From: Daniel Berger <djberg96@...>
Date: 2009-08-09T21:21:31+09:00
Subject: Re: FFI, getutxent question



On Aug 9, 2:54 am, Andrea Fazzi <andrea.fa...@alcacoop.it> wrote:
> Daniel Berger wrote:
> > Info.getstuff # Null Pointer object
>
> Your script runs smoothly on my system (Ubuntu Linux) returning a proper
> NativePointer object. It seems that the issue is related to your
> environment rather then FFI.

Ah, yes. I was on OS X. I tried again on Linux.
>
> > However, the call to getutxent() returns a NULL pointer. I'm guessing I
> > have to tell it what kind of pointer to return, but I wasn't sure how to
> > do that. How do I do that?
>
> You don't. Once you get a proper pointer object you'll use it to
> instantiate the Utmpx class:
>
>   Utmpx.new(getutxent)
>
> The Utmpx class will be a subclass of FFI::Struct that mirrors the
> layout of the C counterpart. Moreover, as I can see from libc reference
> manual[1], utmpx has nested structs within. The good news is that
> Ruby-FFI has support for nested structs. The bad one is that you have to
> define all the nested structs by hand:
>
>   class Pid < FFI::Struct
>     layout ...
>   end

Looks like FFI has a :pid_t type.

I tried this but I'm still having trouble. Part of the problem may be
that I'm not sure how to declare char[] struct members. The docs
indicate there's a :char_array type, but FFI doesn't seem to like it.

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 Utmpx < FFI::Struct
      layout(
         :ut_user, :char, 32
         :ut_id, :char, 4
         :ut_line, :char, 32
         :ut_pid, :pid_t,
         :ut_type, :short,
         :ut_tv, Timeval
      )
   end

   def self.getstuff
      time = nil

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

      time
   end
end

Info.getstuff

 ruby boot_time.rb
boot_time.rb:33:in `[]': NULL Pointer access attempted
(FFI::NullPointerError)
	from boot_time.rb:33:in `getstuff'
	from boot_time.rb:46

Where line 33 is the ent[:ut_type] reference.

Any suggestions?

Regards,

Dan