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From: ekrell@hector.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: symbolic links are a botch
Message-ID: <2636@ulysses.homer.nj.att.com>
Date: Sun, 7-Jun-87 16:40:28 EDT
Article-I.D.: ulysses.2636
Posted: Sun Jun  7 16:40:28 1987
Date-Received: Sun, 7-Jun-87 23:40:30 EDT
References: <7724@brl-adm.ARPA>
Sender: daemon@ulysses.homer.nj.att.com
Reply-To: ekrell@ulysses (Eduardo Krell)
Organization: AT&T Bell Labs, Murray Hill
Lines: 49

In article <7724@brl-adm.ARPA> rbj@icst-cmr.arpa writes:

>I think the key word here is "the". The kernel implements symbolic
>links as you describe below; the csh maintains $cwd symbolicly. If you
>do `cd /foo/bar' where /foo/bar -> /whiz/bang followed by `cd ..',
>$cwd gives /foo while pwd gives /whiz. Thus csh is interpreting `..'
>to mean `back' while the kernel takes it to mean `up'.

So the question is: where should you end up?. Ignore for a moment
the problem of where this special treatment of ".." belongs (shell, kernel,
etc). The question is: if a user does a "cd /foo/bar" and then "cd ..",
is it reasonable to expect to be in /foo whether /foo/bar is a symbolic link
or not?.

>In fact, symbolic links are no worse than when root makes links
>between directories. The possibility exists for loops

The kernel will detect a loop with symbolic links when the count of
symbolic links being encountered by namei() while translating a filename
exceeds some constant (8, I think).

>
>   What would be the best semantics for ..?
>
>Exactly what they are now as far as the kernel is concerned.

The problem ("cd /foo/bar; cd ..") still remains, then. You could argue that
the special treatment for ".." belongs in the shell, but then what about
a program that does a chdir or the C preprocessor searching for "../x.h" ?
We would really have to change all the programs that deal with filenames
and put special code in them to deal with "..". This is clearly wrong.

>what happens when you type `cd ..', I can see several alternatives.
>Possibly the best is to not treat it specially.

Why?. A user that does "cd /foo/bar" and then "cd .." expects to be in /foo.
He/she doesn't know and DOESN'T NEED to know that /foo/bar is a symbolic link.
Naive users don't even know what symbolic links are.

>consider the case of loops, where after `descending' into the same
>directorys a few times and realizing what happened, do you want to 
>retrace your steps in reverse, or go straight up as fast as possible.

Let the kernel deal with the loops. As I said before, after a number of
symbolic links are encounteres, it gives up with an ELOOP error.
    
    Eduardo Krell                   AT&T Bell Laboratories, Murray Hill

    {ihnp4,seismo,ucbvax}!ulysses!ekrell