From: Daniel Berger <djberg96@...>
Date: 2010-01-14T06:56:56+09:00
Subject: Re: ffi, kerberos



Luc Heinrich wrote:
> On 13 janv. 2010, at 16:53, Daniel Berger wrote:
>
> > Now I need to figure out how to create an FFI::MemoryPointer without
> > necessarily knowing the size of the first argument.
>
> The one bit you are missing here is that krb5_context is not a struct. It's a *pointer* to a struct as can be seen in krb5/krb5.h:
>
> typedef struct _krb5_context * krb5_context;
>
> If krb5_context was a struct you wouldn't be able to do this in C:
>
> krb5_context ctx;
> krb5_init_context(&ctx);
>
> Because event the C compiler would not know how to statically allocate without knowing the size of the struct. I assure you that it would complain :)
>
> So, with that in mind you can now simply completely ignore the size of the struct and only use pointers. Like this:
>
> module FFI::Krb5
>     extend FFI::Library
>     ffi_lib "krb5"
>
>     attach_function :krb5_init_context, [:buffer_out], :uint
>     attach_function :krb5_free_context, [:pointer], :void
> end
>
> What you pass to krb5_init_context is really a pointer to a pointer. You can now use that this way:
>
> buf = FFI::Buffer.new(:pointer)
> res = FFI::Krb5.krb5_init_context(buf)
> puts res
>
> First you allocate a buffer which will contain a pointer (the krb5_context, which is, you get it now, a pointer :)), and then you pass this buffer to the krb5_init_context. But now you will probably want to *use* this context so you first have to get it out of the buffer, like this:
>
> ctx = buf.get_pointer(0)
>
> Which allows you to use it in any other krb5_* call, like the krb5_free_context which we have defined:
>
> FFI::Krb5.krb5_free_context(ptr)

Thank you. This was very helpful.

Regards,

Dan