CFFI User Manual - Common Lisp.net

Chapter 6: Foreign Types 24
:boolean &optional (base-type :int)
[Foreign Type]
The :boolean type converts between a Lisp boolean and a C boolean. It canonicalizes
to base-type which is :int by default.
(convert-to-foreign nil :boolean) ⇒ 0
(convert-to-foreign t :boolean) ⇒ 1
(convert-from-foreign 0 :boolean) ⇒ nil
(convert-from-foreign 1 :boolean) ⇒ t
:wrapper base-type &key to-c from-c
[Foreign Type]
The :wrapper type stores two symbols passed to the to-c and from-c arguments. When
a value is being translated to or from C, this type funcalls the respective symbol.
:wrapper types will be typedefs for base-type and will inherit its translators, if any.
Here’s an example of how the :boolean type could be defined in terms of :wrapper.
(defun bool-c-to-lisp (value)
(not (zerop value)))
(defun bool-lisp-to-c (value)
(if value 1 0))
(defctype my-bool (:wrapper :int :from-c bool-c-to-lisp
:to-c bool-lisp-to-c))
(convert-to-foreign nil ’my-bool) ⇒ 0
(convert-from-foreign 1 ’my-bool) ⇒ t
6.3 Defining Foreign Types
You can define simple C-like typedefs through the defctype macro. Defining a typedef is
as simple as giving defctype a new name and the name of the type to be wrapped.
;;; Define MY-INT as an alias for the built-in type :INT.
(defctype my-int :int)
With this type definition, one can, for instance, declare arguments to foreign functions
as having the type my-int, and they will be passed as integers.
More complex types
CFFI offers another way to define types through define-foreign-type, a thin wrapper
macro around defclass. As an example, let’s go through the steps needed to define a
(my-string &key encoding) type. First, we need to define our type class:
(define-foreign-type my-string-type ()
((encoding :reader string-type-encoding :initarg :encoding))
(:actual-type :pointer))
The :actual-type class option tells CFFI that this type will ultimately be passed to and
received from foreign code as a :pointer. Now you need to tell CFFI how to parse a type
specification such as (my-string :encoding :utf8) into an instance of my-string-type.
We do that with define-parse-method: