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

[John Fremlin]

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

> On Mon, Jan 19, 2009 at 2:35 AM, John Fremlin <jf at msi.co.jp> wrote:
>> Duane at Franz has helpfully come up with a decent way to get the real
>> foreign address from a Lisp array.
>>
>> (let ((x (make-array 10 :element-type '(unsigned-byte 8) :allocation :static-reclaimable)))
>>   (ff:fslot-address-typed :unsigned-char :lisp x))
>>
>> We will test a modification of with-shareable-byte-vector to use this
>> mechanism for ,ptr-var, hopefully it will work well.
>
> What's the advantage, compared to using :FOREIGN-ADDRESS?

We can continue to use foreign-address, so that people will be able to
(unportably) take advantage of Allegro's generous handling of Lisp arrays.

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.

We want this buffer to be a shareable byte-vector array.

Here is part of a real example

(cffi:with-foreign-objects ((sockaddr-buf :unsigned-char +sockaddr-max-len+)
			      (iov-buf 'iovec)
			      (msghdr-buf 'msghdr)
			      (cmsg-buf :unsigned-char +cmsg-buf-len+))
  (cffi-sys:with-pointer-to-vector-data (ptr buf)
    (cffi:with-foreign-slots ((base len) iov-buf iovec)
      (setf base ptr
	    len (length buf)))
    ... 

    (recvmsg (socket-fd socket)
			     msghdr-buf
			     (logior MSG_DONTWAIT MSG_ERRQUEUE))

))