require "rubygems"
require "inline"

module Foo
  inline do |builder|
    builder.c "
      long fib(long n) {
        long x = 1, y = 1, i, temp;
        for (i=1; i < n; i++) {
          temp = y; y = x + temp; x = temp;
        }
        return y;
      }"
  end
end

puts Foo.fib(30)

yields error:
fibinline.rb:17: undefined method `fib' for Foo:Module (NoMethodError)