From: Stefano Crocco <stefano.crocco@...>
Date: 2007-01-23T01:18:41+09:00
Subject: Re: ebedded: calling a C function from script.

Alle 16:35, luned� 22 gennaio 2007, Shea Martin ha scritto:
> I have my embedded interpreter running my scipt.  Now I would like my
> script to call function in my C++ program.
>
> i.e.,
>
> int getSomeValue()
> {
>    return 5;
> }
>
> void setSomeValue( int value )
> {
>    _value = value;
> }
>
> How would I call these functions from my ruby script?
>
> ~S

If I understand correctly what you mean, I think you need to create a ruby 
method wrapping that function. If you want to do that at kernel-level, that 
is not inside a class, you should do the following (in your C code) (NOTE: 
all this is untested. I've tried a little project which mixed ruby and C some 
time ago, but soon abandoned it. What I'm telling comes from my memory of 
that attempt and its source code):

1: define a static C function which wraps your own. It must accept at least 
one argument of type VALUE (the first one is self) and return a VALUE (return 
Qnil if you don't need the return value). For instance, a wrapper around your 
getSomeValue() could be something like

static VALUE _getSomeValue(VALUE self){
  //INT2NUM converts from a C int to a ruby number (FixNum or BigNum)
  return INT2NUM(getSomeValue()); 
}

2: Create a ruby method which calls _getSomeValue. To do this, use the 
rb_define_global_function function:

rb_define_global_function("get_some_value", _getSomeValue, 1);

Here the first argument is the name of the ruby method, _getSomeValue is a 
pointer to the C function which implements the method and 1 is the number of 
arguments of the C function.

After calling rb_define_global_function, you'll be able to 
call "get_some_value" from your script:

puts get_some_value
=> 5

If you need more information on this topic, you can refer to the Pickaxe book, 
in the chapter called (at least in the online edition) "Extending ruby". 
Besides, you may find useful the ruby C api 
(http://www.ruby-doc.org/doxygen/current/) and, of course, ruby source code. 
Moreover, today on the mailing list was announced a tutorial on writing ruby 
extensions. It's at http://nanoblog.ath.cx/index.rb?module=readmore&id=8 I 
haven't read it, so I don't know if it can be useful to you (there are 
differences between writing an extension and embedding the interpreter)

I hope this helps

Stefano