(defun reader (stream)
  (let ((result (make-string 10)))
    (file-position stream 1024)
    (read-sequence result stream)
    result))

(defun writer (stream)
  (let ((output "abcde12345"))
    (file-position stream 0)
    (write-sequence output stream)
    (values)))

(defun init-file (stream)
  (file-position stream 0)
  (let ((buffer (make-string 80 :initial-element #\space)))
    (write-sequence buffer stream)
    (write-sequence buffer stream)))

(defmacro with-my-file ((stream path) &body body)
  `(with-open-file (,stream ,path
                            :element-type 'character
                            :direction :io
                            :if-does-not-exist :create
                            :if-exists :append
                            #+ccl :sharing #+ccl :external)
     (init-file stream)
     ,@body))


(with-my-file (stream "/tmp/temp.data")
  (reader stream)
  (writer stream))

(com.informatimago.common-lisp.interactive.browser:cat "/tmp/temp.data")

(with-my-file (stream "/tmp/temp.data")
  (writer stream)
  (reader stream))

(com.informatimago.common-lisp.interactive.browser:cat "/tmp/temp.data")

(with-my-file (stream "/tmp/temp.data")
  (dolist (thread (list (bt:make-thread (lambda () (writer stream)) :name "writer")
                        (bt:make-thread (lambda () (reader stream)) :name "reader")))
    (bt:join-thread thread)))

(com.informatimago.common-lisp.interactive.browser:cat "/tmp/temp.data")

#|
> (load "~/f.lisp")
abcde12345
abcde12345
abcde12345
#P"/Users/pjb/f.lisp"
>
|#