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From: diamond@jit345.swstokyo.dec.com (Norman Diamond)
Newsgroups: comp.lang.c,comp.std.c
Subject: Re: Is it only ANSI C that allow #if to compare symbols?
Keywords: sun C #if
Message-ID: <1991Jan19.040820.1940@tkou02.enet.dec.com>
Date: 19 Jan 91 04:08:20 GMT
References: <1991Jan18.170921.4866@resam.dk>
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Reply-To: diamond@jit345.enet@tkou02.enet.dec.com (Norman Diamond)
Distribution: comp
Organization: Digital Equipment Corporation Japan , Tokyo
Lines: 39

In article <1991Jan18.170921.4866@resam.dk> andrew@resam.dk (Leif Andrew Rump) writes:

>#define	FOLDER	fk
>#ifdef	FOLDER
>  part 1
>#endif
>#if	FOLDER == fk
>  part 2
>#endif
>
>It worked OK for part 1 ... but part 2 get compiled under any circumstances
>
>#ifdef	FOLDER == CASE
>  part 2
>#endif
>
>So this rules out that the presence is equal 1 and absence equal 0!

Absence equals zero, yes.  Presence equals whatever the defined value is.
Now let's see what happens when you test
#if    FOLDER == fk
#if    fk     == fk
#if    0      == fk
#if    0      == 0
which is true.

#ifdef FOLDER == CASE
is syntactically invalid.  Assume you meant #if and let's see:
#if    FOLDER == CASE
#if    0      == CASE
#if    0      == 0

If you want a macro to evaluate to something other than 0, you must give
it a value (a replacement-list) that evaluates as a constant-expression.
Doug Gwyn already posted correct examples.

--
Norman Diamond       diamond@tkov50.enet.dec.com
If this were the company's opinion, I wouldn't be allowed to post it.