From: Clifford Heath <no.spam@...>
Date: 2008-11-11T19:12:33+09:00
Subject: Re: [ANN] FFI 0.1.1 (Foreign Function Interface) for Ruby 1.8.6/7 and 1.9

Charles Oliver Nutter wrote:
> Clifford Heath wrote:
>> callback :handle_message, [ :poi...
>> class Context < FFI:Struct
>>   layout \
>>     :msg_handler, :handle_message, 8,
> There's no support at the moment for callbacks-in-structs

While I'm on a roll, my example above creates a new type (handle_message),
and the ability to define other kinds of types similarly would be excellent.
For example, FreeTDS also has a string struct that contains a char* and a
length, and it uses that type liberally. A typedef would rock, and the above
example would just create a function-pointer typedef.

In a previous life, we implemented a generic structure pack/unpack subsystem.
It had a table for every type containing the alignment and size of that type.
Some architectures need more than one table depending on compile options, but
we supported 30 different architectures using tables whose values were filled
in by the C compiler. The rule of C don't allow re-ordering structure elements,
or at least, the require that two structures that start with the same elements
use the same alignment for those elements.

You can discover the alignment of a type by declaring a struct z { char x; TYPE y; }
and asking for offsetof(struct z, y); Mere byte-counting and padding suffices
to calculate alignment for the members of any structure. Nested structures are
padded to the largest of either the maximum alignment of the members, or some
size specific to the architecture. This can also be populated into the alignment
tables by the C compiler.

If FFI could include such a facility for Struct definitions, that would be magic,
as the current system is almost impossible to use for cross-platform support.
Unless you use the awkward templating system, of course... That requires that
you have the development versions of the target libraries when you want to gem
install a wrapper.

Just some random thoughts, anyhow. I might have time to contribute code to this
effort in December.

Clifford Heath.