CFFI User Manual - Common Lisp.net

Chapter 6: Foreign Types 26
6.5 Optimizing Type Translators
Being based on generic functions, the type translation mechanism described above can add
a bit of overhead. This is usually not significant, but we nevertheless provide a way of
getting rid of the overhead for the cases where it matters.
A good way to understand this issue is to look at the code generated by defcfun.
Consider the following example using the previously defined my-string type:
CFFI> (macroexpand-1 ’(defcfun foo my-string (x my-string)))
;; (simplified, downcased, etc...)
(defun foo (x)
(multiple-value-bind (#:G2019 #:PARAM3149)
(translate-to-foreign x #)
(unwind-protect
(translate-from-foreign
(foreign-funcall "foo" :pointer #:G2019 :pointer)
#)
(free-translated-object #:G2019 #
#:PARAM3149))))
In order to get rid of those generic function calls, CFFI has another set of extensible generic
functions that provide functionality similar to CL’s compiler macros: expand-to-foreigndyn,
expand-to-foreign and expand-from-foreign. Here’s how one could define a myboolean
with them:
(define-foreign-type my-boolean-type ()
()
(:actual-type :int)
(:simple-parser my-boolean))
(defmethod expand-to-foreign (value (type my-boolean-type))
‘(if ,value 1 0))
(defmethod expand-from-foreign (value (type my-boolean-type))
‘(not (zerop ,value)))
And here’s what the macroexpansion of a function using this type would look like:
CFFI> (macroexpand-1 ’(defcfun bar my-boolean (x my-boolean)))
;; (simplified, downcased, etc...)
(defun bar (x)
(let ((#:g3182 (if x 1 0)))
(not (zerop (foreign-funcall "bar" :int #:g3182 :int)))))
No generic function overhead.
Let’s go back to our my-string type. The expansion interface has no equivalent of freetranslated-object;
you must instead define a method on expand-to-foreign-dyn, the
third generic function in this interface. This is especially useful when you can allocate
something much more efficiently if you know the object has dynamic extent, as is the case
with function calls that don’t save the relevant allocated arguments.
This exactly what we need for the my-string type: