Segmentation Fault example from waveOutProc callback for waveOutOpen windows multimedia function.

ffi_callback_segfault.rb

require 'ffi'

class HWAVEOUT < FFI::Struct
  layout :i, :int
end

class WAVEFORMATEX < FFI::Struct
  def initialize
    self[:wFormatTag] = 1
    self[:nChannels] = 1
    self[:nSamplesPerSec] = 44100
    self[:wBitsPerSample] = 16
    self[:cbSize] = 0
    self[:nBlockAlign] = (self[:wBitsPerSample] >> 3) * self[:nChannels]
    self[:nAvgBytesPerSec] = self[:nBlockAlign] * self[:nSamplesPerSec]
  end
  layout(
    :wFormatTag, :ushort,
    :nChannels, :ushort,
    :nSamplesPerSec, :ulong,
    :nAvgBytesPerSec, :ulong,
    :nBlockAlign, :ushort,
    :wBitsPerSample, :ushort,
    :cbSize, :ushort
  )
end

class WAVEHDR < FFI::Struct      
  layout(
    :lpData, :pointer,
    :dwBufferLength, :ulong,
    :dwBytesRecorded, :ulong,
    :dwUser, :ulong,
    :dwFlags, :ulong,
    :dwLoops, :ulong,
    :lpNext, :pointer,
    :reserved, :ulong
  )
end

class Sound
  extend FFI::Library

  ffi_lib :winmm
  
  callback :waveOutProc, [:pointer, :uint, :ulong, :ulong, :ulong], :void
  attach_function :waveOutOpenCallback, :waveOutOpen, [:pointer, :uint, :pointer, :waveOutProc, :ulong, :ulong], :uint
  attach_function :waveOutOpenNull, :waveOutOpen, [:pointer, :uint, :pointer, :ulong, :ulong, :ulong], :uint
  
  WaveOutProc = Proc.new do |hwo, mMsg, dwCallback, dwParam1, dwParam2|
    puts mMsg
    return
  end
  
end

hWaveOut = HWAVEOUT.new
wfx = WAVEFORMATEX.new

puts "waveOutOpen returns 0 (no error) if I don't use the callback form"
puts Sound.waveOutOpenNull(hWaveOut.pointer, -1, wfx.pointer,0,0,0)
puts "but if I use a callback, the moment it returns from the callback, there is a seg fault"
puts "note that the code inside the callback runs, as I am puts-ing the mMsg which is 955"
puts "for WOM_OPEN which tells us the device was opened."
puts "this is on ruby 1.9.3p484 (2013-11-22) [i386-mingw32] running 1.9.3 x86-mingw32 on"
puts "Windows 7"
puts Sound.waveOutOpenCallback(hWaveOut.pointer, -1, wfx.pointer,Sound::WaveOutProc,0,0x30000)