
Date: Thu, 10 Nov 88 19:53:06 PST
From: vorbrueg%bufo.usc.edu@oberon.usc.edu (Jan Vorbrueggen)
Message-Id: <8811110353.AA06403@bufo.usc.edu>
To: info-vax@kl.sri.com
Subject: Forcing code to be executed when logging out
Reply-To: vorbrueg%bufo.usc.edu@oberon.usc.edu (Jan Vorbrueggen)
Organization: University of Southern California, Los Angeles, CA

There is a mostly undocumented hook (at least with V4.x) in VMS
for having code executed when a process is deleted.
(If you're not interested in VMS internals, skip the rest!)

SYS$DELPRC calls SYS$RUNDWN which in turn (after doing some small
things) executes the following:

	MOVL	@#CTL$GL_USRUNDWN,R5
	BEQL	10$
	JSB	(R5)
10$:	MOVL	@#EXE$GL_USRUNDWN,R5
	BEQL	20$
	JSB	(R5)
20$:

Some remarks:
The non-zero value in CTL$GL_USRUNDWN gets there when an image
is linked with a priviledged shareable image (i.e., user system services)
and this image has specified a rundown routine. This information
is deleted at image rundown. SYS$EXAMPLES:USSDISP.MAR contains a
sample file for such a beast with a lot of comments; it also describes
the environment for a rundown routine (with a simple example).

The non-zero value in EXE$GL_USRUNDWN gets there when the system
manager (or some such with CMKRNL) loads the code into (non)paged pool
and writes it's address into that cell. I.e., no support from VMS
for this feature.

The rundown routine is also called for image and DCL rundown, so
if you want to do something only when the process is deleted, you
should check that R7, which contains the access mode, is 1 (exec mode)
or less.

As I have never tried this out, I can't tell you what you're allowed
to do and what not. So you're on your own. Fortunately, access violations
and such in exec mode only cause the process to be deleted, not the
system :-)

Incidentally, something analogous exists for user system services.
The cells are called EXE$GL_USRCHM<K,E>.
This is described in some detail in I&DS.

Jan Vorbrueggen

(visiting USC, usually at the 
Max-Planck-Institute for Brain Research, Frankfurt)