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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: For vs while (was Re: Comparing strings...)
Message-ID: <4044@goanna.cs.rmit.oz.au>
Date: 22 Oct 90 11:25:56 GMT
References: <2205.271700c2@cc.nu.oz.au> <1990Oct13.190106.15615@ux1.cso.uiuc.edu> <8308@scolex.sco.COM>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 34

In article <8308@scolex.sco.COM>, seanf@sco.COM (Sean Fagan) writes:
> #define	while(cond)	for(;cond;)
> 
> (I'm sitting here trying to think of any case where this will break
> [no pun intended 8-)] something; I can't think of any, offhand.)

Here's one:	do i++; while (p(i));

> If you have something such as
> 	while () {
> 		whatever;
> 	}

> this will be an infinite loop given the macro, but a syntax error otherwise.
In gcc it's a syntax error.  In a guide to ANSI C that I have before me, it
says that "a macro argument is ONE or more preprocessing tokens.  I think this
was a nasty thing for the committee to do:  it is the one thing which has
broken most of my code.  Yes, the standard _does_ permit the definition of
macros that look like functions with no arguments:
	#define no_arg_mac() stuff
	... no_arg_mac() ...
unfortunately old preprocessors that were happy with no tokens in an
argument tended to be unhappy with no arguments in a definition.  So I
have to do
	#ifdef	__STDC__
	#define no_arg_mac() stuff
	#else
	#define no_arg_mac(DUMMY) stuff
	#endif
Snarl.  (Naturally, I figured out how to do that _after_ I'd added dummy
arguments to the invocations in the files I really cared about.  Sigh.)

-- 
Fear most of all to be in error.	-- Kierkegaard, quoting Socrates.