[cffi-devel] Re: cffi/allegro crash

[Hoehle, Joerg-Cyril]

Hi,

Luis Oliveira wrote:
>Glad you were able to fix this issue! You should pass (cffi:null- 
>pointer). CLISP does convert NIL to the null pointer which is  
>unfortunate for this reason (Hi
>Joerg!) :-) And it seems Lispworks does too.

>The lack of uniformity is very unfortunate here. This needs to be  
>fixed. We should either a) translate nil->null everywhere, b) don't  
>translate nil->null anywhere or c) offer the two alternatives through  
>two different types.

Surendra Singhi wrote:
>I will vote for clisp's and lispworks approach, it seems least
>surprising to me.
>Another related issue is when a C function returns a null pointer. The
>behavior in that case should be also uniform.

My opinion is:
a) the pointer type should be opaque to users.
b) Users should be pointed at the problematic string <> NIL <> pointer conversion. That's a requirement for documentation of CFFI/UFFI.
c) Use of NIL in CFFI or UFFI is declared non-portable in the documentation.
d) CFFI (and UFFI) provide means to pass "pointers" that the foreign side will see as a NULL pointer -- they already do so.
e) Such a "pointer" may well be represented as NIL on the Lisp side. Nevertheless its opaque.

f) CFFI and UFFI must export a type for the pointer types they use that is suitable for use with type declarations. -- a TODO for CFFI
   Example: (deftype pointer #+allegro number(?) #+clisp(or ffi:foreign-address ffi:foreign-variable ffi:foreign-function null))
g) This type cannot be used for discrimination (by this I mean that the CFFI/UFFI documentation specify this property).  That is (typep foo 'pointer) does not mean that foo is nothing but a pointer.  It could be an integer in Allegro, etc.

h) CFFI does not try to map the implementation's objects into whatever else.   Reasons: Meet the user's expectations. Performance. Don't overuse type transforms.
i) CFFI exports enough primitives that wrap needed functionality.  E.g. POINTER-EQ, IS-NULL-POINTER-P etc.

j) Only acceptable exception to h): Provide a new (non-default) pointer type that does null-pointer-p <-> NIL (or whatever else the user chooses) conversion.  Such a thing has its uses.  Another useful candidate for post-transforms is the "return-type is mostly unsigned int, except for -1" pattern. E.g. read() and the like. -1 is awful as an unsigned int, and using a signed int when its mostly unsigned also feels wrong.

You will recognize a bias towards "least common denominator", i.e. UFFI-style. However, I'd really dislike CFFI on CLISP back-check every NIL and create a foreign-pointer with address 0 instead :-(

Regards,
	Jorg Hohle.