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14.7 Custodians

See Custodians for basic information on the Racket custodian model.

Returns

if

is a

value,

otherwise.

Creates a new custodian that is subordinate to

. When

is directed (via

) to shut down all of its managed values, the new subordinate custodian is automatically directed to shut down its managed values as well.

Closes all file-stream ports, TCP ports, TCP listeners, and UDP sockets that are managed by cust (and its subordinates), and empties all custodian boxes associated with cust (and its subordinates). It also removes cust (and its subordinates) as managers of all threads; when a thread has no managers, it is killed (or suspended; see thread/suspend-to-kill) If the current thread is to be killed, all other shut-down actions take place before killing the thread.

If cust is already shut down, then custodian-shutdown-all has no effect. When a custodian is shut down and it has subordinate custodians, the subordinates are not only shut down, they no longer count as subordinates.

Returns

if

has been shut down with

or if it was a subordinate of a custodian that is shut down,

otherwise.

Added in version 6.11.0.5 of package base.

A parameter that determines a custodian that assumes responsibility for newly created threads, file-stream ports, TCP ports, TCP listeners, UDP sockets, and byte converters.

Returns a list of immediately managed objects (not including

es) and subordinate custodians for

, where

is itself subordinate to

(directly or indirectly). If

is not strictly subordinate to

, the

exception is raised.

If cust has been shut down, the result is '(). If cust was a subordinate of a custodian that was shut down, then it cannot be a subordinate of super.

Memory accounting is normally available, but not in the CGC implementation.

Returns #t if Racket is compiled with support for per-custodian memory accounting, #f otherwise.

Registers a required-memory check if Racket is compiled with support for per-custodian memory accounting, otherwise the

exception is raised.

If a check is registered, and if Racket later reaches a state after garbage collection (see Garbage Collection) where allocating need-amt bytes charged to limit-cust would fail or trigger some shutdown, then stop-cust is shut down.

The stop-cust must be a subordinate custodian of limit-cust.

Registers a limited-memory check if Racket is compiled with support for per-custodian memory accounting, otherwise the

exception is raised.

If a check is registered, and if Racket later reaches a state after garbage collection (see Garbage Collection) where limit-cust owns more than limit-amt bytes, then stop-cust is shut down.

A custodian’s limit is checked only after a garbage collection, except that it may also be checked during certain large allocations that are individually larger than the custodian’s limit. A single garbage collection may shut down multiple custodians, even if shutting down only one of the custodians would have reduced memory use for other custodians.

For reliable shutdown, limit-amt for custodian-limit-memory must be much lower than the total amount of memory available (minus the size of memory that is potentially used and not charged to limit-cust). Moreover, if individual allocations that are initially charged to limit-cust can be arbitrarily large, then stop-cust must be the same as limit-cust, so that excessively large immediate allocations can be rejected with an exn:fail:out-of-memory exception.

New memory allocation will be accounted to the running thread’s managing custodian. In other words, a custodian’s limit applies only to the allocation made by the threads that it manages. See also call-in-nested-thread for a simpler setup.

Examples:

Non-examples:

Returns a

that contains

as long as

has not been shut down. If

is already shut down, the custodian box’s value is immediately removed.

A custodian box is a synchronizable event (see Events). The custodian box becomes ready when its custodian is shut down; the synchronization result of a custodian box is the custodian box itself.

Returns the value in the given

, or

if the value has been removed.

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