eventfd.cr

lib LibC
  fun eventfd(initval : LibC::UInt, flags : LibC::Int) : LibC::Int
end

module FiberNotifier
  struct Notifier
    def initialize(@semaphore = false)
      @eventfd = IO::FileDescriptor.new(LibC.eventfd(0, 0))
      @write_buf = 1u64
      @read_buf = 0u64
    end

    def notify
      LibC.write(@eventfd.fd, pointerof(@write_buf), 8)
    end

    # Fetches all pending notifications, and return the count.
    def listen_all
      @eventfd.wait_readable { }
      reset
    end

    def reset
      LibC.read(@eventfd.fd, pointerof(@read_buf), 8)
      @read_buf
    end
  end

  struct Semaphore
    def initialize
      @eventfd = IO::FileDescriptor.new(LibC.eventfd(0, 1))
      @write_buf = 1u64
      @read_buf = 0u64
    end

    def notify
      LibC.write(@eventfd.fd, pointerof(@write_buf), 8)
    end

    # Decrement value 1 step.
    def listen
      @eventfd.wait_readable { reset }
    end

    def reset
      LibC.read(@eventfd.fd, pointerof(@read_buf), 8)
    end
  end
end

example.cr

notifier = FiberNotifier::Notifier.new
semaphore = FiberNotifier::Semaphore.new

spawn do
  2.times { notifier.notify }
  semaphore.notify
end

spawn do
  p notifier.listen_all
  semaphore.notify
end

semaphore.listen
semaphore.listen