[cffi-devel] Re: CFFI Callbacks on SBCL

[Luís Oliveira]

I'm Cc-ing cffi-devel and sbcl-devel, since I'm sure more people will  
be interested in this and hopefully others will help in figuring this  
out.

On 2005-dec-31, at 13:59, Thomas F. Burdick wrote:
> I noticed in the #lisp logs that you're having problems preserving
> callbacks across image saves.  SBCL already goes to quite a bit of
> effort to make sure this works.  From C's point of view, the callbacks
> will never move.  But because your library is in lisp, and persists
> across image saves, from its point of view, callback objects can move
> at arbitrary times.  Instead of storing SAPs, you should store the
> alien-value object that alien-lambda returns.  If you actually need
> the SAP, get it only when you need it, using alien-sap.

We're not even testing if the callback moves or not. Our libtest is  
not saving the callbacks' address.

Here's the a testcase, using sb-alien (sort of):

luis at nhop:/tmp$ cat sbcl-callback.lisp
(use-package :sb-alien)

(define-alien-routine "qsort" void
   (base system-area-pointer)
   (nmemb int)
   (size int)
   (compar system-area-pointer))

(defparameter *callback*
   (sb-alien::alien-lambda int
       ((a system-area-pointer) (b system-area-pointer))
     (let ((x (sb-sys:signed-sap-ref-32 a 0))
           (y (sb-sys:signed-sap-ref-32 b 0)))
       (cond ((> x y) 1)
             ((< x y) -1)
             (t 0)))))

(defun test ()
   (let ((array (alien-sap (make-alien (unsigned 32) 10))))
     (dotimes (i 10)
       (setf (sb-sys:signed-sap-ref-32 array (* i 4))
             (nth i '(7 2 8 9 6 1 10 3 5 4))))
     (qsort array 10 4 (alien-sap *callback*))
     (dotimes (i 10)
       (format t "~A " (sb-sys:signed-sap-ref-32 array (* i 4))))
     (free-alien (sap-alien array (* (unsigned 32))))))

luis at nhop:/tmp$ sbcl --sysinit /dev/null --userinit /dev/null
This is SBCL 0.9.8.3, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (load "sbcl-callback.lisp")

T
* (test)
1 2 3 4 5 6 7 8 9 10
NIL
* (sb-ext:save-lisp-and-die "test.core")
[undoing binding stack and other enclosing state... done]
[saving current Lisp image into /tmp/test.core:
writing 1584 bytes from the read-only space at 0x01000000
writing 1368 bytes from the static space at 0x05000000
writing 22609920 bytes from the dynamic space at 0x09000000
done]
luis at nhop:/tmp$ sbcl --core test.core
This is SBCL 0.9.8.3, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (test)

debugger invoked on a SB-KERNEL::MEMORY-FAULT-ERROR: memory fault

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
   0: [ABORT] Exit debugger, returning to top level.

(SB-KERNEL::MEMORY-FAULT-ERROR)
0]


Any pointers? Well, I'm off to party. Happy new year!

-- 
Luís Oliveira
http://student.dei.uc.pt/~lmoliv/
Equipa Portuguesa do Translation Project
http://www.iro.umontreal.ca/translation/registry.cgi?team=pt