Path: utzoo!attcan!uunet!husc6!uwvax!rutgers!njin!princeton!phoenix!haahr
From: haahr@phoenix.Princeton.EDU (Paul Gluckauf Haahr)
Newsgroups: comp.std.c
Subject: Re: Variable length arg lists for macros
Summary: not a general solution, but ...
Message-ID: <3604@phoenix.Princeton.EDU>
Date: 7 Sep 88 22:07:25 GMT
References: <438@ucsvc.unimelb.edu.au>
Reply-To: haahr@princeton.edu (Paul Gluckauf Haahr)
Organization: Princeton University, Princeton NJ
Lines: 48

> How do people feel about the idea of preprocessor macros with variable
> length argument lists ?
> 
> However this can not be done with macro calls. Wouldn't it be nice to be
> able to do something like ...
> 
> 
> 	#ifdef TRACE
> 	#define	tracef(s,...)	printf(s,...)
> 	#else
> 	#define	tracef(s,...)	/* nothing */
> 	#endif
> 
> At present, this CANNOT BE DONE, without nesting parentheses (clumsy) or
> using a call to an empty function (inefficient, unless you have inline
> integration in an optimizing compiler).

well, if your trace function has no more arguments than printf,
you can always do

	#ifdef	TRACE
	#define	tracef	printf
	#else
	#define	tracef	1 ? 0 :
	#endif

note the complete absence of parenthese.  if you want arguments,
the syntax gets ugly, but can work.  for example,

	#ifdef	TRACE
	#define	trace(n)	(debug > n) ? 0 : printf
	#else
	#define	trace(n)	1 ? 0 :
	#endif

usage is then

	trace(3)("n = %d\n", n)

making that print to stderr probably requires an additional function
call, but only when tracing is enabled.

[ this trick is not my own.  peter honeyman (now umix!honey) showed me
a variant in 1984 that is in the hdb uucp sources, used to implement
logging for uucico -x<number> ]

paul haahr
princeton!haahr		haahr@princeton.edu