ConditionVariable objects augment class Mutex. Using condition variables, it is possible to suspend while in the middle of a critical section until a resource becomes available.

Example:

  require 'thread'

  mutex = Mutex.new
  resource = ConditionVariable.new

  a = Thread.new {
    mutex.synchronize {
      # Thread 'a' now needs the resource
      resource.wait(mutex)
      # 'a' can now have the resource
    }
  }

  b = Thread.new {
    mutex.synchronize {
      # Thread 'b' has finished using the resource
      resource.signal
    }
  }