gistfile1.txt

class Semaphore
  #
  # Creates a new Semaphore
  #
  def initialize(initvalue = 0)
    @counter = @max  = initvalue
    @waiting = []
    @mutex = Mutex.new
  end

  #
  # Attempts to enter and waits if the semaphore is already full
  #
  def wait
    @mutex.lock
    if (@counter -= 1) < 0
      @waiting.push(Thread.current)
      Thread.stop
    end
  ensure
    @mutex.unlock
  end

  alias down wait

  #
  # Leaves and let another thread in, if there's any waiting
  #
  def signal
    @mutex.lock
    begin
      #if (@counter = [@counter+1,@max].min) <= 0
      if (@counter += 1) <= 0
        t = @waiting.shift
        t.wakeup if t
      end
    rescue ThreadError
      retry
    end
  ensure
    @mutex.unlock
  end

  alias up signal
end

output

sem.rb:5:in `block in <main>'
sleep
/tmp/ruby/lib/ruby/1.9.1/thread.rb:50:in `wait'
sem.rb:6:in `block in <main>'
sleep
/tmp/ruby/lib/ruby/1.9.1/thread.rb:62:in `signal'
sem.rb:5:in `block in <main>'
sleep
/tmp/ruby/lib/ruby/1.9.1/thread.rb:50:in `wait'
sem.rb:6:in `block in <main>'

sem.rb

require "thread"

s = Semaphore.new(1)

t1 = Thread.new { s.down; Thread.stop; s.up }
t2 = Thread.new { s.down; puts "It works" }

puts t1.status
puts t1.backtrace
puts t2.status
puts t2.backtrace

t1.run

puts t1.status
puts t1.backtrace
puts t2.status
puts t2.backtrace