CL-USER> (defun worker (mailbox stream)
           (let ((x 0))
             (loop
              (print x stream)
              (sleep 1)
              (let ((message (sb-concurrency:receive-message-no-hang mailbox)))
                (case (car message)
                  (:set-x (setf x (cadr message)))
                  (:die (return)))))))
WORKER
CL-USER> (defvar *mailbox*
           (sb-concurrency:make-mailbox))
*MAILBOX*
CL-USER> (sb-thread:make-thread #'worker :arguments (list *mailbox* *standard-output*))

0 
#<SB-THREAD:THREAD RUNNING {100BB65D53}>

0 
0 
0 
0 
0 
0 
CL-USER> (sb-concurrency:send-message *mailbox* '(:set-x 123))

NIL

123 
123 
123 
CL-USER> (sb-concurrency:send-message *mailbox* '(:set-x 321))

NIL

321 
321 
321 
321 
CL-USER> (sb-concurrency:send-message *mailbox* '(:die))

321 
NIL