test variables are shared between threads

var_thread_test.rb

# testing out class variables are shared between instances
require 'thread'

class A
  @m = Mutex.new
  @@m = Mutex.new
  M = Mutex.new

  def self.m; @m end
  def self.mm; @@m end
  def self.mmm; M end
end

puts "nil #{nil.object_id}"
@output = {}

def gather_output
  name = "Thread #{Thread.current.object_id}"
  @output[name] = []
  @output[name] << "c #{A}"
  @output[name] << "@m #{A.m}"
  @output[name] << "@@m #{A.mm}"
  @output[name] << "M #{A.mmm}"
end

threads = []
3.times do
  threads << Thread.new(&method(:gather_output))
end

gather_output
threads.map &:join

@output.each_pair do |name, o|
  puts [name, o].inspect
end