[cffi-devel] with-pointer-to-vector-data

[Hoehle, Joerg-Cyril]

Hi,
[Sorry for my coming out of the blue after such a long pause]

rpgoldman at sift.info
>The somewhat confusing manual materials are (section 8.1 of the ACL
>manual):

>Before accessing a Lisp value from C, it should be
>registered first. [...]
>A C function is provided that will return
>a pointer to the Lisp object given its index.
[...]
>If instead one were to pass the registration
>indices of these Lisp objects, one could readily find their new
>addresses using these indices following a call-back to Lisp.

I think this clearly implies that the registered object *can* move if
the C code ever uses a callback to Lisp.  It's *not* pinned in memory.

Scenario
Lisp
     -> C
         (obtain address)
      <-
Lisp...
      -> C
         ;former address can be invalid,
         ;use index and provided function to obtain new address

One can derive from that that if you hold the address for a long time,
including a couple of exchanges between Lisp and C, the original address
*can* be stale.

This gives me just another reason against any sort of
"with-pointer-to-vector-data" exposed to the outside world.
Use that in tight loops inside lowest-level CFFI code (such low-level
that no application programmer ever sees it).
Don't use that in application code!

As I said long time ago, a good block API is more value to CFFI users
than dangerous pinnable vector interfaces.  I haven't followed the CFFI
evolution this year and don't know if that's been done now.

Regards,
	Jorg Hohle.