[SICP Excercise 3.47] Some observation about semaphore

7-14-2014

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

(I’ve Borrowed code above from Weiqun Zhang’s Blog – SICP Exercise 3.47.)

It took some time to understand the structure of semaphore. Here is the problem:

The place of ‘acquire’

Next question that bothered me was that: does the code still work if I replace acquire method as follows?

(define (make-semaphore n)
  (let ((the-mutex (make-mutex))
        (count 0))
    (define (the-semaphore m)
      (cond ((eq? m 'acquire)
             (if (= count n)
                 (the-semaphore 'acquire))
                 (begin
                   (the-mutex 'acquire) ; Added acquire  method here instead
                   (set! count (+ count 1))
                   (the-mutex 'release))))
            ((eq? m 'release)
             (the-mutex 'acquire)
             (if (> count 0)
                 (set! count (- count 1)))
             (the-mutex 'release))))
    the-semaphore))

After a little bit observation I noticed that this code has a problem: