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From: mohamed@hscfvax.UUCP (750025@Mohamed_el_Lozy)
Newsgroups: comp.unix.questions
Subject: Re: Brain-dead Unix tutor needs quick help (-1)
Message-ID: <473@hscfvax.UUCP>
Date: Wed, 18-Nov-87 12:27:31 EST
Article-I.D.: hscfvax.473
Posted: Wed Nov 18 12:27:31 1987
Date-Received: Sat, 21-Nov-87 04:56:12 EST
References: <387@cogen.UUCP>
Reply-To: mohamed@hscfvax.UUCP (750025@Mohamed_el_Lozy)
Organization: Health Sciences Computing Facility, Harvard University
Lines: 37

In article <387@cogen.UUCP> alen@cogen.UUCP (Alen Shapiro) writes:

>One of the topics touches on good C coding practices. I've hit a mental
>block about the reason ()s are used in preprocessor expressions like
>#define XXX (-1).
>
>--alen the Lisa slayer (it's a long story)

Here is a good general example of what can go wrong:

	% cat example.c
	#define RIGHT (4 + 4)	/* RIGHT must be 8 under any circumstances */
	#define WRONG 4 + 4	/* Will WRONG always equal 8? */

	main()
	{
		printf("%d\n", 3*RIGHT);	/* 3*8 = 24 */
		printf("%d\n", 3*WRONG);	/* Does it?? */
	}

	% cc -o example example.c
	% example
	24
	16

To understand what happenned, use the c preprocessor:

	% /lib/cpp example.c
	# 1 "example.c"



	main()
	{
		printf("%d\n", 3*(4 + 4));
		printf("%d\n", 3*4 + 4);	<- ahaaa!!
	}