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11.1 Threads

Concurrency and Synchronization in The Racket Guide introduces threads.

See Threads for basic information on the Racket thread model. See also Futures and Places.

When a thread is created, it is placed into the management of the current custodian and added to the current thread group. A thread can have any number of custodian managers added through thread-resume. The allocation made by a thread is accounted to the thread’s custodian managers. See custodian-limit-memory for examples.

A thread that has not terminated can be garbage collected (see Garbage Collection) if it is unreachable and suspended or if it is unreachable and blocked on only unreachable events through functions such as semaphore-wait, semaphore-wait/enable-break, channel-put, channel-get, sync, sync/enable-break, or thread-wait. Beware, however, of a limitation on place-channel blocking; see the caveat in Places.

In GRacket, a handler thread for an eventspace is blocked on an internal semaphore when its event queue is empty. Thus, the handler thread is collectible when the eventspace is unreachable and contains no visible windows or running timers.

A thread can be used as a synchronizable event (see Events). A thread is ready for synchronization when thread-wait would not block; the synchronization result of a thread is the thread itself.

Calls

with no arguments in a new thread of control. The

procedure returns immediately with a

value. When the invocation of

returns, the thread created to invoke

terminates.

Creates a nested thread managed by

to execute

. (The nested thread’s current custodian is inherited from the creating thread, independent of the

argument.) The current thread blocks until

returns, and the result of the

call is the result returned by

.

The nested thread’s exception handler is initialized to a procedure that jumps to the beginning of the thread and transfers the exception to the original thread. The handler thus terminates the nested thread and re-raises the exception in the original thread.

If the thread created by call-in-nested-thread dies before thunk returns, the exn:fail exception is raised in the original thread. If the original thread is killed before thunk returns, a break is queued for the nested thread.

If a break is queued for the original thread (with break-thread) while the nested thread is running, the break is redirected to the nested thread. If a break is already queued on the original thread when the nested thread is created, the break is moved to the nested thread. If a break remains queued on the nested thread when it completes, the break is moved to the original thread.

If the thread created by call-in-nested-thread dies while itself in a call to call-in-nested-thread, the outer call to call-in-nested-thread waits for the innermost nested thread to complete, and any breaks pending on the inner threads are moved to the original thread.

Immediately suspends the execution of

if it is running. If the thread has terminated or is already suspended,

has no effect. The thread remains suspended (i.e., it does not execute) until it is resumed with

. If the

does not solely manage

(i.e., some custodian of

is not the current custodian or a subordinate), the

exception is raised, and the thread is not suspended.

Resumes the execution of

if it is suspended and has at least one custodian (possibly added through

, as described below). If the thread has terminated, or if the thread is already running and

is not supplied, or if the thread has no custodian and

is not supplied, then

has no effect. Otherwise, if

is supplied, it triggers up to three additional actions:

• If benefactor is a thread, whenever it is resumed from a suspended state in the future, then thd is also resumed. (Resuming thd may trigger the resumption of other threads that were previously attached to thd through thread-resume.)

• New custodians may be added to thd’s set of managers. If benefactor is a thread, then all of the thread’s custodians are added to thd. Otherwise, benefactor is a custodian, and it is added to thd (unless the custodian is already shut down). If thd becomes managed by both a custodian and one or more of its subordinates, the redundant subordinates are removed from thd. If thd is suspended and a custodian is added, then thd is resumed only after the addition.

• If benefactor is a thread, whenever it receives a new managing custodian in the future, then thd also receives the custodian. (Adding custodians to thd may trigger adding the custodians to other threads that were previously attached to thd through thread-resume.)

Terminates the specified thread immediately, or suspends the thread if

was created with

. Terminating the main thread exits the application. If

has already terminated,

does nothing. If the

does not solely manage

(i.e., some custodian of

is not the current custodian or a subordinate), the

exception is raised, and the thread is not killed or suspended.

Unless otherwise noted, procedures provided by Racket (and GRacket) are kill-safe and suspend-safe; that is, killing or suspending a thread never interferes with the application of procedures in other threads. For example, if a thread is killed while extracting a character from an input port, the character is either completely consumed or not consumed, and other threads can safely use the port.

Registers a break with the specified thread. The optional

value indicates the kind of break to register, where

,

, and

correspond to interrupt, hang-up, and terminate breaks respectively. If breaking is disabled in

, the break will be ignored until breaks are re-enabled. See

for details.

Causes the current thread to sleep until at least secs seconds have passed after it starts sleeping. A zero value for secs simply acts as a hint to allow other threads to execute. The value of secs can be a non-integer to request a sleep duration to any precision; the precision of the actual sleep time is unspecified.

Returns

if

has not terminated and is not suspended,

otherwise.

Returns #t if thd has terminated, #f otherwise.

Blocks execution of the current thread until

has terminated. Note that

deadlocks the current thread, but a break can end the deadlock if breaking is enabled and if the thread is the main thread or otherwise accessible; see

.

Unless thd was created with thread/suspend-to-kill, a (thread-wait thd) may potentially continue even if thd is otherwise inaccessible, because a custodian shut down could terminate the thread. As a result, a thread blocking with thread-wait normally cannot be garbage collected (see Garbage Collection). As a special case, however, (thread-wait thd) blocks without preventing garbage collection of the thread if thd is the current thread, since the thread could only continue if a break escapes from the wait.

A thread waiting on the result of (thread-dead-evt thd) normally cannot itself be garbage collected, unless thd was created with thread/suspend-to-kill, along the same lines as waiting via thread-wait. However, there is no special case for waiting on the result of (thread-dead-evt thd) where thd is the current thread.

For a given thd, thread-dead-evt always returns the same (i.e., eq?) result.

Returns a

(see

) that becomes

when

is running. (If

has terminated, the event never becomes ready.) If

runs and is then suspended after a call to

, the result event remains ready; after each suspend of

a fresh event is generated to be returned by

. The result of the event is

, but if

is never resumed, then reference to the event does not prevent

from being garbage collected (see

).

If thd was created with thread/suspend-to-kill, then waiting on (thread-suspend-evt thd) prevents garbage collection of the waiting thread in the same way as (thread-dead-evt another-thd) for a another-thd created via thread. Furthermore, since the event result is thd, waiting on (thread-suspend-evt thd) prevents garbage collection of thd.

Each thread has a mailbox through which it can receive arbitrary messages. In other words, each thread has a built-in asynchronous channel.

See also Buffered Asynchronous Channels.

Queues

as a message to

without blocking. If the message is queued, the result is

. If

stops running—

as in

—

before the message is queued, then

is called (through a tail call) if it is a procedure to produce the result, or

is returned if

is

.

Receives and dequeues a message queued for the current thread, if any. If no message is available,

blocks until one is available.

Receives and dequeues a message queued for the current thread, if any, or returns #f immediately if no message is available.

Pushes the elements of lst back onto the front of the current thread’s queue. The elements are pushed one by one, so that the first available message is the last element of lst.

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