From: Daniel Berger <djberg96@...>
Date: 2009-08-07T17:22:20+09:00
Subject: FFI, sysctl, pointer question

Hi,

I'm trying to get better at FFI, but I'm getting stumped on how to 
create and pass pointers to functions.

In the example below, in the Sys::Uptime.seconds method, I'm trying to 
figure out how to create and pass a pointer for the mib (a 2-element 
integer array) and the timeval size (i.e. where I have a '?' instead of 
actual code).

What should the actual code be?

module Sys
    class Uptime
       extend FFI::Library

       attach_function :time, [:pointer], :ulong
       attach_function :sysctl,
          [:pointer, :uint, :pointer, :pointer, :pointer, :uint], :int

       CTL_KERN = 1 # Kernel
       KERN_BOOTTIME = 21 # Time kernel was booted

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

       # How do I create the mib? How do I pass the address of tv.size?
       def self.seconds
          tv  = Timeval.new
          mib = [CTL_KERN, KERN_BOOTTIME]

          # What's the proper way to call this?
          if sysctl(?, 2, tv, ?, nil, 0) != 0
             raise SystemCallError, 'sysctl()'
          end

          time(nil) - tv[:tv_sec]
       end
    end
end

Regards,

Dan