A weak box is similar to a normal box (see section 3.9),
but when the automatic memory manager can prove that the content
value of a weak box is only reachable via weak boxes, the content of
the weak box is replaced with #f.
(make-weak-boxv) returns a new weak box that initially
(weak-box-valueweak-box) returns the value contained in
weak-box. If the memory manager has proven that the previous
content value of weak-box was reachable only through weak
boxes, then #f is returned.
(weak-box?v) returns #t if v is a weak box,
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 will
executors, weak boxes, weak hash table keys, and custodians. 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.
Calling the will-execute or will-try-execute
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 thread whose sole job is to execute wills for a particular
(make-will-executor) returns a new will executor with no
(will-executor?v) returns #t if v is a will
executor, #f otherwise.
(will-registerexecutor v proc) registers the value v
with the will procedure proc in the will executor
executor. When v is proven unreachable, then the
procedure proc is ready to be called with v as its
argument via will-execute or will-try-execute.
(will-executeexecutor) 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 will-execute call. If no will is ready for
immediate execution, will-execute blocks until one is ready.
(will-try-executeexecutor) is like will-execute if a
will is ready for immediate execution. Otherwise, #f is
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 unreachable once again 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.
If the content value of a weak box (and/or a key in a weak hash table)
is registered with a will executor, the weak box's content is not
changed to #f (and/or the weak hash table entry is not
removed) until all wills have been executed for the value and the
value has been proven unreachable again.
(collect-garbage) 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.
(current-memory-use[custodian]) 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 current-memory-use.) If
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.
(dump-memory-stats) dumps information about memory usage to the
(low-level) standard output port.