Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!rutgers!sunybcs!boulder!hao!oddjob!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.unix.wizards
Subject: Re: /bin/test and stat(2)
Message-ID: <9013@mimsy.UUCP>
Date: Thu, 15-Oct-87 19:13:39 EDT
Article-I.D.: mimsy.9013
Posted: Thu Oct 15 19:13:39 1987
Date-Received: Sat, 17-Oct-87 10:23:27 EDT
References: <9767@brl-adm.ARPA> <673@unmvax.unm.edu> <6569@brl-smoke.ARPA>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 47

>In article <673@unmvax.unm.edu> mike@turing.unm.edu.UUCP (Michael
>I. Bushnell) writes:
>>But "" is NOT "\0".

In article <6569@brl-smoke.ARPA>, gwyn@brl-smoke.ARPA (Doug Gwyn ) writes:
>What the hell are you talking about?

Colloquial usage confounds and confuses.  To communicate in C:

	""		A `zero length string', actually an array
			containing one byte, the ASCII NUL (code 0/0 in
			ASCII tables) (this assumes your machine uses
			ASCII).  This is also called `the null string'.
			The code NUL is also called the `null character'.

	(char *)0	A null-valued string, or a nil string, or
	(char *)NULL	the nil pointer to char.  

Note that both of these use the word `null'.  We have three things
called *null*, namely NULL, NUL, and "".  Only the last is a
*string*.  When properly cast, the first `looks like a string' from
the outside.  NUL is a character and never looks like a string.

Now then, one `null' that is an invalid pathname is (char *)NULL.
This is a value that is guaranteed not to point to any valid data
object.  On a 4BSD Vax, it happens to point to an (invalid of
course) data object that is nonetheless accessible *and* starts
with a zero byte, so that it looks for all the world just like the
former null string.  *This is mere happenstance*.

The other `null' that is (sometimes) an invalid pathname is the
null string, "".  This is a perfectly valid pointer, pointing to
a zero byte---the character NUL.

On some machines, therefore, open("") and open((char *)NULL) do
the same thing.  This is an accident of the implementation.  They
are not required to do the same thing.  On a 4BSD machine, the
former is required to work, and acesses the current directory.  On
some 4BSD machines, the latter happens to do the same; on others,
it returns an EFAULT error.

Summary: the *null string* `""' is a pathname.  The *nil pointer
to char* `(char *)NULL' is not a pathname.  The *NUL character* in
position 0/0 on ASCII charts is not even a string.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris