From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX" 10-APR-1993 06:26 10-APR-1993 06:26:00.00
To:	Info-VAX@KL.SRI.COM
CC:	
Subj:	Re: Interrupt Based Inter Process Communications

In article <555E03A012FF228848@MAX.U.WASHINGTON.EDU>, DEREK@U.WASHINGTON.EDU writes:
> Wayne (wpearson@kean.ucs.mun.ca) asked:
> 
>>Hi,
>>   I wish to implement interrupt based communications between several
>>   processes on a VAX 8250 running VMS version 5.1. A signalling process
>>   would generate an interrupt causing several other processes to retrieve
>>   and process data from a common global section.
> 
> To which both Jerry Leicther and Jim Agnew replied, each with a different
> approach.  The first thing that came to my mind, which I didn't see
> suggested elsewhere, is to use the ENQ/DEQ system services with blocking
> (or notify) ASTs.
> 
> Here, each reader of the global section would obtain a, say, CR mode lock
> on some resource name.  The writer would obtain a NL mode lock on the same
> resource.  When the writer has data to pass to the readers, it simply
> converts the NL mode lock to EX mode.  Since the readers are blocking the
> writer, and since they have requested blocking notification, the ENQW
> service will issue ASTs to all of the readers.  Each reader would then
> convert their CR mode lock to NL, and then attempt to convert it back
> to CR mode.  This would place all of the readers on the conversion waiting
> to be granted queue *behind* the writer.  Once the writer gets the EX
> mode lock, it may simply convert it back to NL mode.  (This is a matter
> of *when* you decide the writer can write.  It may be that you use this
> technique simply to send signals to the readers, and *not* to control
> access to the global section.)
> 
> Voila.
> 
> This also gets around a potential problem of mailboxes for this type
> of application.  Namely, that only one reader can read the mailbox.  *IF*,
> however, you only want one reader to process each message, then the
> mailbox will work fine.   Although this seems to be what one would
> normally want to do, it doesn't match the scenario you describe.  (Perhaps
> each receiver in your scheme does something different with the data?)
> 
And still another solution.....

You can use a global section threaded with known size packets
and use the LIB$INSQUE and LIB$REMQUE which will allow easy 
insertion of datagrams within a region, and even be protected
by multiple processor type machines.   You can use event
flags, kernal mode ASTs, locks, etc. to wake up the receiving
processs.   

Earl Lakia			Email:  LAKIA@IPACT.COM
IPACT Inc.			Voice:  219-464-7212
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