midi_in 2.rb

require 'ffi'

# http://msdn.microsoft.com/en-us/library/aa383751(VS.85).aspx
# DWORD appears to be an :int (32 bits that is, so :int should work well)
# HWND appears to be a :pointer see this thread for why you should not actually read from the value it points to. You may as well just use a :long or :ulong.
# LPDWORD is a pointer (the P standing for pointer)
# LPARAM appears to be a long (hence the L)
# WPARAM appears to be a long (i.e. 64 bits on 64bit OS).
# BOOL is an :int

module WinMM
  extend FFI::Library

  ffi_lib 'winmm'
  ffi_convention :stdcall

  CALLBACK_FUNCTION = 0x00030000

  # void CALLBACK MidiInProc(
  #   HMIDIIN hMidiIn,
  #   UINT wMsg,
  #   DWORD_PTR dwInstance,
  #   DWORD_PTR dwParam1,
  #   DWORD_PTR dwParam2
  # );
  callback :midiInProc, [:ulong, :uint, :int, :int, :int], :void
  MidiInProcCallback = Proc.new do |hmidiin, wmsg, dwintance, dwparam1, dwparam2|
    p [hmidiin, wmsg, dwintance, dwparam1, dwparam2]
  end
  # MidiInProcCallback = FFI::Function.new(:void, [:ulong, :uint, :int, :int, :int]) do |hmidiin, wmsg, dwintance, dwparam1, dwparam2|
  #   puts hmidiin, wmsg, dwintance, dwparam1, dwparam2  
  # end

  # MMRESULT midiInOpen(
  #   LPHMIDIIN lphMidiIn,
  #   UINT uDeviceID,
  #   DWORD_PTR dwCallback,
  #   DWORD_PTR dwCallbackInstance,
  #   DWORD dwFlags
  # )
  attach_function :midiInOpen, [ :pointer, :uint, :midiInProc, :int, :int ], :int

  # MMRESULT midiInClose(
  #   HMIDIIN hMidiIn
  # );
  attach_function :midiInClose, [ :ulong ], :int

  # MMRESULT midiInStart(
  #     HMIDIIN hMidiIn
  # );
  attach_function :midiInStart, [ :ulong ], :int
  
  # MMRESULT midiInStop(
  #     HMIDIIN hMidiIn
  # );
  attach_function :midiInStop, [ :ulong ], :int
  
  # MMRESULT midiInReset(
  #     HMIDIIN hMidiIn
  # );
  attach_function :midiInReset, [ :ulong ], :int

  # UINT midiInGetNumDevs(void);
  attach_function :midiInGetNumDevs, [], :uint

  # UINT midiOutGetNumDevs(void);
  attach_function :midiOutGetNumDevs, [], :uint

  # MMRESULT midiInGetDevCaps(
  #   UINT_PTR uDeviceID,
  #   LPMIDIINCAPS lpMidiInCaps,
  #   UINT cbMidiInCaps
  # );
  # attach_function :midiInGetDevCaps, [:uint, :pointer, :uint], :uint
end

#===---------------------------------------------------------------------------

require 'pp'



trap("INT") { exit }

device_ptr = FFI::MemoryPointer.new(:ulong)

WinMM.midiInOpen(device_ptr, 0, WinMM::MidiInProcCallback, 0, WinMM::CALLBACK_FUNCTION)

device_handle = device_ptr.read_long

WinMM.midiInStart(device_handle)

at_exit do
  WinMM.midiInStop(device_handle)
  WinMM.midiInClose(device_handle)
end
system("PAUSE")

result.txt

...\Ruby\midi>ruby "midi_in 2.rb"
[5014440, 961, 0, 0, 0]
Press any key to continue . . .
[5014440, 962, 0, 0, 0]