"

 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.

"

Queue subclass:#SharedQueue

	instanceVariableNames:'accessProtect dataAvailable spaceAvailable'

	classVariableNames:''

	poolDictionaries:''

	category:'Kernel-Processes'

!

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

"

! !

!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 ok|

    ok := false.

    [ok] whileFalse:[

	[tally == 0] whileTrue:[

	    dataAvailable wait

	].

	accessProtect critical:[

	    "

	     this check is needed, since another process may

	     have read the value in the meantime ...

	    "

	    tally == 0 ifFalse:[

		value := super next.

		tally == (contentsArray size - 1) ifTrue:[

		    spaceAvailable signal

		].

		ok := true

	    ]

	]

    ].

    ^ 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."

    |ok|

    ok := false.

    [ok] whileFalse:[

	[tally == contentsArray size] whileTrue:[

	    spaceAvailable wait

	].

	accessProtect critical:[

	    tally == contentsArray size ifFalse:[

		super nextPut:anObject.

		tally == 1 ifTrue:[

		    dataAvailable signal

		].

		ok := true

	    ]

	]

    ].

    ^ anObject

!

readSemaphore

    "return the semaphore which is signalled when data is available

     for reading."

    ^ dataAvailable

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

!

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 ok|

    ok := false.

    [ok] whileFalse:[

        [tally == 0] whileTrue:[

            dataAvailable wait

        ].

        accessProtect critical:[

            "

             this check is needed, since another process may

             have read the value in the meantime ...

            "

            tally == 0 ifFalse:[

                value := super removeLast.

                tally == (contentsArray size - 1) ifTrue:[

                    spaceAvailable signal

                ].

                ok := true

            ]

        ]

    ].

    ^ value

    "Created: 22.6.1996 / 18:50:12 / cg"

!

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.

    accessProtect := Semaphore forMutualExclusion.

    dataAvailable := Semaphore new.

    spaceAvailable := Semaphore new

! !

version

! !