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
) 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
#f is returned.
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 trggering 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 thread whose sole job is to execute wills for a particular
executor v proc
) registers the value
with the will procedure
proc in the will executor
v is proven unreachable, then the
proc is ready to be called with
v as its
) 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
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 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
#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.
) 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
cuostdian 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.
) dumps information about memory usage to the
(low-level) standard output port.