Source code in the common programming language

;;; Demonstration of references by swapping two variables using a function rather than a macro
;;; Needs http://paste.lisp.org/display/71952
(defun swap (ref-left ref-right)
  ;; without with-refs we would have to write this:
  ;; (psetf (deref ref-left) (deref ref-right)
  ;;        (deref ref-right) (deref ref-left))
  (with-refs ((l ref-left) (r ref-right))
    (psetf l r r l)))

(defvar *x* 42)
(defvar *y* 0)

(swap (ref *x*) (ref *y*))

;; *y* -> 42
;; *x* -> 0


(use-package :ffi)

(defmacro def-libc-call-out (name &rest args)
  `(def-call-out ,name
     (:language :stdc)
     #-cygwin(:library "libc.so.6")
     #+cygwin (:library "cygwin1.dll")
     ,@args))

(progn
  (def-libc-call-out errno-location
    #-cygwin (:name "__errno_location")
    #+cygwin (:name "__errno")
    (:arguments)
    (:return-type (c-pointer int)))

  (defun get-errno ()
    (let ((loc (errno-location)))
      (foreign-value loc)))

  (defun set-errno (value)
    (let ((loc (errno-location)))
      (setf (foreign-value loc) value)))

  (defsetf get-errno set-errno)

  (define-symbol-macro errno (get-errno)))