From: Daniel Berger <djberg96@...>
Date: 2010-01-14T00:53:27+09:00
Subject: Re: ffi, kerberos



On Jan 12, 6:52 pm, Heesob Park <pha...@gmail.com> wrote:
> Hi,
>
> 2010/1/13 Daniel Berger <djber...@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

This almost got me there. I had to modify it somewhat to account for
the fact that I'm on Snow Leopard (x64), so I replaced :uint
with :long, 4 with 8 and ptr.read_int with ptr.read_long.

Now I need to figure out how to create an FFI::MemoryPointer without
necessarily knowing the size of the first argument.

Thanks for the help!

Regards,

Dan