#!/usr/bin/env ruby
require 'thread'

m = Mutex.new
x = 1
t = Thread.new do
  # ここは子のみが実行するところ
  5.times do
    m.synchronize {
      x=5; sleep 0.2
    }
  end
end
while t.alive?
  m.synchronize {
    STDERR.printf("x=%dに1足すと:", x)
    sleep 0.05
    x += 1
    STDERR.printf("%d\n", x)
  }
end
t.join