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Subj:	re: Is it possible to talk without GRPNAM

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Date: Thu,  1 Nov 90 10:52:34 EDT
From: Jerry Leichter <leichter@LRW.COM>
Subject: re: Is it possible to talk without GRPNAM
To: INFO-VAX@KL.SRI.COM
Message-Id: <9011011627.AA20518@BULLDOG.CS.YALE.EDU>
X-Envelope-To: info-vax@kl.sri.COM

	If I log into two separate terminals with the same account, Is there
	any way to communicate between them using a mailbox without having the
	GRPNAM (I think that is the spelling) priv set?

Sure.  The ability to communicate through a mailbox is dependent only on the
access rights you need to get to that mailbox.  If both processes are running
under the same UIC, they will both gain access via the OWNER protection of
the mailbox, which normally will be set to allow access.  You can set the
protection to allow or deny access to GROUP, WORLD, and SYSTEM, or you can be
even more selective an use an ACL.

So, where does GRPNAM come in?  In order to communicate through a mailbox, you
have to know WHICH mailbox to communicate through.  Normally, you define a
logical name in some appropriate logical table accessible to all the processes
that need to use the mailbox.  In general, the logical is put in the job table
for temporary mailboxes (and is thus accessible throughout the job tree) and
in the system table for permanent ones.  That won't work here, since there is
normally no table shared by the two independently-started jobs.  There are at
least three alternatives you can use, however:

	- Have the creator of the mailbox store its number in a file.  Every-
		one else reads the file.  Since the file need only be accessed
		when a process wants to open the mailbox, the overhead should
		not be excessive.

	- Create a system-wide table accessible to both processes.  This
		requires privileges, but need only be done once, probably
		at system startup.  The table would be a system-wide table
		accessible only to the particular UIC involved.  The logical
		defining the mailbox can be placed directly into that table
		by simply redefining LNM$TEMPORARY_MAILBOX.

	- Hack approach.  LNM$JOB translates to an a system-wide table, owned
		by your UIC, of the form LNM$JOB_xxxxxxxx, where the x's are
		hex digits.  (They actually are the hex representation of the
		address of the JIB - Job Information Block - for the job.  It
		would have made things SO much simpler if they were the hex
		representation of the PID of the process at the top of the job
		tree....)  So, while you normally have no reason to do it, you
		CAN access the job tables of any of your jobs on the system -
		IF you can find them.  There's no easy way to find the JIB's
		of your other jobs, nor can you do wildcard access to a logi-
		cal name table from a program; but you CAN from DCL with the
		SHOW LOGICAL command.  Try SHOW LOGICAL/STRUCTURE for the
		quickest look - it shows you all tables that you have access
		to, so automatically sorts out which JOB_xxx tables are yours.

							-- Jerry