Ruby Semaphore Implementation

semaphore.rb

require 'monitor'
class Semaphore
  def initialize(maxval = nil)
    maxval = maxval.to_i unless maxval.nil?
    raise ArgumentError.new("Semaphores must use a positive maximum value or have no maximum!") if maxval and maxval <= 0
    @max   = maxval || -1
    @count = 0
    @mon   = Monitor.new
    @dwait = @mon.new_cond
    @uwait = @mon.new_cond
  end
  
  def count; @mon.synchronize { @count } end
  
  def up!(number = 1)
    if (number > 1)
      number.times { up!(1) }
      count
    else
      @mon.synchronize do
        @uwait.wait while @max > 0 and @count == @max
        @dwait.signal if @count == 0
        @count += 1
      end
    end
  end
  
  def down!(number = 1)
    if (number > 1)
      number.times { down!(1) }
      count
    else
      @mon.synchronize do
        @dwait.wait while @count == 0
        @uwait.signal if @count == @max 
        @count -= 1
      end
    end
  end
end

On more than one occasion I've been bothered by the fact that there's no standard implementation of semaphores in ruby. Sometimes a semaphore really is a more expressive synchronization metaphor. So- even though this implementation is making a more primitive synchronization object out of more complex ones, I've still found it useful enough to share.

Thanks a bunch! This works well. In my project I also added the following aliases, which provide the more classical terminology:

alias_method :wait, :down!
alias_method :signal, :up!

Thanks or sharing this gist. I was looking around for something like that.

What kind of code is "@dwait.wait while @count == 0"??!!

@ldcl289 it's been a long time since I wrote this gist, but basically you can't decrement a semaphore lower than 0. Semantically, when that happens the calling thread is supposed to be blocked and go to the wait queue.

An 'if' condition would have sufficed, but a 'while' statement is also valid and protects against modifications to this class that might call @dwait.broadcast (waking up more than the appropriate number of waiters).