On this page:
10.1.1 Creating Threads
thread
thread?
current-thread
thread/ suspend-to-kill
call-in-nested-thread
10.1.2 Suspending, Resuming, and Killing Threads
thread-suspend
thread-resume
kill-thread
break-thread
sleep
thread-running?
thread-dead?
10.1.3 Synchronizing Thread State
thread-wait
thread-dead-evt
thread-resume-evt
thread-suspend-evt
10.1.4 Thread Mailboxes
thread-send
thread-receive
thread-try-receive
thread-receive-evt
thread-rewind-receive
Version: 4.2.1

10.1 Threads

See Threads for basic information on the PLT Scheme thread model.

When a thread is created, it is placed into the management of the current custodian and added to the current thread group (see Thread Groups). A thread can have any number of custodian managers added through thread-resume.

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 semaphore-wait, semaphore-wait/enable-break, channel-put, channel-get, sync, sync/enable-break, or thread-wait.

In MrEd, 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.

All constant-time procedures and operations provided by MzScheme are thread-safe because they are atomic. For example, set! assigns to a variable as an atomic action with respect to all threads, so that no thread can see a “half-assigned” variable. Similarly, vector-set! assigns to a vector atomically. The hash-set! procedure is not atomic, but the table is protected by a lock; see Hash Tables for more information. Port operations are generally not atomic, but they are thread-safe in the sense that a byte consumed by one thread from an input port will not be returned also to another thread, and procedures like port-commit-peeked and write-bytes-avail offer specific concurrency guarantees.

10.1.1 Creating Threads

(thread thunk)  thread?
  thunk : (-> any)
Calls thunk with no arguments in a new thread of control. The thread procedure returns immediately with a thread descriptor value. When the invocation of thunk returns, the thread created to invoke thunk terminates.

(thread? v)  thread?
  v : any/c
Returns #t if v is a thread descriptor, #f otherwise.

Returns the thread descriptor for the currently executing thread.

(thread/suspend-to-kill thunk)  thread
  thunk : (-> any)
Like thread, except that “killing” the thread through kill-thread or custodian-shutdown-all merely suspends the thread instead of terminating it.

(call-in-nested-thread thunk [cust])  any
  thunk : (->any)
  cust : custodian? = (current-custodian)
Creates a nested thread managed by cust to execute thunk. (The nested thread’s current custodian is inherited from the creating thread, independent of the cust argument.) The current thread blocks until thunk returns, and the result of the call-in-nested-thread call is the result returned by thunk.

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.

10.1.2 Suspending, Resuming, and Killing Threads

(thread-suspend thd)  void?
  thd : thread?
Immediately suspends the execution of thd if it is running. If the thread has terminated or is already suspended, thread-suspend has no effect. The thread remains suspended (i.e., it does not execute) until it is resumed with thread-resume. If the current custodian does not manage thd (and none of its subordinates manages thd), the exn:fail:contract exception is raised, and the thread is not suspended.

(thread-resume thd [benefactor])  void?
  thd : thread?
  benefactor : (or/c thread? custodian? #f) = #f
Resumes the execution of thd if it is suspended and has at least one custodian (possibly added through benefactor, as described below). If the thread has terminated, or if the thread is already running and benefactor is not supplied, or if the thread has no custodian and benefactor is not supplied, then thread-resume has no effect. Otherwise, if benefactor is supplied, it triggers up to three additional actions:

(kill-thread thd)  void?
  thd : thread?
Terminates the specified thread immediately, or suspends the thread if thd was created with thread/suspend-to-kill. Terminating the main thread exits the application. If thd has already terminated, kill-thread does nothing. If the current custodian does not manage thd (and none of its subordinates manages thd), the exn:fail:contract exception is raised, and the thread is not killed or suspended.

Unless otherwise noted, procedures provided by MzScheme (and MrEd) 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.

(break-thread thd)  void?
  thd : thread?
Registers a break with the specified thread. If breaking is disabled in thd, the break will be ignored until breaks are re-enabled (see Breaks).

(sleep [secs])  void?
  secs : nonnegative-number? = 0
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 non-integral to request a sleep duration to any precision; the precision of the actual sleep time is unspecified.

(thread-running? thd)  any
  thd : thread?
Returns #t if thd has not terminated and is not suspended, #f otherwise.

(thread-dead? thd)  any
  thd : thread?
Returns #t if thd has terminated, #f otherwise.

10.1.3 Synchronizing Thread State

(thread-wait thd)  void?
  thd : thread?
Blocks execution of the current thread until thd has terminated. Note that (thread-wait (current-thread)) deadlocks the current thread, but a break can end the deadlock (if breaking is enabled; see Breaks).

(thread-dead-evt thd)  evt?
  thd : thread?
Returns a synchronizable event (see Events) that is ready if and only if thd has terminated. Unlike using thd directly, however, a reference to the event does not prevent thd from being garbage collected (see Garbage Collection). For a given thd, thread-dead-evt always returns the same (i.e., eq?) result.

(thread-resume-evt thd)  evt?
  thd : thread?
Returns a synchronizable event (see Events) that becomes ready when thd is running. (If thd has terminated, the event never becomes ready.) If thd runs and is then suspended after a call to thread-resume-evt, the result event remains ready; after each suspend of thd a fresh event is generated to be returned by thread-resume-evt. The result of the event is thd, but if thd is never resumed, then reference to the event does not prevent thd from being garbage collected (see Garbage Collection).

(thread-suspend-evt thd)  evt?
  thd : thread?
Returns a synchronizable event (see Events) that becomes ready when thd is suspended. (If thd has terminated, the event will never unblock.) If thd is suspended and then resumes after a call to thread-suspend-evt, the result event remains ready; after each resume of thd created a fresh event to be returned by thread-suspend-evt.

10.1.4 Thread Mailboxes

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

See also Buffered Asynchronous Channels.

(thread-send thd v [fail-thunk])  any
  thd : thread?
  v : any/c
  fail-thunk : (or/c (-> any) #f)
   = (lambda () (raise-mismatch-error ....))
Queues v as a message to thd without blocking. If the message is queued, the result is #<void>. If thd stops running – as in thread-running? – before the message is queued, then fail-thunk is called (through a tail call) if is a procedure to produce the result, or #f is returned if fail-thunk is #f.

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

Returns a constant synchronizable event (see Events) that becomes ready when the synchronizing thread has a message to receive. The event result is itself.

(thread-rewind-receive lst)  void?
  lst : list?
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.