Path: utzoo!attcan!uunet!brunix!gvr
From: gvr@brunix (George V. Reilly)
Newsgroups: comp.lang.c
Subject: Re: A question of style
Summary: Oops!  Typos!
Message-ID: <22150@brunix.UUCP>
Date: 4 Dec 89 03:33:29 GMT
References: <547@mars.Morgan.COM> <1989Nov30.001947.14883@aqdata.uucp> <3552@convex.UUCP> <22139@brunix.UUCP>
Sender: news@brunix.UUCP
Reply-To: gvr@panda.UUCP (George V. Reilly)
Organization: Brown University Department of Computer Science
Lines: 33


In article <22139@brunix.UUCP> gvr@panda.UUCP (George V. Reilly) [I] wrote:
: Because of the two uses of the comma as a parameter separator and as a
: sequential-expression separator, you can occasionally get unexpected
: results.  Consider:
: 
: #define single(list) printf list
: #define double(list) printf(list)
: 
: main()
: {
: 	single("%d %d %d", 1, 2, 3);
: 	double("%d %d %d", 1, 2, 3);
: }
: 
: The expansion of |single()| will yield |printf("%d %d %d", 1, 2, 3)|, while
: the expansion of |double()| will yield |printf(("%d %d %d", 1, 2, 3))|.
: The argument to the second printf will be treated as four comma-separated
: expressions which evaluate to the last expression (3), ultimately
: yielding |printf(3)|, which will probably cause a segmentation fault
: and a core dump.  That's what happened to me the other day, at least.

Oops, I goofed.  There should have been two sets of parentheses each
for the invocations of |single()| and |double()|, thus:

	single(("%d %d %d\n", 1, 2, 3));
	double(("%d %d %d\n", 1, 2, 3));

Thanks to Rich Salz for pointing out that my original posting got
argument-mismatch errors from cpp.
------
George V. Reilly			gvr@cs.brown.edu
uunet!brunix!gvr   gvr@browncs.bitnet	Box 1910, Brown U, Prov, RI 02912