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Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ucbvax!ulysses!dgk
From: dgk@ulysses.UUCP
Newsgroups: comp.unix.wizards
Subject: symbolic links are a botch
Message-ID: <2629@ulysses.homer.nj.att.com>
Date: Thu, 4-Jun-87 14:31:07 EDT
Article-I.D.: ulysses.2629
Posted: Thu Jun  4 14:31:07 1987
Date-Received: Sat, 6-Jun-87 09:39:16 EDT
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 88
Keywords: file system, file name resolution


If you type
	cd /usr/include/sys
	echo cwd=$cwd
	/bin/pwd
	cd ..
	echo cwd=$cwd
	/bin/pwd
from csh with BSD 4.3 running on our vax you get:
	cwd=usr/include/sys
	/usr/sys/h
	cwd=/sys
	/usr/sys

If you do the equivalent
	cd /usr/include/sys
	/bin/pwd
	echo cwd=$PWD
	cd ..
	echo cwd=$PWD
	/bin/pwd
from the Korn shell with BSD 4.3 running on our vax you get:
	cwd=/usr/include/sys
	/usr/include/sys
	cwd=/usr/include
	/usr/include

If you do the nearly equivalent
	cd /usr/include/sys
	pwd
	/bin/pwd
	cd ..
	pwd
	/bin/pwd
from the 4.3 Bourne shell you get:
	usr/include/sys
	usr/include/sys
	/usr/sys/h
	/sys

I think that the reason for this discrepancy is that the implementation
of symbolic links on BSD Unix is a botch. 

In order to use a system with symbolic links without being surprised,
you have to be aware of all symbolic links to directories.  The reason
is that the kernel does not keep track of the device-inodes as you
descend the file tree, so it does not know how to walk back up.
For example, if I add a directory of include files named /usr/include/local
and one of my include files includes ""../stat.h", it will not work on
BSD but will work on Sys V.

Can you imagine what Unix would have been like if .. were not handled
specially at mount points?  You would have to know where each of the
mount points were.  Two systems could be compatible only if they had
identical mount points.

I recently added symbolic links to my version of System V Release 3
kernel. I made sure that .. was always handled like an operator that
moves you up one level.  The changes were not complex and I have not
run into any problems at all.  The user again sees the file system as
a tree structure which preserves the original simplicity of the Unix model. 
By the way, I did modify find so that you can specify a virtual walk
or a physical walk.  I virtual walk follows symbolic links.  The virtual
walk keeps track of device-inodes so that I never descend through the
same physical directory more than once.  Therefore, a symbolic line to
/ poses no problem.

( Now some guru might say that . and .. are just names that have
i-nodes associated with them just like any other name and therefore, the
i-node should be followed.  If this were true, then as root I should
be able to edit my directory and change the inode of . to change its
meaning.  What do you think really happens?)

I am conducting a poll on this issue and I am interested in hearing your views.
I will post the results to this newsgroup on the net.

What would be the best semantics for ..?

Do you think that BSD has done it right or have I done it right?

What code would break if .. were always treated as an operator?

Disclaimer:  I am not affiliated with the System V product, I am involved
in research only and these are my own.  They do not imply any direction
for future releases of System V.

David Korn
{ihnp4,decvax}ulysses!dgk