Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!mailrus!tut.cis.ohio-state.edu!ukma!gatech!udel!mmdf
From: Bruno_Kieba.SBDERX@xerox.com
Newsgroups: comp.os.minix
Subject: Re: Semaphores
Message-ID: <2864@louie.udel.EDU>
Date: 3 Jun 88 16:44:23 GMT
Article-I.D.: louie.2864
Sender: mmdf@udel.EDU
Lines: 31

Pat,
I would interested to see the source code of your semaphore implementation.  I
have myself implemented semaphores in MINIX but not as sophisticated as yours.
I wrote it for educational purpose only.  I implemented mine in the file system
of MINIX (because this is one area I am most familiar with) with three system
calls seminit, semwait and semsignal. To test it, I created a dummy file called
'data'.   First I do a 'initsem' (calling seminit) to initialise two semaphores
SPACE and WORK.  A producer (running in the background) does a semwait on a
semaphore SPACE, opens the file 'data', writes a number contained in a variable
into the file, increments the number contained in the variable by two for the
next  round, closes the file and does a semsignal on a  WORK semaphore.  A
consumer does a semwait on the WORK semaphore, opens the file 'data', retrieves
the number from it, displays it to the terminal screen, closes the file and does
a semsignal on the SPACE semaphore.  A consumer also runs in the background.
Both, producers and consumers need to run in the background otherwise they
cannot be killed  while they are executing.  The kill signal doesn't get caught.
The implementation works for one or more producers and consumers. Processes
waiting on a semaphore are blocked on the appropriate semaphore (FIFO) queue and
awaken when there is work to do (after a signal on the appropriate semaphore).
I used the 'trick' of Andy to do the implementation.  The only problem with my
implementation is that any process can initialise the WORK and SPACE semaphores
and thus totally screw up the blocked processes waiting on them.   I read about
the concept of keys UNIX uses. Maybe this is one way to do it.  I am interested
in the way you implemented your semaphores.  It will also help me to get more
familiar with the memory manager.  
The back pages of PC XENIX documentation show a few examples of semaphores in
order to test them.  The UNIX System Programming book from Keith Haviland and
Ben Salama (1987 Addison-Wesley Publishing Company, Inc.) has some examples for
shared memory with shmget and shmop system calls.  Good to look at.
Hoping that you can send me the source,
Bruno