[Openmcl-devel] strange(?) hash-table-count on weak hash table

Jared C. Davis jared at cs.utexas.edu
Thu Jun 19 03:19:00 UTC 2014


Hi,

I was experimenting with weak hash tables and have run into something I don't
understand.  Here's a small example, taken on 1.10-dev-r16074M-trunk
(LinuxX8664).

------------------

(defparameter *ht* (make-hash-table :weak :key :test 'eq))
(defparameter *key* (cons 1 2))
(setf (gethash *key* *ht*) 17)

(hash-table-count *ht*) ;; 1, as I expected
(gc)

(hash-table-count *ht*) ;; 1, as I expected since *key* is obviously reachable

nil ;; try to clear out *, **, ***
nil
nil

(gc)
(hash-table-count *ht*) ;; 1, as I expected since *key* is still reachable
(setf *key* nil)        ;; The (cons 1 2) should now be unreachable, right?
(gc)                    ;; So this should free the hash table entry

(hash-table-count *ht*) ;; 2 --- er, what the heck?!?

------------------

I had expected the final line above to report 0 instead of 2.  I wouldn't have
been entirely surprised by a count of 1, but how in the world did the count
increase?  Is this expected?

Thanks,
Jared


-- 
Jared C. Davis <jared at cs.utexas.edu>
11410 Windermere Meadows
Austin, TX 78759
http://www.cs.utexas.edu/users/jared/


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