[TUHS] FD 2

[Warner Losh]

On Sun, Jan 29, 2023, 5:25 PM Phil Budne <phil at ultimate.com> wrote:

> Ron Natalie wrote:
> > Warner Losh wrote:
> > >"The main loop of the shell went as follows:
> > >1) The shell closed all its open files, then opened the terminal
> > >special file for standard input and output (file descriptors 0 and 1).
> > >
> > Unfortunately, the source code says otherwise.   None of shells V6, PWB,
> > V7 do anything like is mentioned above.   They assume 0 and 1 (and 2)
> > are already open.
> > The only fd redirection they do is when you do pipes or redirection on
> > the command line.
> >
> > Where this is done is, as I posted earlier, in /etc/init.    Init opens
> > the tty device and dups it to 1 and then invokes either the shell (if
> > we're in single user mode) or getty for interactive mode.
> > This was done in V6 and PWB (1).   In V7, init added a second dup for
> > file descriptor 2.
>
> I think the text Warner quoted applied to only the pre-fork version of
> the system, since it's preceeded by:
>
>   "Processes (independently executing entities) existed very early in
>   PDP-7 Unix. There were in fact precisely two of them, one for each
>   of the two terminals attached to the machine. There was no fork,
>   wait, or exec. There was an exit, but its meaning was rather
>   different, as will be seen.
>
> and ends with:
>
>   "4) The command did its work, then terminated by calling exit. The
>   exit call caused the system to read in a fresh copy of the shell
>   over the terminated command, then to jump to its start (and thus in
>   effect to go to step 1)."
>
> In the (late) PDP-7 Unix we have sources for, FDs 1 & 2 are
> established in init at labels init1 (for the user on the console tty)
> and init2 (for the user on the display & display keyboard) after forking
> and doing the job of the login executable:
>
> https://github.com/DoctorWkt/pdp7-unix/blob/master/src/cmd/init.s



It looks like 0 & 1 to me. Am I missing something?

Warner

>
> A remaining mystery about the set of PDP-7 Unix sources we have is
> that cat.s (and only cat.s?) writes error messages to FD 8, but I
> haven't seen any place that sets FD 8:
>
> https://github.com/DoctorWkt/pdp7-unix/blob/master/scans/cat.s#L43
> https://github.com/DoctorWkt/pdp7-unix/blob/master/scans/cat.s#L45
> https://github.com/DoctorWkt/pdp7-unix/blob/master/scans/cat.s#L51
>
> Listing scan at:
>
> https://www.tuhs.org/Archive/Distributions/Research/McIlroy_v0/06-5-12.pdf#page=21
>
>
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