how to send sysex with core midi and ruby ffi.rb

require 'ffi'

class MIDISysexSendRequest < FFI::Struct
  layout  :destination,         :pointer,
          :data,                :pointer,
          :bytes_to_send,       :uint32,
          :complete,            :int,
          :reserved,            [:char, 3],
          :completion_proc,     :pointer,
          :completion_ref_con,  :pointer
end

module C
  extend FFI::Library
  ffi_lib '/System/Library/Frameworks/CoreMIDI.framework/Versions/Current/CoreMIDI'

  attach_function :MIDIGetDevice, [:int], :pointer
  attach_function :MIDIDeviceGetEntity, [:pointer, :int], :pointer
  attach_function :MIDIEntityGetDestination, [:pointer, :int], :pointer
  attach_function :MIDISendSysex, [:pointer], :int

end

# hardcode it to my IAC driver and bus 1 entity
device = C.MIDIGetDevice( 0 )
entity = C.MIDIDeviceGetEntity( device, 0 )
end_point = C.MIDIEntityGetDestination( entity, 0 )

CALLBACK = FFI::Function.new( :void, [:pointer] ) do | sysex_request_ptr |
  # it never prints out anything.. hmm...
  puts sysex_request_ptr[:complete]
end

# lets pretend we are sending a GS reset to a roland
hex = %w(F0 41 10 42 12 40 00 7F 00 41 F7)
sysex_data = hex.map{ |h| h.hex }
format = "C" * sysex_data.size

bytes = FFI::MemoryPointer.new FFI.type_size(:char) * sysex_data.size
bytes.write_string sysex_data.pack(format)


request = MIDISysexSendRequest.new
request[:destination] = end_point
request[:data] = bytes
request[:bytes_to_send] = sysex_data.size
request[:complete] = 0
request[:completion_proc] = CALLBACK
request[:completion_ref_con] = request

C.MIDISendSysex(request)