Practice 3.47

基于互斥元

(define (make-semaphore n)
  (let ((mutex (make-mutex)))
    (define (the-semaphore m)
      (cond ((eq? 'acquire)
             ((mutex 'acquire)
                 (if (> n 0)
                     (begin (set! n (- n 1))
                            (mutex 'release))
                     (begin (mutex 'release)
                            (the-semaphore 'acquire)))))
            ((eq? 'release)
             (begin (mutex 'acquire)
                    (set! n (+ n 1))
                    (mutex 'release)))))
    the-semaphore))

基于原子的test-and-set!

(define (make-semaphore n)
  (let ((count n))
    (define (the-semaphore m)
      (cond ((eq? 'acquire)
             (if (test-and-set! count)
                 (the-semaphore 'acquire)))
            ((eq? m 'release) (release! count))))
    the-semaphore))

(define (release! count)
  (set! count (+ count 1)))

(define (test-and-set! count)
  (if (> 0 count)
      (begin (set! count (- count 1))
             false)
      true))