Path: utzoo!attcan!uunet!mcvax!ukc!stc!datlog!dlhpedg!cpm
From: cpm@datlog.co.uk (Paul Merriman)
Newsgroups: comp.os.minix
Subject: Re: Semaphores
Message-ID: <785@dlhpedg.co.uk>
Date: 9 Jun 88 15:35:18 GMT
References: <1832@silver.bacs.indiana.edu>
Sender: news@dlhpedg.co.uk
Reply-To: cpm@.co.uk (Paul Merriman)
Organization: FSD@Data Logic Ltd, Queens House, Greenhill Way, Harrow, London.
Lines: 35


 	In response to the request for a simple test to semaphores :-

	As well as the Producer/Consumer type problem semaphores are useful
for providing synchronization between two or more  processes. A simple
test and illustration of this use would be to create two (very !) short 
prcesses - one which prints odd numbers and the other even numbers. Create
two semaphores (ODD and EVEN ?) and initialize them to 1 and 0 respectively.
Then 'wait's and 'signal's can be used to ensure that the processes output 
their numbers alternately (i.e. synchronizes the processes ) e.g.


	Process 1
	---------

	Wait(ODD)
	Print next odd number in sequence (1,3,5..)
	Signal(EVEN)


	Process 2
	---------

	WAIT(EVEN)
	Print next even number in sequence (2,4,6..)
	SIGNAL(ODD)


	While I'm on the subject I was interested to see that you implemented
the semaphores in mm. A nicer, but harder idea would be to put it in the fs
and then use file names to name your semaphores !