[TUHS] Sockets and the true UNIX

Chris Torek torek at torek.net
Sat Sep 23 04:14:31 AEST 2017


>Kulp did the signal work in the kernel and added the code to the c shell
>to use it (or had Eric Cooper, who was working for him as a student at
>the time do fg/bg). He did his work on 3BSD and set the mods to Berkeley,
>where Joy took them and integrated them into 4.1 BSD.

Kulp's name (and IIASA) appeared in several source files, I think;
I know I came across them.

Job control is a little overly complicated but there's only one
really big mistake in it, in my opinion, and that is that the
process group IDs are not allocated in the kernel.  This is what
led to all the "session group leader" cruft in POSIX.

Job control ought to be independent of tty devices: each shell
pipeline is a "job", and the shell should group them by having the
kernel create an initial empty(ish -- the shell is a member of
both its original pgroup and this one it just created, until it
detaches from it) job, then pushing each process it forks into
that pgroup.

I'll use "fd" as a name here since I think that's one way to
return them (they're refcounted like file descriptors) but not
the only way, and it probably makes more sense to return a
pgroup ID directly and have a pgclose()...

    // in a shell:
    fd = new_pgroup(); // kernel allocates pgroup

    for (all commands in a pipeline) {
        pid = fork();
        if (pid == 0) {
            // child
            pid = getpid();
            setpgrp(pid, fd);
            ... all the other stuff we do ...
            exec(childprocess)
        }
        if (pid < 0) ... clean up ...
        // still in the shell - set up the rest of the pipe using
        // the same pgroup
    }
    close(fd); // we're done with the pgroup, it goes away
    // when the last pid we created above also exits

Note that this would mean that anything that creates login sessions
would have to have this same kind of code (create a pgroup, fork,
push child into pgroup, exec shell).  But that's already true for
job control in general: init or getty needs to be job-control-aware,
and this makes their code simpler.

Chris



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