gistfile1.rb

require 'rubygems'
require 'ffi'

class Info
  extend FFI::Library

  BOOT_TIME = 2

  attach_function :setutxent, [], :void
  attach_function :getutxent, [], :pointer
  attach_function :endutxent, [], :void

  class Timeval < FFI::Struct
    layout( :tv_sec, :long, 
            :tv_usec, :long )
  end

  class ExitStatus < FFI::Struct
    layout( :e_termination, :short,
            :e_exit, :short )
  end

  class Utmpx < FFI::Struct
    layout( :ut_type, :short, 
            :ut_pid, :pid_t,
            :ut_line, [:char, 32],
            :ut_id, :char,
            :ut_user, [:char, 32],
            :ut_host, [:char, 256],
            :ut_exit, ExitStatus,
            :ut_session, :long,
            :ut_tv, Timeval )
    
  end

  def self.getstuff
    begin
      setutxent
      ent = Utmpx.new(getutxent())
      time = Time.at(ent[:ut_tv][:tv_sec], ent[:ut_tv][:tv_usec]) if ent[:ut_type] == BOOT_TIME
    ensure
      endutxent
    end
  end
end

puts "Boot time: #{Info.getstuff}"