From: Heesob Park <phasis@...>
Date: 2010-01-13T10:52:12+09:00
Subject: Re: ffi, kerberos

Hi,

2010/1/13 Daniel Berger <djberg96@gmail.com>:
> Hi,
>
> I'm trying to wrap two function calls from kerberos, krb5_init_context
> and krb5_free_context. Here are the prototypes for both:
>
> krb5_init_context(krb5_context * context) => krb5_error_code
> krb5_free_context(krb5_context context) => void
>
> krb5_context is a struct, but unfortunately I don't know what its
> members are.
>
> From the docs:
>
>     The krb5_context structure is designed to hold all per thread
> state. All
>     global variables that are context specific are stored in this
> structure,
>     including default encryption types, credentials-cache (ticket
> file), and
>     default realms.
>
>     The internals of the structure should never be accessed directly,
> func-
>     tions exist for extracting information.
>
> All I know is that it's 8 bytes in size.
>
> I tried something like this, but it didn't work:
>
> require 'ffi'
>
> module Krb5
>  class Context
>    extend FFI::Library
>
>    class Error < StandardError; end
>
>    ffi_lib 'krb5'
>
>    attach_function :krb5_init_context, [:string], :uint
>    attach_function :krb5_free_context, [:string], :void
>
>    def initialize
>      @ctx = FFI::MemoryPointer.new(:pointer, 8)
>      ret = krb5_init_context(@ctx)
>
>      if ret > 0
>        raise Error, "krb5_init_context() failed"
>      end
>
>      if block_given?
>        begin
>          yield self
>        ensure
>          free
>        end
>      end
>    end
>
>    def free
>      krb5_free_context(@ctx)
>    end
>  end
> end
>
> k = Krb5::Context.new
> k.free
>
> I can use a plain string buffer, but then it segfaults on the 'free'
> call.
>
> Any suggestions?
>

In my test platform, sizeof(krb5_context) is 4.
Here is a working code:

require 'ffi'

module Krb5
 class Context
   extend FFI::Library

   class Error < StandardError; end

   ffi_lib 'krb5'

   attach_function :krb5_init_context, [:pointer], :uint
   attach_function :krb5_free_context, [:uint], :void

   def initialize
     ptr = FFI::MemoryPointer.new(:char, 4)
     ret = krb5_init_context(ptr)
     if ret > 0
       raise Error, "krb5_init_context() failed"
     end
     @ctx = ptr.read_int

     if block_given?
       begin
         yield self
       ensure
         free
       end
     end
   end

   def free
     krb5_free_context(@ctx)
   end
 end
end

k = Krb5::Context.new
k.free


Regards,

Park Heesob