Trying to use getmntinfo with ffi

getmntinfo.rb

require 'ffi'

class Filesystem
  extend FFI::Library
  ffi_lib FFI::Library::LIBC

  attach_function(:getmntinfo, [:pointer, :int], :int)
  attach_function(:strerror, [:int], :string)

  class Statfs < FFI::Struct
    layout(
      :f_otype, :short,
      :f_oflags, :short,
      :f_bsize, :long,
      :f_iosize, :long,
      :f_blocks, :long,
      :f_bfree, :long,
      :f_bavail, :long,
      :f_files, :long,
      :f_ffree, :long,
      :f_fsid, :long,
      :f_owner, :long,
      :f_reserved1, :short,
      :f_type, :short,
      :f_flags, :long,
      :f_reserved2, [:long,2],
      :f_fstypename, [:char, 16],
      :f_mntonname, [:char,16],
      :f_mntfromname, [:char,16],
      :f_reserved3, :char,
      :f_reserved4, [:long,4]
    )
  end

  def self.mounts
    mntbuf = FFI::MemoryPointer.new(Statfs)

    num = getmntinfo(mntbuf, 2)

    if num == 0
      raise Error, 'getmntinfo() function failed: ' + strerror(FFI.errno)
    end

    0.upto(num){ |n|
      ptr = mntbuf.get_pointer(n)
      mnt = Statfs.new(ptr)
      p mnt[:f_otype]
    }
  end
end

Filesystem.mounts