[Ecls-list] Safety of vector addresses.

[David Brown]

I was wondering about the stability of item->vector.self pointers in
ECL.  As I understand, the conservative GC never moves blocks of
memory around, but I wanted to make sure this is correct.

Basically, I'm looking to implement the shareable-byte-vector
interface in CFFI, and wanted to make sure what I was doing was safe.

Are there any plans to ever add a GC that can relocate, or is using a
conservative GC kind of in line with the embedable part of ECL?

This is the code I'm thinking of putting into CFFI, into
cffi-ecl.lisp:

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

;;; ECL, built with the Boehm GC never moves allocated data.
(defun %vector-address (vector)
   "Return the address of VECTOR's data."
   (check-type vector (vector (unsigned-byte 8)))
   (ffi:c-inline (vector) (object) :unsigned-long
                 "(unsigned long) #0->vector.self.b8"
                 :side-effects nil
                 :one-liner t))

(defmacro with-pointer-to-vector-data ((ptr-var vector) &body body)
   "Bind PTR-VAR to a foreign pointer to the data in VECTOR."
   `(let ((,ptr-var (make-pointer (%vector-address ,vector))))
      , at body))

Previously defined
(defun make-pointer (address)
   "Return a pointer pointing to ADDRESS."
   (ffi:make-pointer address :void))

Thanks,
David