From: Paul Brannan <pbrannan@...>
Date: 2002-10-22T03:54:24+09:00
Subject: rb_gc_register_address problem

We ran into a problem today with the garbage collector (caused by our
own bug).  We had a function that did something like this:
  std::vector<char *> foo;
  void func(int argc, VALUE * argv, VALUE self) {
    foo.push_back(STR2CSTR(argv[0]));
    rb_global_variable(&argv[0]);
  }

This is a bug, because once the function returns, argv[0] is invalid,
and global_List contains an invalid VALUE* that may or may not point to
the VALUE we want to keep global.  When the GC tries to mark this
VALUE*, it ends up marking an invalid object.

So my question is: why does rb_gc_register_address() store VALUE*'s
instead of VALUE's?  I suppose that if it stored a VALUE, then it would
be possible to register an object as a global but not keep a reference
to that object; this may not be desirable.  In our case, though, we just
wanted the string to remain valid.

If I do have a bug like this in the future, what techniques can I use to
track it down?

Paul