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From: rwl@uvacs.UUCP (Ray Lubinsky)
Newsgroups: net.lang.c
Subject: Re: Re: Can #define `token-string' be empty?
Message-ID: <1914@uvacs.UUCP>
Date: Sun, 24-Feb-85 14:39:59 EST
Article-I.D.: uvacs.1914
Posted: Sun Feb 24 14:39:59 1985
Date-Received: Wed, 27-Feb-85 21:02:18 EST
References: <800003@acf4.UUCP> <151@cci-bdc.UUCP>
Organization: U.Va. CS in Charlottesville, VA
Lines: 37

> > Is
> > 	#define	X
> > valid, i.e., can the `token-string' mentioned on page 207 of K&R be empty?
> 
> Yes.  the default value of X is 1.
> 
> 					larry...

   This is also the way I've understood #define, but I made a little program
just to test it out.  Fact is, X is not literally given a default value of 1.
You can really only say that it has the value of "defined" as far as the
pre-processor (/lib/cpp) is concerned.  Consider the following C program which
I fed to /lib/cpp :

C source:				Output from preprocessor:
===================================	=======================================
#include <stdio.h>			...  (stuff from the include file here)
#define	X

main()					main()
{					{
	printf("This is X: %d\n",X);		printf("This is X: %d\n",);
}					}

   Clearly, X was not considered to be 1 (or anything), and when I attempted to
used the test  #if X == 1  , the preprocessor complained of syntax error (since
it translated the line into  #if  == 1  .

   Empty #define's should be used only in connection with #ifdef and #ifndef
tests.  Otherwise they have little usefulness.

------------------------------------------------------------------------------

Ray Lubinsky		     University of Virginia, Dept. of Computer Science
			     uucp: decvax!mcnc!ncsu!uvacs!rwl

--------------  The mind is buddha.  The mind is not buddha.  ----------------