chickadee » srfi-18 » condition-variable-signal!

If there are threads blocked on the condition-variable, the scheduler selects a thread and unblocks it. condition-variable-signal! returns an unspecified value.

    ; an implementation of a mailbox object of depth one; this
    ; implementation behaves gracefully when threads are forcibly
    ; terminated using thread-terminate! (the "abandoned mutex"
    ; exception will be raised when a put! or get! operation is attempted
    ; after a thread is terminated in the middle of a put! or get!
    ; operation)
 
    (define (make-empty-mailbox)
      (let ((mutex (make-mutex))
            (put-condvar (make-condition-variable))
            (get-condvar (make-condition-variable))
            (full? #f)
            (cell #f))
 
        (define (put! obj)
          (mutex-lock! mutex)
          (if full?
              (begin
                (mutex-unlock! mutex put-condvar)
                (put! obj))
              (begin
                (set! cell obj)
                (set! full? #t)
                (condition-variable-signal! get-condvar)
                (mutex-unlock! mutex))))
 
        (define (get!)
          (mutex-lock! mutex)
          (if (not full?)
              (begin
                (mutex-unlock! mutex get-condvar)
                (get!))
              (let ((result cell))
                (set! cell #f) ; avoid space leaks
                (set! full? #f)
                (condition-variable-signal! put-condvar)
                (mutex-unlock! mutex)
                result)))
 
        (lambda (msg)
          (case msg
            ((put!) put!)
            ((get!) get!)
            (else (error "unknown message"))))))
 
    (define (mailbox-put! m obj) ((m 'put!) obj))
    (define (mailbox-get! m) ((m 'get!)))