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From: apn@nonvon.UUCP
Newsgroups: comp.bugs.4bsd
Subject: Re: C preprocessor question
Message-ID: <1493@nonvon.UUCP>
Date: Wed, 28-Jan-87 19:33:10 EST
Article-I.D.: nonvon.1493
Posted: Wed Jan 28 19:33:10 1987
Date-Received: Fri, 30-Jan-87 04:41:37 EST
References: <818@wjvax.wjvax.UUCP>
Organization: NONVON Systems Computer Research Group
Lines: 41

in article <818@wjvax.wjvax.UUCP>, brett@wjvax.UUCP (Brett Galloway) says:
> 
> I have a question about the C preprocessor.  I have the following code
> fragment, which fails to compile on my system (4.2BSD):
> 
> #define	GROUP(group,subgroup)	(((group) << 8) | (subgroup))
> 
> #if	GROUP(0,0)
> #endif
> 
> The #if chokes for some reason.  Can anyone in comp.lang.c see my error?
> If there is no error, is this a known bug of the 4.2BSD cpp?
> -- 

	I don't think this should work....

	try running 'cc -E file.c' for some clues.
	this will give you a output essentially a file
	which is what your program looks like to the actual
	c compiler after macros are expanded.

CPP evaluates the macro 'GROUP' with arguments 0,0
to be:

(((0) << 8) | ( 0 ))

which is not a valid c line if it is stand alone.

be carefull.. CPP is for just that , preprocessing.... i.e.
#if's are compile time options and not
run time options. 


-- 
	Alex P Novickis

	UUCP:   {seismo, sun, ihnp4, 'etc' }!ptsfa!nonvon!apn

{* Only those who attempt the absurd   ...   will achieve the impossible   *}
{* I think... I think it's in my basement... Let me go upstairs and check. *}
{*                                                      -escher            *}