Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!ucbvax!ucsfcgl!pixar!rta
From: rta@pixar.UUCP (Rick Ace)
Newsgroups: comp.unix.wizards
Subject: Re: transferring processes under csh
Summary: TOPS-20 has the notion of a "job"; UNIX does not
Message-ID: <2364@pixar.UUCP>
Date: 1 Sep 88 18:45:49 GMT
References: <1074@imagine.PAWL.RPI.EDU> <1264@mcgill-vision.UUCP> <5709@cg-atla.UUCP>
Organization: Pixar -- Marin County, California
Lines: 49

In article <5709@cg-atla.UUCP>, duane@cg-atla.UUCP (Andrew Duane) writes:
> In article <1264@mcgill-vision.UUCP>, mouse@mcgill-vision.UUCP (der Mouse) writes:
> > In article <1074@imagine.PAWL.RPI.EDU>, hiebeler@pawl23.pawl.rpi.edu (David Hiebeler) writes:
> > > Does anyone know of a way to transfer processes between different
> > > incantations of csh?
> > 
> > [stuff about environments, terminal types being changed,
> >  signals required, etc.]
> > 					der Mouse
> 
> From my days in college, I remember TOPS-20 (Twenex) being able
> to do just about this. There was the DETACH command that
> detached the current login session from your terminal (there
> was still a command shell running, but not logged in). From
> this same "not-login" shell, there was an ATTACH command that
> would reattach a previously detached session.
> 
> Does anyone know how TOPS-20 handled the issues mentioned above?
> Most of them are non-trivial, but at least one system figured
> them out reasonably.
> 
> Andrew L. Duane (JOT-7)  w:(508)-658-5600 X5993  h:(603)-434-7934

TOPS-20 had a the concept of a process, similar to that of a process
under UNIX.  TOPS-20 also had the notion of a "job", which is a set
of processes all sharing a common ancestor and that ancestor.  In
addition to a process-private context data structure, all processes
of a job shared a common job-private data structure called the JSB.
In the JSB resided a handle on the job's "controlling terminal".
When a process chose to write to its "terminal", output was done
to the controlling terminal's handle appearing in the JSB.  The
ATTACH/DETACH feature was simple (in concept) because by changing
one location in the JSB, you could redirect the terminals for all
processes in the job.  A job with appropriate privilege could
manipulate certain items in another job's JSB.

Roadblocks to making ATTACH work under UNIX:
1. UNIX has no formal concept of a "job" and no per-job database.
   (This is not the only approach, but it worked well for ATTACH.)
2. Determining which processes belong to which terminals is a nebulous task.
3. Processes have direct handles (file descriptors) on terminals;
   they do not go indirect a job-common location.  About the
   closest thing to a "controlling terminal" is /dev/tty, but
   that's not what you'll find at stdin/stdout/stderr.

Rick Ace
Pixar
3240 Kerner Blvd, San Rafael CA 94901
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