"

 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:libbasic' }"

Object subclass:#Semaphore

	instanceVariableNames:'count waitingProcesses lastOwnerID name'

	classVariableNames:''

	poolDictionaries:''

	category:'Kernel-Processes'

!

!Semaphore 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

"

    Semaphores are used to synchronize processes providing a nonBusy wait

    mechanism. A process can wait for the availability of some resource by

    performing a Semaphore>>wait, which will suspend the process until the

    resource becomes available. Signalling is done by (another process performing) 

    Semaphore>>signal.

    If the resource has been already available before the wait, no suspending is

    done, but the resource immediately allocated.

    There are also semaphores for mutual access to a critical region

    (Semaphore>>forMutualExclusion and Semaphore>>critical:).

    Additional protocol is provided for oneShot semaphores, 

    (#signalOnce) and for conditional signalling (#signalIf).

    You can also attach semaphores to external events (such as I/O arrival or

    timer events). 

    This is done by telling the Processor to signal the semaphore 

    under some condition.

    See 'Processor>>signal:afterSeconds:', 'Processor>>signal:onInput:' etc.

    See examples in doc/coding (found in the CodingExamples-nameSpace).

    Warning/Note/Hint:

        a Semaphore-forMutualExclusion does NEVER allow for the critical

        region to be entered twice - NOT EVEN by the same process.

        That means, that a recursive attempt to enter that section leads

        to a deadlock.

        Use a RecursionLock instead of a semaphore to avoid this.

    [instance variables:]

        count                   <SmallInteger>          the number of waits, that will go through

                                                        without blocking.

                                                        Incremented on #signal; decremented on #wait.

        waitingProcesses        <OrderedCollection>     waiting processes - will be served first

                                                        come first served when signalled.

        lastOwnerID             <SmallInteger>          a debugging aid: set when count drops

                                                        to zero to the current processes id.

                                                        Helps in finding deadlocks.

        name                    <String>                a debugging aid: an optional userFriendly

                                                        name; helps to identify a semaphore easier.

    [see also:]

        SemaphoreSet RecursionLock Monitor

        SharedQueue Delay 

        Process ProcessorScheduler

    [author:]

        Claus Gittinger

"

!

examples

"

    two processes synchronizing on a sema:

                                                        [exBegin]

        |sema thread1 thread2|

        sema := Semaphore new.

        thread1 := [

                        Transcript showCR:'here is thread 1; now waiting ...'.

                        sema wait.

                        Transcript showCR:'here is thread 1 again.'.

                   ] newProcess.

        thread2 := [

                        Transcript showCR:'here is thread 2; delaying a bit ...'.

                        Delay waitForSeconds:5.

                        Transcript showCR:'here is thread 2 again; now signalling the sema'.

                        sema signal.

                        Transcript showCR:'here is thread 2 after the signalling.'.

                  ] newProcess.

        thread1 priority:7.

        thread2 priority:6.

        thread1 resume.

        thread2 resume.

                                                        [exEnd]

    semaphore for critical regions:

                                                        [exBegin]

        |accessLock|

        accessLock := Semaphore forMutualExclusion.

        [

            5 timesRepeat:[

                Delay waitForSeconds:2.

                accessLock critical:[

                    Transcript showCR:'thread1 in critical region'.

                    Delay waitForSeconds:1.

                    Transcript showCR:'thread1 leaving critical region'.

                ].

            ]

        ] forkAt:5.

        [

            5 timesRepeat:[

                Delay waitForSeconds:1.

                accessLock critical:[

                    Transcript showCR:'thread2 in critical region'.

                    Delay waitForSeconds:2.

                    Transcript showCR:'thread2 leaving critical region'.

                ].

            ]

        ] forkAt:4.

                                                        [exEnd]

    a deadlock due to recursive enter of a critical region:

                                                        [exBegin]

        |accessLock block|

        accessLock := Semaphore forMutualExclusion.

        block := [:arg |

                    Transcript showCR:'about to enter'.

                    accessLock critical:[

                        Transcript showCR:'entered - doing action'.

