rif rif@MIT.EDU writes:

I have a foreign object, points to a vector of (C) double-floats:

(defstruct (fnv-double (:constructor make-fnv-double-internal)) (foreign-pointer nil :read-only t) (size 0 :type fixnum :read-only t))

I can create and manipulate it. I need to know how to define (with defcfun) an interface to a C function which expects a vector of double-floats. My attempts have been along the lines of:

(defmethod translate-to-foreign (val (f fnv-double)) (fnv-foreign-pointer f))

The two arguments to TRANSLATE-TO-FOREIGN are the Lisp object to be converted, and the name of the foreign type to convert to. First, declare a foreign typedef for :POINTER (since you will ultimately be returning a pointer from the type translator) that you use to hang your type translator on:

(defctype fnv-double :pointer)

Then you can specialize TRANSLATE-TO-FOREIGN on both the Lisp value and the foreign type (using an EQL specializer):

(defmethod translate-to-foreign ((value fnv-double) (name (eql 'fnv-double))) (fnv-foreign-pointer value))

It might be confusing to call both the Lisp and foreign types FNV-DOUBLE, but it is legal because foreign type names are a separate namespace from Lisp types (viva Lisp-N!). It is important to remember the distinction between the two though.

and then declaring something like

(defcfun ("foo" :%foo) :double (x fnv-double))

should just work!

(This is untested and I haven't really been doing any CFFI hacking lately, so I apologize if I missed something obvious here.)

James