format_message.rb

require 'ffi'
module MyFFI
  FORMAT_MESSAGE_FROM_SYSTEM = 0x00001000

  extend FFI::Library
  ffi_lib 'kernel32'
  ffi_convention :stdcall

  attach_function :FormatMessageA, [:int, :pointer, :int, :int, :string, :int, :pointer], :int
end

msg = " " * 1024
p len =  MyFFI.FormatMessageA(MyFFI::FORMAT_MESSAGE_FROM_SYSTEM, nil, 6, 0, msg, 1024, nil)
p msg[0..len-3] # remove \r\n\000

results.txt

D:\work\jruby-dev\samples\FFI>jruby formatMessage.rb
24
"                      "

D:\work\jruby-dev\samples\FFI>ruby -rrubygems formatMessage.rb
24
"The handle is invalid."