                        arg value

                    ].

                    Transcript showCR:'left region'.

                 ].

        block value:[].                 'this works'.

        block value:[block value:[] ].  'this deadlocks'.

                                                        [exEnd]

    Avoid the deadlock by using a RecursionLock instead:

                                                        [exBegin]

        |accessLock block|

        accessLock := RecursionLock new.

        block := [:arg |

                    Transcript showCR:'about to enter'.

                    accessLock critical:[

                        Transcript showCR:'entered - doing action'.

                        arg value

                    ].

                    Transcript showCR:'left region'.

                 ].

        block value:[].                 'this works'.

        block value:[block value:[] ].  'this deadlocks'.

                                                        [exEnd]

"

! !

!Semaphore class methodsFor:'instance creation'!

forMutualExclusion

    "create & return a new semaphore which allows exactly one process to

     wait on it without blocking. This type of semaphore is used

     for mutual exclusion from critical regions (see #critical:).

     Also see RecursionLock, to avoid deadlock in case of recursive entered

     critical regions."

    ^ super new setCount:1; name:'criticalRegionSema'

    "Modified: / 17.6.1998 / 16:23:09 / cg"

!

new

    "create & return a new semaphore which blocks until a signal is sent"

    ^ super new setCount:0

!

new:n

    "create & return a new semaphore which allows n waits before

     blocking"

    ^ super new setCount:n

! !

!Semaphore methodsFor:'friend-class interface'!

checkAndRegisterProcess:process

    "interface for SemaphoreSet.

     If the semaphore is available, decrement it and return true.

     Otherwise register our process to be wakened up once the semaphore is available

     and return false..

    "

    "

     bad ST/X trick (needs change, when multiProcessor support is added):

     this works only since interrupts are only serviced at 

     message send and method-return time ....

     If you add a message send into the ifTrue:-block, things will

     go mad ... (especially be careful when adding a debugPrint-here)

    "

    count ~~ 0 ifTrue:[

	count := count - 1.

	count == 0 ifTrue:[

	    lastOwnerID := Processor activeProcessId.

	].

	^ true

    ].

    (waitingProcesses identityIndexOf:process) == 0 ifTrue:[

	waitingProcesses add:process.

    ].

    ^ false

    "Modified: 14.12.1995 / 10:32:17 / stefan"

    "Modified: 10.1.1997 / 21:42:18 / cg"

!

unregisterProcess:process

    "interface for SemaphoreSet.

     Unregister our process from the Semaphore"

    waitingProcesses removeIdentical:process ifAbsent:[].

    "Created: 14.12.1995 / 10:31:50 / stefan"

    "Modified: 1.2.1997 / 12:11:22 / cg"

! !

!Semaphore methodsFor:'printing & storing'!

displayString

    "return a string to display the receiver - include the

     count for your convenience"

    ^ self class name , '(' , count printString , ' name: ' , (name ? 'unnamed') , ')'

    "Modified: 28.6.1997 / 16:21:09 / cg"

!

name

    "return the semaphores userFriendly name"

    ^ name

!

name:aString

    "set the semaphores userFriendly name"

    name := aString

! !

!Semaphore methodsFor:'private accessing'!

initSignals

    "set the count of the semaphore to zero.

     provided for ST-80 compatibility."

    count := 0

    "Created: 17.2.1997 / 11:31:19 / cg"

!

setCount:n

    "set the count of the semaphore;

     thats the number of possible waits, without blocking"

    waitingProcesses := OrderedCollection new:2.

    count := n

    "Modified: 17.2.1997 / 11:36:40 / cg"

! !

!Semaphore methodsFor:'queries'!

count

    "return the number of 'already-counted' trigger events.

     Thats the number of waits which will succeed without blocking"

    ^ count

    "Created: 23.1.1997 / 02:55:58 / cg"

!

lastOwnerId

    "return the processId of the last owning process

     (the one which counted to zero).

