sem.rb

#! ruby19

require 'thread'

Thread.abort_on_exception = true

def log(msg)
  printf "%-30p %s\n", Thread.current, msg
end

class Semaphore
  def initialize(val)
    raise ArgumentError, "Wrong value %p" % val unless val >= 0
    @val = val
    @lock = Mutex.new
    @positive = ConditionVariable.new
  end

  def acquire
    @lock.synchronize do
      while @val == 0
        @positive.wait(@lock)
      end

      @val -= 1
    end
  end

  def release
    @lock.synchronize do
      @val += 1
      @positive.signal
    end
  end

  def synchronize
    acquire
    begin
      yield
    ensure
      release
    end
  end
end

# locking
puts <<MSG

Locking test

MSG

sem = Semaphore.new 2

th = (1..5).map do
  Thread.new do
    tc = Thread.current

    10.times do
      log "try"

      sem.synchronize do
        log "got"
      end

      log "released"
    end
  end
end

th.each {|t| t.join }


# signalling
puts <<MSG

Signalling test

MSG

sem = Semaphore.new 0

th = Thread.new do
  5.times do
    sem.acquire
    log "got it"
  end
end

10.times do
  log "signalling"
  sem.release
end

th.join