From: Luc Heinrich <luc@...>
Date: 2010-01-14T01:55:46+09:00
Subject: Re: ffi, kerberos

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)

-- 
Luc Heinrich - luc@honk-honk.com