.gitignore

*.o
*.so
*.bundle
Makefile

bar.c

#include "ruby.h"

static VALUE Foo;
static VALUE Bar;

static VALUE 
print_string(VALUE self, VALUE string) {
  printf("%s\n", 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);
}

extconf.rb

require 'mkmf'

$CFLAGS << " -Wall"

create_makefile('foo')

foo.rb

module Foo
  class Bar
    def original_ruby_method
      "original"
    end
  end
end

require 'foo.so'

test.rb

$:.unshift 'lib'
$:.unshift 'ext'

require 'foo'

bar = Foo::Bar.new

bar.print_string "hello" #defined in C (bar.c)

puts bar.original_ruby_method #defined in Ruby (foo.rb)