auto_string.rb

require 'rubygems'
require 'ffi'

module Hello
  extend FFI::Library

  attach_function 'puts', [ :string ], :int
end

Hello.puts("1")
Hello.puts(1)

# jmettraux:~/tmp/ffi$ ruby auto_string.rb
# 1
# (eval):3:in `call1': Invalid String value (ArgumentError)
# from (eval):3:in `puts'
# from auto_string.rb:12

# jmettraux:~/tmp/ffi$ ruby19 auto_string.rb
# 1
# (eval):3:in `call1': Invalid String value (ArgumentError)
# from (eval):3:in `puts'
# from auto_string.rb:12:in `<main>'

# jmettraux:~/tmp/ffi$ jruby-1.1.6 auto_string.rb
# 1
# 1