SAP #X7FFFF6E97FF0)
    aref: #.(SB-SYS:INT-SAP #X7FFFF6E97FF4)
    NIL

This looks right to me.  I made a new branch libffi; the old branch fsbv
will be abandoned.  Note that generation of a pointer from mem-aref for
structures is only supported when using the deprecated "bare" structures
form, for compatibility reasons; otherwise you should use mem-aptr.
Please checkout this branch and see if it works correctly for you and looks
right.
I agree as Martin said the other examples are incorrectly written, so I'm
not bothering with them.

Head:       0fec6ff839 - Update manual to include mem-aptr

Liam


On Fri, Feb 24, 2012 at 7:34 AM, Martin Simmons <martin at lispworks.com>wrote:

> 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
> > >
> >
>
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