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From: dbh@doc.ic.ac.uk (Denis Howe)
Newsgroups: comp.sys.acorn
Subject: Re: C release 3 preprocessor bug ??
Message-ID: <DBH.91May17123726@asun4.doc.ic.ac.uk>
Date: 17 May 91 11:37:26 GMT
References: <1991May16.125325.15323@doc.ic.ac.uk>
Organization: Dept. of Computing, Imperial College, London, UK
Lines: 44
Nntp-Posting-Host: asun4.doc.ic.ac.uk
In-Reply-To: mybg@doc.ic.ac.uk's message of Thu, 16 May 1991 12:53:25 GMT

In article <1991May16.125325.15323@doc.ic.ac.uk> Michael Ben-Gershon
<mybg@doc.ic.ac.uk> asks:
>Am I crazy, or is there a subtle bug in C release 3?

You cannot be that crazy if you use an Arc, but then again, you are at
Imperial ;-)

My understanding of the C pre-processor which agrees with that of cpp
of SunOS 4.1 here and HP-UX (both /lib/cpp and /lib/cpp.ansi) is that
an undefined macro is only 0 in pre-processor evaluated expressions
(after #if, as opposed to #ifdef) so if UNDEFINED is not defined:

	#if UNDEFINED == 0
	This will be included.
	#endif
or
	#if UNDEFINED+1 == 1

(also true) so

	#if UNDEFINED
	This will not be included.
	#else
	But this will.
	#endif

If a macro is defined as 0 then it is defined!  So
	
	#define ZERO 0
	#ifdef ZERO
	This will be included.
	#endif

To turn off
	#ifdef VAX
use
	#undef VAX

after the point where VAX is defined or simpler still, just don't define it!
Does this help?
--
Denis Howe <dbh@doc.ic.ac.uk>      C-C
H558A Imperial College London     C-C-C
   +44 (71) 589 5111 x5064         N=N