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Documentation

Thread-safe queues with timeout.

mailbox-timeout-condition?

[procedure] (mailbox-timeout-condition? OBJ) -> boolean

Is the OBJ a mailbox timeout condition?

A mailbox timeout condition is a composite condition of 'exn, 'mailbox, and 'timeout conditions.

The 'exn condition has properties of 'location, 'message, 'arguments, and 'call-chain.

The 'mailbox condition has properties of 'box.

The 'timeout condition has properties of 'time, and 'value.

The addition of the timeout properties as 'exn 'arguments is deprecated.

mailbox-timeout-exception?

[procedure] (mailbox-timeout-exception? OBJ) -> boolean

Synonym of mailbox-timeout-condition?.

make-mailbox

[procedure] (make-mailbox [NAME]) -> mailbox

Returns a new mailbox object.

NAME: * ; default (gensym 'mailbox)

identify this mailbox.

mailbox?

[procedure] (mailbox? OBJ) -> boolean

Is the OBJ a mailbox?

mailbox-name

[procedure] (mailbox-name MAILBOX) -> *

Returns the name of the MAILBOX.

mailbox-empty?

[procedure] (mailbox-empty? MAILBOX) -> boolean

If there are no queued objects in the MAILBOX, then this procedure returns #t, otherwise it returns #f.

mailbox-count

[procedure] (mailbox-count MAILBOX) -> integer

Returns the number of queued objects for the MAILBOX.

mailbox-waiting?

[procedure] (mailbox-waiting? MAILBOX) -> boolean

Is any thread waiting for the MAILBOX?

mailbox-waiters

[procedure] (mailbox-waiters MAILBOX) -> list

Returns a list of the threads waiting for the MAILBOX.

mailbox-send!

[procedure] (mailbox-send! MAILBOX OBJ)

Queues the data object OBJ. If any threads exist that are waiting for input on MAILBOX, the execution of the first one will be resumed. The data will be read out of a mailbox in the same order in which is written in (in FIFO manner).

mailbox-receive!

[procedure] (mailbox-receive! MAILBOX [TIMEOUT [DEFAULT]]) -> *

If there is any data in the MAILBOX, then the first object will be removed and returned as the result. If the mailbox is currently empty, the current thread will suspended until data is available.

TIMEOUT is a SRFI-18 time object or the real number of seconds.

Should TIMEOUT be specified and occur the DEFAULT, if supplied, will be returned. Otherwise a mailbox timeout exception will be signaled for the calling thread. The DEFAULT value cannot be (void).

mailbox-wait!

[procedure] (mailbox-wait! MAILBOX [TIMEOUT])

Similar to mailbox-receive!, but does not remove the received result from the queue of pending data.

TIMEOUT is a SRFI-18 time object or the real number of seconds.

Should TIMEOUT be specified and occur a mailbox timeout exception will be signaled for the calling thread.

mailbox-push-back!

[procedure] (mailbox-push-back! MAILBOX OBJ)

Pushes the data object OBJ into the first position of a mailbox.

mailbox-push-back-list!

[procedure] (mailbox-push-back-list! MAILBOX OBJS)

Pushes the list of objects OBJS back into the mailbox, so that (car OBJS) becomes the next receivable item.

make-mailbox-cursor

[procedure] (make-mailbox-cursor MAILBOX) -> mailbox-cursor

Returns an object which can enumerate a mailbox.

Multiple cursors can scan, and mutate, the same mailbox.

mailbox-cursor?

[procedure] (mailbox-cursor? OBJ) -> boolean

Is the OBJ a mailbox-cursor?

mailbox-cursor-mailbox

[procedure] (mailbox-cursor-mailbox MAILBOX-CURSOR) -> mailbox

Returns the mailbox object associated with the mailbox cursor.

mailbox-cursor-next

[procedure] (mailbox-cursor-next MAILBOX-CURSOR [TIMEOUT [DEFAULT]]) -> *

Returns the next object in the mailbox queue, waiting if necessary.

The mailbox queue is scanned from oldest to newest.

TIMEOUT is a SRFI-18 time object or the real number of seconds.

mailbox-cursor-rewind

[procedure] (mailbox-cursor-rewind MAILBOX-CURSOR)

Position the cursor at the oldest message in the mailbox.

mailbox-cursor-extract-and-rewind

[procedure] (mailbox-cursor-extract-and-rewind MAILBOX-CURSOR)

Remove from the associated mailbox queue the last object returned by mailbox-cursor-next and position the cursor at the oldest message in the mailbox.

The extraction is not performed without a previous call to mailbox-cursor-next.

mailbox-cursor-rewound?

[procedure] (mailbox-cursor-rewound? MAILBOX-CURSOR) -> boolean

Is the MAILBOX-CURSOR positioned at the start of the mailbox queue?

mailbox-cursor-unwound?

[procedure] (mailbox-cursor-unwound? MAILBOX-CURSOR) -> boolean

Is the MAILBOX-CURSOR positioned at the end of the mailbox queue?

Usage

(require-extension mailbox)

(require-library mailbox)
...
(import mailbox)

Examples

(define (consumer ch)
  (make-thread
    (lambda ()
      (let loop ()
        (print (current-thread) ": reading " (mailbox-receive! ch))
        (loop) ) ) ) )

(define ch (make-mailbox))
(thread-start! (consumer ch))
(for-each
  (lambda (x)
    (print (current-thread) ": writing " x)
    (mailbox-send! ch x) )
  '(33 44 55 hello) )

Notes

Cannot wait with timeout on the primordial thread. So mailbox-wait!, mailbox-receive!, and mailbox-cursor-next must not be used with a timeout value when the (current-thread) is the ##sys#primordial-thread.
A "deadlock" situation is possible when using mailbox-wait!, mailbox-receive!, or mailbox-cursor-next should there be no other threads "runnable." The problem is when the mailbox is empty the current-thread is suspended (indefinite block). When the scheduler looks for the next ready thread and one is not available it signals "deadlock." Using a TIMEOUT will allow the calling thread to unblock eventually.
A mailbox-cursor must live in an environment where mailbox entries are added and removed asynchronously. The cursor does not see a snapshot of the mailbox. The current state of the mailbox is queried.
mailbox-timeout-exception? will be deprecated.

Author

felix winkelmann

Kon Lovett

Requirements

check-errors condition-utils record-variants setup-helper

Version history

2.3.1: Fix name type.
2.3.0: Add types.
2.2.3
2.2.2: Expand mailbox-timeout-condition.
2.2.1
2.2.0: Included inline-type-checks.scm so all includes are egg-local.
2.1.3: Fix for resuming a thread when mailbox empty; reported by Jeronimo Pellegrini. Added mailbox-cursor-unwound?. The mailbox-name can be an arbitrary Scheme object.
2.1.2
2.1.1
2.1.0: Needs "check-errors" extension.
2.0.0: Port to hygienic Chicken.

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

 Redistributions of source code must retain the above copyright notice, this list of conditions and the following
   disclaimer.
 Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following
   disclaimer in the documentation and/or other materials provided with the distribution.
 Neither the name of the author nor the names of its contributors may be used to endorse or promote
   products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.