[cffi-devel] shareable-byte-vectors for Allegro

[John Fremlin]

"Luís Oliveira" <luismbo at gmail.com> writes:

> On Mon, Jan 19, 2009 at 5:50 AM, John Fremlin <jf at msi.co.jp> wrote:
>> The advantage of the change is that ptr-var is now actually a foreign
>> pointer as it is in all other cffi-ports.
>>
>> You might not want to pass the ptr-var directly to a function, but
>> actually use it as a foreign pointer.
>>
>> For a concrete example, using the recvmsg(2) system call, you pass a
>> structure containing an iovec, which is itself a structure containing
>> the base address and length of the buffer into which the message body
>> should be written.
>
> That definitely looks like a legitimate use-case. Let us know when you
> figure out what the first argument to FF:FSLOT-ADDRESS-TYPED should
> be.

Here is the new magic based on fresh clues from Duane . . . This one
seems to work. A thorough test suite would be great though!

(defun make-shareable-byte-vector (size)
  "Create a Lisp vector of SIZE bytes which can be passed to
;WITH-POINTER-TO-VECTOR-DATA."
  (make-array size :element-type '(unsigned-byte 8) :allocation :static-reclaimable))

(defmacro with-pointer-to-vector-data ((ptr-var vector) &body body)
  "Bind PTR-VAR to a foreign pointer to the data in VECTOR. Not safe
except with vectors allocated by MAKE-SHAREABLE-BYTE-VECTOR and
possibly arrays of type simple-array (unsigned-byte 8) (*)."
;;; An array allocated in static-reclamable is a non-simple array in
;;; the normal Lisp allocation area, pointing to a simple array in the
;;; static-reclaimable allocation area. Therefore we have to get out
;;; the simple-array to find the pointer to the actual contents.
  (with-unique-names (simple-vec)
  `(excl:with-underlying-simple-vector (,vector ,simple-vec)
     (let ((,ptr-var (ff:fslot-address-typed :unsigned-char :lisp ,simple-vec)))
       , at body))))