[cffi-devel] Converting foreign structures with CFFI generic functions

[Joerg-Cyril.Hoehle at t-systems.com]

Hi,

Luís Oliveira wrote:
>I agree that DEFCSTRUCT* is a lousy name, but DEFCSTRUCT-BY-VALUE
>is misleading, since there's more going on.

I recommend you look again at CLISP's FFI
http://www.clisp.org/impnotes/dffi.html
It is essentially call by value.  It will convert (de-/serialize)
arbitrarily nested structures to and from Lisp, for instance a full linked
list of objects given a single pointer (and hang on a circular list...).
Passing stuff by reference (i.e. using the type (C-POINTER XYZ) was a late addition.

Note that automated return by value is not void of problems.  Old APIs use
structs or &out pointers with flags indicating which fields are valid and
you better not convert back an invalid char* (or random bits as a float).
Modern APIs tend to do better here, esp. those that have been influenced
by remote design thoughts (RPC, network transmission etc.).

The old Haskell/Direct had a domain specific language to express
such dependencies and I've found myself suggesting some for CLISP
(e.g. a :guard to prevent dereferencing some slots, as in
(def-c-fun foobar (:return-type int) ...
(errorstr (:type c-string) (:out :alloca) (:guard (zerop return)))
=> foobar returns type (values integer (or string null))
  with integer return value being not 0 => NIL as second value, no string
(or the converse, I forgot how I originally conceived the guard)

Regards,
	Jörg Höhle