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From: richard@aiva.ed.ac.uk (Richard Tobin,                           JANET: R.Tobin@uk.ac.ed             )
Newsgroups: comp.lang.c
Subject: Re: Distinguished pointers (was Re: Weird syscall returns)
Message-ID: <123@aiva.ed.ac.uk>
Date: Mon, 3-Aug-87 11:56:14 EDT
Article-I.D.: aiva.123
Posted: Mon Aug  3 11:56:14 1987
Date-Received: Sat, 8-Aug-87 08:58:58 EDT
References: <1158@copper.TEK.COM> <8317@utzoo.UUCP>
Reply-To: richard@uk.ac.ed.aiva (Richard Tobin)
Distribution: comp
Organization: AI Applications Institute, Edinburgh University
Lines: 33

In article <8317@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>Put the following code fragment into a file and include it in your library:
>
>	char myio_no;
>	char myio_null;	
>
>And then include this in the include file for your library:
>
>	#define	NOFILE		((FILE *)&myio_no)
>	#define	NULLFILE	((FILE *)&myio_null)

Is this valid in all Ansi-conforming implementations?  That is, is a
comparison like

    if(file == NOFILE)

valid?  My copy of the draft C standard says (apropos of pointer comparisons):

"If the objects pointed to are not members of the same aggregate object, the
result is undefined"

and that seems to apply here.  I assume this restriction is to allow segmented
architectures to just compare the segment offset (or is there another reason?).
Of course, if the routines can be arranged to always return pointers from
the same array, then the out-of-band values could be in the same array, and
all would be well.

My copy of the C standard is a little (18 months) out of date, so maybe this
has changed.
-- 
Richard Tobin,                           JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,               ARPA:  R.Tobin%uk.ac.ed@cs.ucl.ac.uk
Edinburgh University.                    UUCP:  ...!ukc!ed.ac.uk!R.Tobin