From: Jeff Davis <ruby@...>
Date: 2008-07-07T02:24:42+09:00
Subject: Re: mysterious memory corruption, very confused

On Mon, 2008-06-30 at 15:47 +0900, Seebs wrote:
> On 2008-06-30, Roger Pack <rogerpack2005@gmail.com> wrote:
> >> I get random data corruption when trying to execute queries.
> > valgrind might tell you if memory is being tramped.
> 
> It is.  There's a loop of
> 	VALUE x;
> 	char **foo = malloc(buncha char *);
> 
> 	for (big list of things) {
> 		x = rb_obj_as_string(y);
> 		foo[i] = GetStringValue(x);
> 	}
> 
> The idiom of using rb_obj_as_string, and then using the value, is common in
> the Ruby source.  It works.  ... It works *as long as you don't allocate
> anything more before you're done with it*.  What ends up happening is that,
> if enough of the objects in question need a new string allocated by
> rb_obj_as_string, sooner or later you end up invoking the garbage collector.
> Now, since there's only one x, the garbage collector assumes the current
> rb_obj_as_string() return is in use, *and the others aren't*.  So it might,
> if it wants the space, free one... And then the memory gets reused.

Thanks again for the detailed analysis.

To be clear, you're saying that the new object created by
rb_obj_as_string() can be freed as soon as I allocate any new ruby
object?

Is this documented behavior? To be safe, should I always assume any
object that I allocate in C land lives only until the next object is
allocated (unless it's referenced by some other object Ruby knows about,
of course)?

Regards,
	Jeff Davis