From: Andrea Fazzi <andrea.fazzi@...>
Date: 2009-08-09T17:54:28+09:00
Subject: Re: FFI, getutxent question

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.

> 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

  ...

  class Utmpx < FFI::Struct
    layout :ut_type, :short,
           :ut_pid, Pid
    ...
  end

Regards,
Andrea
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