Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!sdd.hp.com!apollo!smv
From: smv@apollo.HP.COM (Steve Valentine)
Newsgroups: comp.sys.apollo
Subject: Re: reason to buy Apollos
Message-ID: <4b937d4d.20b6d@apollo.HP.COM>
Date: 14 Jul 90 01:50:00 GMT
References: <9007131342.AA25868@pan.ssec.honeywell.com>
Sender: root@apollo.HP.COM
Reply-To: smv@apollo.HP.COM (Steve Valentine)
Organization: Hewlett-Packard Company, Apollo Division; Chelmsford, MA
Lines: 41

Apollo (among others?) lobbied the 1003.1 committee to permit the leading double
slash convention.

From POSIX 1003.1 (the ugly green book):

	2.3 General Terms

	pathname.
	... Multiple successive slashes are considered the same as one slash.
	A pathname that begins with two successive slashes may be
	interpreted in an implementation-defined manner, although more
	than two leading slashes shall be treated as a single slash. ...

In POSIX-ese, 'may' means that the implementation is permitted, but not
required, to treat a leading double slash specially.  This means that in order
to be POSIX 1003.1 compliant, an application program must not assume that two
leading slashes will be collapsed into one, as is traditional in UNIX systems.

This means that to be on the safe side you should start paths with three slashes
when constructing a path from (possibly null) components, to wit:

A shell command such as:

pathname=/$TOPDIR/$MIDDLEDIR/$LEAF

will result in a potentially troublesome path if $(TOPDIR) happens to be null.

To POSIX-proof the script, add a two more slashes:

pathname=///$TOPDIR/$MIDDLEDIR/$LEAF

Note that adding one slash is insufficient because $(TOPDIR) might not be null.
A POSIX 1003.1 compliant OS will always collapse the three slashes into one.

The 'implementation-defined' keyword tells the POSIX-ese literate that, the
implementation's compliance document must specify what the implementation does
if a pathname with two leading slashes is encountered.
//node/install/doc/apollo/os.v.10.2__posix is the compliance doc for sr10.2.
Steve Valentine - smv@apollo.hp.com
Hewlett-Packard Company, Apollo Systems Division, Chelmsford, MA
Hermits have no peer pressure. -Steven Wright