From: "Suraj N. Kurapati" <sunaku@...>
Date: 2008-04-06T01:13:49+09:00
Subject: Re: unable to set $0 from C extension

Suraj N. Kurapati wrote:
> I tried several approaches to set $0 from my C extension but none
> have succeeded in providing a modifiable $0 to Ruby code.  In
> particular, any modifications to $0 from Ruby code are raising a
> "RuntimeError: $0 not initialized".

I dug around the Ruby source code some more and found the
ruby_prog_init() function which is called by ruby_init() to define
$0 as a hooked variable.

So the solution was to simply redefine the hooked handler for $0
after ruby_init() was called.  The new handler would treat $0 as a
pure Ruby value and would allow it to be modified without restriction:

  static VALUE gProgName = Qnil;

  static void setProgName(VALUE name, ID id)
  {
      gProgName = rb_obj_as_string(name);
      rb_obj_taint(gProgName);
  }

  main()
  {
      // ...

      ruby_init();

      //
      // override Ruby's hooked handlers for $0 so that $0 can be
      // treated as pure Ruby value and modified without restriction
      //

      rb_define_hooked_variable("$0", &gProgName, 0, setProgName);

      rb_define_hooked_variable("$PROGRAM_NAME", &gProgName, 0,
                                setProgName);

      // ...
  }

Cheers.