ffi and private declarations

ffi_private.rb

require 'ffi'

class Foo
  class << self
    extend FFI::Library
    ffi_lib FFI::Library::LIBC

    private

    attach_function 'puts', [:string], :int

    public

    def hello
      puts "Hello, World!"
    end
  end
end

p Foo.methods(false) # => why is 'puts' here?