OpenEuphoria

Re: Garbage Collection

• Posted by Robert Craig <rds at RapidEuphoria.com> Jun 28, 2005
• 658 views

Vincent wrote:
> I just concluded after reading a bunch of stuff online, that Robert should
> just stick with
> either pure reference counting or use a hybrid reference counting & trace
> garbage collector
> to handle cyclic structures.
> 
> The problem with reference counting besides implementing mutex locks properly
> is, objects
> within cyclic structures will never obtain a zero reference count, and the
> unused block
> will never be freed).
> 
> But Rob claims Euphoria ignores cyclic structures of pointers. Are the garbage
> blocks
> still freed by a conversion, or just left to build up?

In parser.e where assignments to subscripted variables are handled,
there's a tiny bit of code that prevents cycles. So the cycle problem
they always talk about doesn't exist. Cycles do not happen.

Here's how a cycle could occur (without precautions):

     A[5] = A

You might just store a pointer to A in A[5].
That means A[5] will point to A, but A will also
contain a pointer to A[5], since A[5] is an element of the
sequence A. That's circular. The ref count on A will
never drop to zero, because A[5] points at it. The ref count
on A[5] will never drop to zero, because A points at it.

Euphoria avoids this problem by making a copy
of A (call it A_TEMP) and pointing A[5] at the *copy*, rather than
at the original sequence that A[5] is a part of. So there's
no cycle. A points to A[5] which points to A_TEMP, and A_TEMP[5]
points wherever A[5] originally pointed (if anywhere).

This costs some time because a copy of A is made, but this
is a rare case. Not many people do A[5] = A. It's strange code.
But the bottom line is Euphoria can use reference counting
without worrying about cycles.

Regards,
   Rob Craig
   Rapid Deployment Software
   http://www.RapidEuphoria.com