Trying to get sysctlbyname working with ffi

ffi_test.rb

require 'ffi'

class Foo
  extend FFI::Library
  ffi_lib('c')

  attach_function :sysctlbyname,
    [:string, :pointer, :pointer, :pointer, :size_t],
    :int

  def self.freq
    ptr  = FFI::MemoryPointer.new(:int)
    size = FFI::MemoryPointer.new(:int)

    if sysctlbyname("hw.cpufrequency", ptr, size, nil, 0) < 0
      raise "sysctlbyname function failed"
    end

    p ptr.read_int # => 0 ?
  end
end

Foo.freq