weak key hash versus display

weak key hash versus display

[Kevin Ryde]

In guile 1.6.1 or the cvs on a recent i386 debian, I noticed that
doing a display of a weak key hash table can seemingly prevent a key
from being garbage collected.  For instance

	(define h (make-weak-key-hash-table 7))

	(define k "mykey")
	(hash-set! h (string-copy k) 12345)

	(display (hash-ref h k)) (newline)
	(display h) (newline)
	(gc)
	(display (hash-ref h k)) (newline)

run with "guile -s foo.scm" produces

	12345
	#wh(() () () () () ((mykey . 12345)) ())
	12345

whereas I might have expected the gc to have collected the entry just
set, making the second hash-ref give #f rather than 12345.  This is
what happens if the (display h) is not present.

I don't really know if this is a bug, or ignorance on my part, but it
seemed more than a little strange.


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Re: weak key hash versus display

[Mikael Djurfeldt]

Kevin Ryde <user42@zip.com.au> writes:

> In guile 1.6.1 or the cvs on a recent i386 debian, I noticed that
> doing a display of a weak key hash table can seemingly prevent a key
> from being garbage collected.  For instance
>
> 	(define h (make-weak-key-hash-table 7))
>
> 	(define k "mykey")
> 	(hash-set! h (string-copy k) 12345)
>
> 	(display (hash-ref h k)) (newline)
> 	(display h) (newline)
> 	(gc)
> 	(display (hash-ref h k)) (newline)
>
> run with "guile -s foo.scm" produces
>
> 	12345
> 	#wh(() () () () () ((mykey . 12345)) ())
> 	12345
>
> whereas I might have expected the gc to have collected the entry just
> set, making the second hash-ref give #f rather than 12345.  This is
> what happens if the (display h) is not present.

And if you keep (display h) but add

  (display <some deep list-structure>),
  
hash-ref will give #f again.

The reason is that in order to avoid infinite loops due to circular
data-structures print functions need to keep track of printed
references.  These are stored in a "print-state" attached to the
port.  After displaying h, the print-state contains the reference to
the pair (mykey . 12345) which protects it from GC.

We should probably replace this vector with a weak vector, or (perhaps
more efficient) clear the references from the vector.

> I don't really know if this is a bug, or ignorance on my part, but it
> seemed more than a little strange.

Well, it's not really a bug.  The Guile GC doesn't make any guarantees
that objects will get GC:d.  For example, if the C stack happens to
contain an integer which happens to coincide with a reference on the
heap, that object won't get GC:d.  A conservative GC only behaves
nicely in a statisticial sense.

Thanks for your observation.

Best regards,
Mikael Djurfeldt


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Re: weak key hash versus display

[Kevin Ryde]

Mikael Djurfeldt <djurfeldt@nada.kth.se> writes:
>
> We should probably replace this vector with a weak vector, or (perhaps
> more efficient) clear the references from the vector.

Sounds wise, to avoid any chance of a program being badly bloated by
big structure kept around just because it was printed.


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