A weak box is similar to a normal box (see section 3.11),
but when the automatic memory manager can prove that the content
value of a weak box is only reachable via weak references, the
content of the weak box is replaced
#f. A weak reference is a reference through
a weak box, through a key reference in a weak hash table
(see section 3.14), through a value in an ephemeron where the
value can be replaced by
#f (see section 13.2), or
through a custodian (see section 9.2).
)returns the value contained in
weak-box. If the memory manager has proven that the previous content value of
weak-boxwas reachable only through a weak reference, then
An ephemeron is similar to a weak box (see section 13.1), except that
an ephemeron contains a key and a value; the value can be extracted from the ephemeron, but the value is replaced by
#fwhen the automatic memory manager can prove that either the ephemeron or the key is reachable only through weak references (see section 13.1); and
nothing reachable from the value in an ephemeron counts toward the reachability of an ephemeron key (whether for the same ephemeron or another), unless the same value is reachable through a non-weak reference, or unless the value's ephemeron key is reachable through a non-weak reference (see section 13.1 for information on weak references).
In particular, an ephemeron can be combined with a weak hash table (see section 3.14) to produce a mapping where the memory manager can reclaim key-value pairs even when the value refers to the key. An example is shown below.
;; This weak map is like a weak hash table, but ;; without the key-in-value problem: (define (make-weak-map) (
make-hash-table'weak)) SPACE (define (weak-map-put! m k v) (
hash-table-put!m k (make-ephemeron k (
boxv)))) SPACE (define (weak-map-get m k def-v) (let ([v (
hash-table-getm k (lambda () #f))]) (if v (let ([v (ephemeron-value v)]) (if v (
unboxv) def-v)) def-v))) SPACE (define m (make-weak-map)) (define k (
list1 2)) (weak-map-put! m k k) (weak-map-get m k #f) ; =>
'(1 2)(set! k #f) list is eventually GCed even if
A will executor manages a collection of values and associated will procedures. The will procedure for each value is ready to be executed when the value has been proven (by the automatic memory manager) to be unreachable, except through weak references (see section 13.1) or as the registrant for other will executors. A will is useful for triggering clean-up actions on data associated with an unreachable value, such as closing a port embedded in an object when the object is no longer used.
procedure executes a will that is ready in the specified will
executor. Wills are not executed automatically, because certain
programs need control to avoid race conditions. However, a program
can create a thread whose sole job is to execute wills for a particular
executor v proc
)registers the value
vwith the will procedure
procin the will executor
vis proven unreachable, then the procedure
procis ready to be called with
vas its argument via
procargument is strongly referenced until the will procedure is executed.
)invokes the will procedure for a single ``unreachable'' value registered with the executor
executable. The value(s) returned by the will procedure is the result of the
call. If no will is ready for immediate execution,
blocks until one is ready.
If a value is registered with multiple wills (in one or multiple executors), the wills are readied in the reverse order of registration. Since readying a will procedure makes the value reachable again, the will must be executed and the value must be proven again unreachable through only weak references before another of the wills is readied or executed. However, wills for distinct unreachable values are readied at the same time, regardless of whether the values are reachable from each other.
A will executor's register is held non-weakly until after the
corresponding will procedure is executed. Thus, if the content value
of a weak box (see section 13.1) is registered with a will
executor, the weak box's content is not changed to
all wills have been executed for the value and the value has been
proven again reachable through only weak references.
) forces an immediate garbage collection. Since
MzScheme uses a ``conservative'' garbage collector, some effectively
unreachable data may remain uncollected (because the collector cannot
prove that it is unreachable). This procedure provides some control
over the timing of collections, but garbage will obviously be
collected even if this procedure is never called.
) returns an estimate of the
number of bytes of memory occupied by reachable data from
custodian. (The estimate is calculated without performing
an immediate garbage collection; performing a collection generally
decreases the number returned by
custodian is not provided, the estimate is a total reachable
from any custodians. Unless MzScheme is compiled with special support
for memory accounting, the estimate is the same (i.e., all memory)
for any individual custodian.