From: Konrad Meyer <konrad@...>
Date: 2007-08-10T02:57:09+09:00
Subject: Re: SWIG question: how to return an array from C to Ruby ?

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On Thursday 09 August 2007 03:20:04 am Joachim (M=FCnchen) wrote:
> I need to make some C functions available to Ruby. One of them sets an
> array of doubles. I want to return this array to Ruby.
>=20
> On the C side:
>=20
> data_reader.c:
> #include "hardwaredriver.h"
> ...
> void read_data( double *data )
> {
>     for( i=3D0; i<8; ++i )
>         data[i] =3D .... // get values from hardware driver
> }
>=20
> On the Ruby side, I would like to obtain the following
>=20
> irb> require "data_reader"
> true
> irb> arr =3D Data_reader.read_data
> [ 0.0, 0.1, ... ] # arr now contains eight Floats
>=20
>=20
> Question 1:
> Do I understand correctly that there are basically two different ways
> to proceed ?:
>    (a) wrapping the C code with SWIG,
>    (b) converting the C code to an explicit Ruby extension as
> described in the Book: http://www.rubycentral.com/pickaxe/ext_ruby.html
>=20
> IF the answer is yes, THEN

While I don't know much (anything) about SWIG, C code needs to be wrapped
as an extension to Ruby or indirectly wrapped via Inline for it to be used
by ruby code.

> Question 2:
> How do I return an array through C->SWIG-> Ruby ?
>=20
> Web search gave me some hints on how to do it in the opposite
> direction, using typemap(in); but it seems not trivial to invert
> to typemap(out).
>=20
> Thanks in advance, Joachim

I'd recommend Inline for this, as it provides nice ruby->C parameter
conversions. To return ruby array data, you need a variable of type VALUE,
which is a generic ruby object container. It's also probably easiest if you
know how long the array will be in advance (in fact I don't know what to do
otherwise).

ex:
  #define ARR_LEN 31

  VALUE ary;
  double data[ARR_LEN];

  ... /* do your hw->data[] processing here */

  ary =3D rb_ary_new2(ARR_LEN);
  for(i =3D 0; i < ARR_LEN; i++)
    RARRAY(ary)->ptr[i] =3D rb_float_new(data[i]);
  RARRAY(ary)->len =3D ARR_LEN;
  return ary;

Something like that ought to do the trick. Happy ruby-ing!

=2D-=20
Konrad Meyer <konrad@tylerc.org> http://konrad.sobertillnoon.com/

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