CFFI User Manual - Common Lisp.net

Chapter 4: An Introduction to Foreign Interfaces and CFFI 20
(with-slots (pointer error-buffer c-strings) handle
(curl-easy-cleanup pointer)
(foreign-free error-buffer)
(mapc #’foreign-string-free c-strings)))
(define-foreign-type easy-handle-type ()
()
(:actual-type :pointer)
(:simple-parser easy-handle))
(defmethod translate-to-foreign (handle (type easy-handle-type))
"Extract the pointer from an easy-HANDLE."
(slot-value handle ’pointer))
While we changed some of the Lisp functions defined earlier to use CLOS slots rather
than hash tables, the foreign functions work just as well as they did before.
The greatest strength, and the greatest limitation, of the type translator comes from its
generalized interface. As stated previously, we could define all foreign function calls in terms
of the primitive foreign types provided by CFFI. The type translator interface allows us to
cleanly specify the relationship between Lisp and C data, independent of where it appears
in a function call. This independence comes at a price; for example, it cannot be used to
modify translation semantics based on other arguments to a function call. In these cases,
you should rely on other features of Lisp, rather than the powerful, yet domain-specific,
type translator interface.
4.12 What’s next?
CFFI provides a rich and powerful foundation for communicating with foreign libraries; as
we have seen, it is up to you to make that experience a pleasantly Lispy one. This tutorial
does not cover all the features of CFFI; please see the rest of the manual for details. In
particular, if something seems obviously missing, it is likely that either code or a good
reason for lack of code is already present.
Implementor’s note: There are some other things in CFFI that might deserve
tutorial sections, such as free-translated-object, or structs. Let us know which ones
you care about.