Thank you matz!
That exactly what I forget: calling ruby_init() first!
I should have known this, because I did similar work for  python, where I
DO have to call Py_initialize() first.

Thank you very much!
I will try again.


From matz:
>Hmm, eval'ing Ruby's expression is very easy by
>
>  rb_eval_string(char *str);
>
>You need to call ruby_init() first, though.