Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!ginosko!uunet!hsi!mfci!karzes
From: karzes@mfci.UUCP (Tom Karzes)
Newsgroups: comp.lang.c
Subject: Re: This one bit me today
Message-ID: <1066@m3.mfci.UUCP>
Date: 7 Oct 89 15:26:19 GMT
References: <2432@hub.UUCP> <832@crdos1.crd.ge.COM> <1067@polari.UUCP>
Sender: karzes@mfci.UUCP
Reply-To: karzes@mfci.UUCP (Tom Karzes)
Organization: Multiflow Computer Inc., Branford Ct. 06405
Lines: 26

In article <1067@polari.UUCP> 6sigma@.UUCP (Brian Matthews) writes:
>That's why anyone who knows what they're doing will parenthesize each
>argument in the expansion of the macro and use liberal whitespace, thusly:
>
>#define xavg(m)	(sum + 3) / (m)
>
>Then things like xavg (*p) and xavg (x + y) work as expected.

It's also why people who *really* know what they're doing parenthesize
both the arguments and the top level of their expression macros, thusly:

#define xavg(m)         ((sum + 3) / (m))

Otherwise if you write something like:

    a / xavg(b)

your buggy macro would give you:

    a / (sum + 3) / (b)

rather than:

    a / ((sum + 3) / (b))

Which, as you can see, has an entirely different meaning.