From dhesi@cirrus.com Tue Jan 14 21:34:48 1992
From: dhesi@cirrus.com (Rahul Dhesi)
Newsgroups: comp.unix.misc
Subject: Re: rmdir("foo/") returns EINVAL - why?
Keywords: rmdir SunOS BSD
Date: 14 Jan 92 21:44:20 GMT
Organization: Cirrus Logic Inc.

In <1992Jan13.151341.2303@dcs.glasgow.ac.uk> sinclair@dcs.glasgow.ac.uk
(Duncan Sinclair) writes:

>In SunOs 4.1, when I append a '/' or even a "/." to the end of a
>directory used as an argument to rmdir(2), I get EINVAL. ("The directory
>referred to by path is the current directory, `.'.")

Two facts to keep in mind:

1. In a UNIX filesystem, the root directory has a slash in its name.

2. When the infamous kernel routine "namei" encounters a null filename
component in a pathname, it inteprets it as referring to the current
directory.

The combination of 1 and 2 above makes more sense than 2 alone.  It would
be nice if in every case the sequence

    directory_name   slash   filename

would yield a syntactically valid pathname.  But if the directory_name
happens to be the root directory, you end up with a pathname that looks
like:

    //name

So it makes sense for the kernel to squeeze out multiple consecutive
slashes.  Generalizing this, it makes sense for the kernel to treat a null
filename component as if it were ".".  So "//name" is treated like /./name,
which makes sense.  Extending the reasoning, "name/" should be treated as
"name/.".

Examples of how useful this is.

1.   In a makefile:

	  ROOTDIR = /
	  BINDIR = usr/bin

	  install:  myprg
		  mv myprg $(DESTDIR)/$(BINDIR)/myprg

     The final destination expands to //usr/bin which still makes sense.

2.   Interactively, when you move a file to another directory, if you
     mistype the name of the destination directory, you can end up simply
     renaming the file.  To avoid any problems, specify the destination
     directory name with a trailing slash.

     % mv xyzzy newdir     # is newdir a directory or a filename?
     % mv xyzzy newdir/    # "newdir/" means "newdir/." -- must be dir

     Not all UNIX derivatives get this right.  But the only correct way to
     interpret a trailing slash is as if it were "/.".  I believe that
     SunOS 4.x, 4.3BSD, and 4.3BSD-tahoe get it right, while SVR2 and
     possibly SVR3 get it wrong.

Also try:

     % cd ""

If your kernel is interpreting null filename components correctly, and your
shell leaves the directory name alone, the above cd will leave you where you
are, i.e., in ".".  Any other behavior is wrong.

However, you may find that /bin/sh complains about a bad directory (as it
does on my SunOS 4.1.1 machine).  This is an example of "wrong" but
desirable behavior, as it protects you against shell scripts like this
one:

     #! /bin/sh
     NEEDEDDIR=/some/path/name
     cd "$NEEDEDIR"; find . -mtime +1 -exec rm -f {} \;

Due to the misspelling, the last line in the script is really

     cd ""; find . -mtime +1 -exec rm -f {} \;

which could be disastrous, so you can't blame the shell for protecting you.
The *kernel*, however, should always interpret a null filename component as
referring to the current directory.
-- 
Rahul Dhesi <dhesi@cirrus.COM>
UUCP:  oliveb!cirrusl!dhesi
"You're even nuttier than we've come to expect of you." -- Doug Gwyn