Find port number not in use

gistfile1.lisp

(defun port-available-p (port)
  (handler-case (let ((socket (usocket:socket-listen "127.0.0.1" port :reuse-address t)))
                  (usocket:socket-close socket))
    (usocket:address-in-use-error (e) (declare (ignore e)) nil)))

;; Find port number not in use from 50000 to 60000.
(loop for port from (+ 50000 (random 1000)) upto 60000
      if (port-available-p port)
        return port)

;=> 50177