ffi_utmp_example.rb

require 'time'
require 'rubygems'
require 'ffi'

module Utmp
  extend FFI::Library

  UT_HOSTSIZE = 256
  UT_LINESIZE = 32
  INIT_PROCESS = 5
  NEW_TIME = 3
  RUN_LVL = 1
  EMPTY = 0
  UT_NAMESIZE = 32
  ACCOUNTING = 9
  BOOT_TIME = 2
  LOGIN_PROCESS = 6
  OLD_TIME = 4
  DEAD_PROCESS = 8
  USER_PROCESS = 7

  class ExitStatus < FFI::Struct
    layout :e_termination, :short,
      :e_exit, :short

  end

  class UtTv < FFI::Struct
    layout :tv_sec, :int32,
      :tv_usec, :int32
  end

  class Utmp < FFI::Struct
    layout :ut_type, :short,
      :ut_pid, :int,
      :ut_line, [:char, UT_LINESIZE],
      :ut_id, [:char, 4],
      :ut_user, [:char, UT_NAMESIZE],
      :ut_host, [:char, UT_HOSTSIZE],
      :ut_exit, ExitStatus,
      :ut_session, :int32,
      :ut_tv, UtTv,
      :ut_addr_v6, [:int32,4],
      :__unused, [:char, 20]
  end

  attach_function :setutent, [], :void
  attach_function :endutent, [], :void
  attach_function :getutent, [], :pointer
end

begin
loop do
  ptr = Utmp.getutent
  ut = Utmp::Utmp.new(ptr)
  puts "User: #{ut[:ut_user]}"
  puts "Host: #{ut[:ut_host]}"
  puts "Pid: #{ut[:ut_pid]}"
  puts "Time: #{Time.at(ut[:ut_tv][:tv_sec]).to_s}"
end
rescue FFI::NullPointerError
  nil
end

Rakefile.rb

desc "generate FFI constants"
task :ffi_consts do
  require 'rubygems'
  require 'ffi'
  require 'ffi/tools/const_generator'
  consts = FFI::ConstGenerator.new("Utmp") do |cg|
    cg.include 'utmp.h'
    process_constants = %w{
        EMPTY RUN_LVL BOOT_TIME NEW_TIME OLD_TIME INIT_PROCESS
        LOGIN_PROCESS USER_PROCESS DEAD_PROCESS ACCOUNTING
        UT_LINESIZE UT_NAMESIZE UT_HOSTSIZE
    }
    process_constants.each { |c|
      cg.const c
    }
  end
  puts "module Utmp"
  consts.constants.each_pair do |k,v|
    puts "  #{k} = #{v.converted_value}"
  end
  puts "end"
end