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COPYRIGHT (c) 1993 by Claus Gittinger
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All Rights Reserved
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This software is furnished under a license and may be used
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only in accordance with the terms of that license and with the
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inclusion of the above copyright notice. This software may not
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be provided or otherwise made available to, or used by, any
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other person. No title to or ownership of the software is
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hereby transferred.
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"
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LinkedList subclass:#Semaphore
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instanceVariableNames:'count waitingProcesses'
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classVariableNames:''
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poolDictionaries:''
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category:'Kernel-Processes'!
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Semaphore comment:'
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COPYRIGHT (c) 1993 by Claus Gittinger
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All Rights Reserved
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Semaphores are used to synchronize processes providing a nonBusy wait
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mechanism. A process can wait for the availability of some resource by
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performing a Semaphore-wait, which will suspend the process until the
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resource becomes available which is signalled by (another process performing)
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Semaphore-signal.
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If the resource has been alrady available before the wait, no suspending is
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done, but the resource immediately allocated.
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See samples in doc/coding.
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%W% %E%
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'!
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!Semaphore class methodsFor:'instance creation'!
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new
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"create & return a new semaphore which blocks until a signal is sent"
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^ super new setCount:0
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!
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new:n
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"create & return a new semaphore which allows n waits before
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blocking"
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^ super new setCount:n
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!
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forMutualExclusion
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"create & return a new semaphore which allows exactly one process to
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wait on it without blocking"
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^ self new:1
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! !
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!Semaphore methodsFor:'private accessing'!
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setCount:n
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count := n
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! !
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!Semaphore methodsFor:'wait & signal'!
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wait
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"wait for the semaphore"
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|current|
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"
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need a while-loop here, since more than one process may
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wait for it and another one may also wake up.
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Thus count is not always non-zero after returning from
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suspend.
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"
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[count == 0] whileTrue:[
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current := Processor currentProcess.
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waitingProcesses isNil ifTrue:[
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waitingProcesses := OrderedCollection with:current
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] ifFalse:[
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waitingProcesses add:current
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].
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current suspend
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].
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count := count - 1
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!
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signal
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"waking up waiters"
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count := count + 1.
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waitingProcesses notNil ifTrue:[
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waitingProcesses isEmpty ifFalse:[
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waitingProcesses removeFirst resume
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]
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]
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!
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critical:aBlock
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"evaluate aBlock as a critical region; the receiver must be
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created using Semaphore-forMutualExclusion"
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|value|
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self wait.
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value := aBlock value.
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self signal.
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^ value
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! !
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