"

 COPYRIGHT (c) 1993 by Claus Gittinger

	      All Rights Reserved

 This software is furnished under a license and may be used

 only in accordance with the terms of that license and with the

 inclusion of the above copyright notice.   This software may not

 be provided or otherwise made available to, or used by, any

 other person.  No title to or ownership of the software is

 hereby transferred.

"

"{ Package: 'stx:libbasic2' }"

Queue subclass:#SharedQueue

	instanceVariableNames:'dataAvailable spaceAvailable accessLock'

	classVariableNames:''

	poolDictionaries:''

	category:'Kernel-Processes'

!

!SharedQueue class methodsFor:'documentation'!

copyright

"

 COPYRIGHT (c) 1993 by Claus Gittinger

	      All Rights Reserved

 This software is furnished under a license and may be used

 only in accordance with the terms of that license and with the

 inclusion of the above copyright notice.   This software may not

 be provided or otherwise made available to, or used by, any

 other person.  No title to or ownership of the software is

 hereby transferred.

"

!

documentation

"

    SharedQueues provide a safe mechanism for processes to communicate.

    They are basically Queues, with added secure access to the internals,

    allowing use from multiple processes (i.e. the access methods use

    critical regions to protect against confusion due to a process

    switch within a modification).

    Also, sharedQueues can be used for synchronization, since a reading

    process will be blocked when attempting to read an empty queue, while

    a writer will be blocked when attempting to write into a full queue.

    For nonBlocking read, use #isEmpty; for nonBlocking write, use #isFull.

    See samples in doc/coding.

    [author:]

        Claus Gittinger

    [see also:]

        Semaphore

        Process

        CodingExamples::SharedQueueExamples

"

!

examples

"

    |queues readers writers seqNumber accessLock accessLock2

     numbersStillToReceive|

    seqNumber := 1.

    accessLock := Semaphore forMutualExclusion.

    accessLock2 := Semaphore forMutualExclusion.

    numbersStillToReceive := BooleanArray new:100000 withAll:true.

    queues := (1 to:10) collect:[:i | SharedQueue new].

    readers := (1 to:10) collect:[:i |

                                    [   |num|

                                        10000 timesRepeat:[

                                            num := (queues at:i) next.

                                            accessLock2 critical:[

                                                (numbersStillToReceive at:num) ifFalse:[

                                                    self halt:(num printString , ' received twice')

                                                ] ifTrue:[

                                                    numbersStillToReceive at:num put:false.

                                                ].

                                            ].

                                            'num printCR.'.

                                        ].

                                    ] fork

                                 ].

    writers := (1 to:10) collect:[:i |

                                    [   |num|

                                        10000 timesRepeat:[

                                            accessLock critical:[

                                                num := seqNumber.

                                                seqNumber := seqNumber + 1.

                                            ].

                                            (queues at:i) nextPut:num.

                                        ]

                                    ] fork

                                 ].

    readers do:[:aReader | aReader waitUntilTerminated].

    ' any left ? '.

    (numbersStillToReceive includes:true) ifTrue:[

        self halt:'oops - not all numbers received'

    ]

"

! !

!SharedQueue methodsFor:'accessing'!

next

    "return the next value in the queue; if it its empty, wait 'til

     something is put into the receiver.

     When the datum has been removed, signal space-availability to

     writers"

    |value|

    dataAvailable wait.

    accessLock critical:[

        value := super next.

    ].

    spaceAvailable signal.

    ^ value.

!

nextIfEmpty:exceptionValue

    "return the next value in the queue; if it its empty do not wait, but return

     the value of exceptionValue.

     When a datum has been removed, signal space-availability to writers"

    |value anyRemoved|

    accessLock critical:[

        self isEmpty ifTrue:[

            value := exceptionValue value

        ] ifFalse:[

            value := super next.

            anyRemoved := true.

        ]

    ].

    anyRemoved == true ifTrue:[spaceAvailable signal].

    ^ value.

!

nextPut:anObject

    "enter anObject into the queue; wait for available space, if

     the queue is full. After the put, signal availablity of a datum

     to readers."

    spaceAvailable wait.

    accessLock critical:[

        super nextPut:anObject.

    ].

    dataAvailable signal.

    ^ anObject.

!

nextPutFirst:anObject

    spaceAvailable wait.

    accessLock critical:[

        super nextPutFirst:anObject.

    ].

    dataAvailable signal.

    ^ anObject.

!

removeAll

    "remove all elements in the queue; do not wait, but

     synchronize access to the queue.

     If the queue was full before, signal space-availability to writers.

     This can be used to flush queues in multi-process applications,

     when cleanup is required."

    |count|

    count := 0.

    [(dataAvailable waitWithTimeout:0) notNil] whileTrue:[

        count := count + 1.

    ].    

    accessLock critical:[

        super removeAll.

    ].

    count timesRepeat:[spaceAvailable signal].

!

removeIdentical:anElement ifAbsent:exceptionalValue

    |value noSuchElement|

    noSuchElement := false.

    accessLock critical:[

        value := super removeIdentical:anElement ifAbsent:[noSuchElement := true]

    ].

    noSuchElement ifTrue:[

        ^ exceptionalValue value.

    ].

    ^ value.

!

removeLast

    "return the last value in the queue; if it its empty, wait 'til

     something is put into the receiver.

     When the datum has been removed, signal space-availability to

     writers"

    |value|

    dataAvailable wait.

    accessLock critical:[

        value := super removeLast.

    ].

    spaceAvailable signal.

    ^ value.

! !

!SharedQueue methodsFor:'accessing-internals'!

accessLock

    "return the critical access-semaphore which is used internally to syncronize access"

    ^ accessLock

!

readSemaphore

    "return the semaphore which is signalled when data is available

     for reading."

    ^ dataAvailable

    "Modified: 16.12.1995 / 13:47:11 / cg"

!

withAccessLockedDo:aBlock

    "evaluate aBlock while access via next/nextPut are blocked."

    accessLock critical:aBlock

!

writeSemaphore

    "return the semaphore which is signalled when the queue has space

     for writing."

    ^ spaceAvailable

    "Modified: 16.12.1995 / 13:47:07 / cg"

! !

!SharedQueue methodsFor:'initialization'!

init:size

    "initialize the receiver for size entries"

    super init:size.

    dataAvailable := Semaphore new name:'shared q-read'.

    spaceAvailable := (Semaphore new:size) name:'shared q-write'.

    accessLock := RecursionLock new.

    "Modified: 25.1.1997 / 00:19:45 / cg"

! !

!SharedQueue class methodsFor:'documentation'!

version

! !