From: Martin DeMello <martindemello@...>
Date: 2009-06-16T03:00:53+09:00
Subject: Re: wrapping a C struct[] constant

On Mon, Jun 15, 2009 at 11:00 PM, Joel
VanderWerf<vjoel@path.berkeley.edu> wrote:
> Martin DeMello wrote:
>>
>> another basic thing I missed is the
>> ability to define a typemap and then have a ruby class
>> serialised/deserialised to a C struct. (Is that what Cgenerator's
>> CShadow is all about?). Are these not common use cases?
>
> Would this be like swig in reverse? Input a ruby class with type
> annotations, output C header, accessors, etc?

I actually meant like a simplified swig - you have a C struct
(arbitrary nesting of structs and arrays, actually), and you have a
ruby class that you define a typemap for and then have it
serialise/deserialise. Basically you know that you can mirror C
structures in ruby with a bit of gruntwork, all you need is a tool to
do the gruntwork for you

> I'd like to do that for the
> special case of BitStruct classes (fixed-length, packed fields, usually
> numeric or character, stored in a ruby String using pack/unpack), so that
> once you define a network protocol or other binary format in ruby, you can
> share your code with C programmers (and via swig with other languages). That
> seems feasible, and I did actually do this with a predecessor of BitStruct,
> using cgen. But how would you handle a more general case, like a class with
> a hash or array attribute?

Network protocols were one of the use cases I was thinking of, except
that you usually see them defined in C. I wanted the ability to use
the structs from ruby, without going through C the way
data_wrap_struct makes you do. Not asking for magic, just the ability
to make the common case trivial.

> Including CShadow in your class lets you define the structure of the T_DATA
> object in ruby, in terms of accessors of various types. When your code runs,
> it uses cgenerator to build an extension that takes care of all the boiler
> plate functions: mark, free, accessors with type checking/conversion, alloc,
> init, inspect, marshal, and yaml. Plus it handles inheritance and sets up
> introspection methods. [See example below.]

Okay, that's most of what I was looking for :) If it can convert an
array of structs into a ruby array, that's *all* of what I'm looking
for. Need to look through the CGenerator docs some more - great piece
of work!

martin