How do I extend my ruby class with a c extension?

Question

If I have Foo::Bar written in Ruby, and I want to add a method to Bar as a C extension. Right now when I create Foo::Bar in C like this:

static VALUE Foo;
static VALUE Bar;

static VALUE 
print_string(VALUE self, VALUE string) {
  printf("%s", StringValuePtr(string));
  return Qnil;
}

void Init_foo() {
    Foo = rb_define_module("Foo");
    Bar = rb_define_class_under(Foo, "Bar", rb_cObject);
    rb_define_method(Bar, "print_string", print_string, 1);
}

But the problem is:

ruby-1.9.2-p180 :001 > require 'ext/foo'   #=> ["Foo"]
ruby-1.9.2-p180 :002 > f = Foo::Bar.new   #=> #<Foo::Bar:0x000001046bce48>
ruby-1.9.2-p180 :003 > f.original_ruby_method
NoMethodError: undefined method `original_ruby_method' for #<Foo::Bar:0x000001046bce48>

So I am essentially overwriting the original Foo::Bar. How do I extend it and not overwrite it?

Checkout the gem called 'RubyInline'. It is used by the gem called 'image_science' which makes calls to c code to manipulate images. — Teddy, Jun 22 '11 at 16:05
Yeah I know about RubyInline, but would rather not use it since it doesn't support all data types. I also am going to be writing alot of C, and it would be nice to keep it all separate. — Jeremy Smith, Jun 22 '11 at 16:09
I assume you actually have defined Foo::Bar#original_ruby_method somewhere in your Ruby code there - which isn't seen in your example? — thomthom, Mar 08 '12 at 14:12

score 3 · Answer 1 · answered Jun 22 '11 at 16:29

3

I figured out a way to solve this issue.

void Init_foo() {
    rb_eval_string("require './lib/foo'");
    VALUE Bar = rb_path2class("Foo::Bar");
    rb_define_method(Bar, "print_string", print_string, 1);
}

answered Jun 22 '11 at 16:29

Jeremy Smith

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score 1 · Accepted Answer · answered Jun 22 '11 at 18:05

1

An alternative to your solution, where you require your Ruby code from your C extension, would be to require the extension from the Ruby code.

Add require 'foo.so' or require 'ext/foo.so' (depending on where your compiled library ends up) to lib/foo.rb, then in the client code just call require 'foo' as normal (assuming lib is on your load path).

I think doing it this way round is clearer and more common.

Note that you can use the .so suffix even if you're platform produces something else, i.e. it'll still work on a mac when the actual file is a .bundle.

answered Jun 22 '11 at 18:05

matt

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That does look cleaner, but when I try that I get: "NameError: Bar is already defined" – Jeremy Smith Jun 22 '11 at 20:06
@Jeremy I can't reproduce that error - what is the line it's being raised from? Have you got your `require 'foo.so'` at the top or bottom of the rb file? – matt Jun 22 '11 at 20:59
@Jeremy this is what I've got, if you want a look: https://gist.github.com/1041242 – matt Jun 22 '11 at 21:21

score 1 · Answer 3 · answered Mar 08 '12 at 14:11

A function a bit hidden away - I had to dig into the source to find it, but you can use rb_const_get to get references to existing modules and classes.

void Init_foo() {
  Foo = rb_const_get( rb_cObject, rb_intern("Foo");
  Bar = rb_const_get( Foo, rb_intern("Bar");
  rb_define_method(Bar, "print_string", print_string, 1);
}

If you want to ensure the class/module is created if it doesn't exist:

void Init_foo() {
  if ( rb_const_defined( rb_cObject, rb_intern("Foo") ) )
    Foo = rb_const_get( rb_cObject, rb_intern("Foo");
  else
    Foo = rb_define_module("Foo");

  if ( rb_const_defined( Foo, rb_intern("Bar") ) )
    Bar = rb_const_get( Foo, rb_intern("Bar");
  else
    Bar = rb_define_class_under(Foo, "Bar", rb_cObject);

  rb_define_method(Bar, "print_string", print_string, 1);
}

This is the correct way to build on an existing constant. – Leigh McCulloch Jul 29 '16 at 03:58 — Leigh McCulloch, Jul 29 '16 at 03:58

How do I extend my ruby class with a c extension?

3 Answers3