     May be very useful in debugging deadLock situations"

    ^ lastOwnerID

    "Created: 24.1.1997 / 23:09:33 / cg"

!

numberOfWaitingProcesses

    "return the number of processes waiting on the receiver"

    ^ waitingProcesses size

    "Created: 3.5.1996 / 18:06:27 / cg"

!

waitingProcesses

    "return the processes waiting on the receiver"

    ^ waitingProcesses

    "Created: 18.7.1996 / 20:53:33 / cg"

!

wouldBlock

    "return true, if the receiver would block the activeProcess

     if a wait was performed. False otherwise."

    ^ count == 0

! !

!Semaphore methodsFor:'testing'!

isEmpty

    "ST80 compatibility - return true if there are no waiters"

    ^ waitingProcesses isEmpty

    "Created: / 3.5.1996 / 18:06:27 / cg"

    "Modified: / 18.6.1998 / 16:07:38 / cg"

! !

!Semaphore methodsFor:'wait & signal'!

clear

    "clear the semaphores count"

    count := 0

!

critical:aBlock

    "evaluate aBlock as a critical region; the receiver must be

     created using Semaphore>>forMutualExclusion"

    |retVal gotSema|

    [

        gotSema := self wait.

        retVal := aBlock value.

    ] valueOnUnwindDo:[

        "/ be careful - the unwind may occur both while waiting

        "/ AND while evaluating the block.

        gotSema notNil ifTrue:[self signal].

    ].

    self signal.

    ^ retVal

    "

      the example below is stupid (it should use a SharedQueue,

      or at least a Queue with critical regions).

      Anyhow, it demonstrates how two processes lock each other

      from accessing coll at the same time

     |sema coll|

     sema := Semaphore forMutualExclusion.

     coll := OrderedCollection new:10.

     [

        1 to:1000 do:[:i |

            sema critical:[

                coll addLast:i.

                (Delay forSeconds:0.1) wait.

            ]

        ]

     ] forkAt:4.

     [

        1 to:1000 do:[:i |

            sema critical:[

                coll removeFirst.

                (Delay forSeconds:0.1) wait.

            ]

        ]

     ] forkAt:4.

    "

    "Modified: / 16.4.1996 / 10:00:46 / stefan"

    "Modified: / 21.7.1998 / 17:45:26 / cg"

!

signal

    "waking up (the first) waiter.

     Q: should this be the highest prio waiter ?"

    |wasBlocked|

    wasBlocked := OperatingSystem blockInterrupts.

    [

        |p|

        count := count + 1.

        p := waitingProcesses removeFirstIfAbsent:nil.

        p notNil ifTrue:[

            Processor resume:p.

        ].

    ] valueNowOrOnUnwindDo:[

        wasBlocked ifFalse:[OperatingSystem unblockInterrupts].

    ]

    "Modified: / 4.2.1998 / 21:01:07 / cg"

!

signalForAll

    "signal the semaphore for all waiters.

     This can be used for process synchronization, if multiple processes are

     waiting for a common event."

    |wasBlocked|

    waitingProcesses notEmpty ifTrue:[

	wasBlocked := OperatingSystem blockInterrupts.

	[

	    |p|

	    "/ first, make them all runnable, but do not schedule

	    "/ (in case one has higher prio and goes into a wait

	    "/  immediately again.)

	    [waitingProcesses notEmpty] whileTrue:[

		p := waitingProcesses removeFirst.

		count := count + 1.

		Processor makeRunnable:p.

	    ].

	] valueNowOrOnUnwindDo:[

	    wasBlocked ifFalse:[OperatingSystem unblockInterrupts].

	].

	"/

	"/ now, reschedule

	"/

	Processor reschedule

    ]

    "Modified: / 5.2.1998 / 10:40:26 / cg"

!

signalIf

    "signal the semaphore, but only if being waited upon.

     This can be used for one-shot semaphores (i.e. not remembering

     previous signals)"

    |wasBlocked|

    waitingProcesses notEmpty ifTrue:[

	wasBlocked := OperatingSystem blockInterrupts.

	[

	    waitingProcesses notEmpty ifTrue:[

		self signal

	    ].