연습문제 3.47

a.

<blockquote>(define (make-semaphore-mtx maximal)
  (let ((count maximal)
        (mutex (make-mutex)))
    (define (the-sema m)
      (cond ((eq? m 'release)
              (mutex 'acquire)
              (unless (= count maximal)
                (set! count (+ 1 count)))
              (mutex 'release))
            ((eq? m 'acquire)
              (mutex 'acquire)
              (cond
                ((> count 0)
                  (set! count (- count 1))
                  (mutex 'release))
                (else
                  (mutex 'release)
                  (the-sema 'acquire))))
            (else
              (error "Unknown request -- " m))))
    the-sema))</blockquote>
b.
<blockquote>(define (loop-test-and-set! cell)
  (if (test-and-set! cell)
    (loop-test-and-set! cell)
    '()))

(define (make-semaphore-ts maximal)
  (let ((count maximal)
        (guard (cons #f '())))
    (define (the-sema m)
      (cond ((eq? m 'release)
              (loop-test-and-set! guard)
                (unless (= count maximal)
                  (set! count (+ 1 count)))
              (clear! guard))
            ((eq? m 'acquire)
              (cond
                (loop-test-and-set! guard)
                ((> count 0)
                  (set! count (- count 1))
                  (clear! guard))
                (else
                  (clear! guard)
                  (the-sema 'acquire))))
            (else
              (error "Unknown request -- " m))))
    the-sema))
</blockquote>