From: 13 <one.three@...>
Date: 2007-03-31T21:53:33+09:00
Subject: How to crash Ruby by not registering global C variable ?

Hi list,

On page 284 of Pickaxe, 2nd edition there is a passage:

> If you create a Ruby object from C and store it in a C global variable without exporting it to Ruby, you must at least tell the garbage collector about it, lest ye be reaped inadvertently.

    static VALUE obj;
    /* ... */
    obj = rb_ary_new();
    rb_global_variable(obj);

I would like to reproduce this issue, but don't know how. As far as I
understand, if rb_global_variable(obj) is not called then once the GC
will try to free up garbage memory it will try to destroy obj. GC will
fail and something interesting should happen :)

I tried to implement this bug. While writing this e-mail I started to
think that I misunderstood what Dave Thomas is stating and that I'm
going the wrong way. But here is my code, in case I'm doing something
right:

reg_global.c:

  1 #include <ruby.h>
  2
  3 static VALUE str;
  4
  5 int
  6 main(int argc, char **argv)
  7 {
  8     ruby_init();
  9     ruby_init_loadpath();
 10
 11     /* Create a string variable end export it to ruby */
 12     str = rb_str_new2("FooBar");
 13     rb_define_variable("$value", &str);
 14
 15     rb_require("reg_global.rb");
 16
 17     ruby_finalize();
 18     return 0;
 19 }

reg_global.rb:

  1 # this script should be called from embedded ruby interpreter
  2
  3 def wrapper
  4   p "value: #{$value}, id: #{$value.object_id}"
  5   ObjectSpace.define_finalizer($value, lambda { |id| p "Finalizing #{id}" })
  6   $value = nil
  7 end
  8
  9 wrapper
 10
 11 ObjectSpace.garbage_collect

When I execute reg_global I see the following (nothing special):

"value: FooBar, id: 76750"
"Finalizing 76750"

I expected Ruby to crash after this line:

 11 ObjectSpace.garbage_collect

Has anyone some hints on how to reproduce the bug described in paragraph above ?

--
Martins