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From: $VK0%CLVM.BITNET@WISCVM.ARPA (Valdis Kletnieks)
Newsgroups: net.sources
Subject: Re: unifdef processor
Message-ID: <2533@brl-smoke.ARPA>
Date: Thu, 24-Jul-86 11:32:01 EDT
Article-I.D.: brl-smok.2533
Posted: Thu Jul 24 11:32:01 1986
Date-Received: Thu, 24-Jul-86 21:40:14 EDT
Sender: news@brl-smoke.ARPA
Lines: 54

Hmmm. I see in the last 3 or 4 days a NUMBER of people who claim that running
a program through the C preprecessor will produce the desired results.

'Taint so, guys...

Consider -

#include <stdio.h>
#ifdef mach1
#define NBITS 16   /* this is a poor old PDP-11 */
#endif
#ifdef mach2
#define NBITS 12   /* PDP-8 */
#endif
#ifdef mach3
#define NBITS 32   /* a real machine like a Vax or such */
#endif
....
...

     for(i=0;i<NBITS;i++) { /* some code here */ }

.....
....

Feed this to your C-preprocessor.

(a) That <stdio.h> produces LOTS of line of blanks and other stuff..
(b) You end up with LOTS of 'magic numbers' like {8,12,16,32} hard-coded
    in the code.

For REAL fun, try feeding kernel source that does a LOT of bit masking using
    #define'd masks through the preprocessor, and THEN try to understand
    what's going on.  I thought that AVOIDING such stuff was the reason that
    the C-preprocessor was INVENTED.

What the people were ASKING for here was something that would ONLY strip out
the irrelevant code from stuff like the above, leaving only the 1 appropriate
#define, and LEAVE THE REST OF THE CODE ALONE.


                      Valdis Kletnieks
                      Systems Programmer
                      Educational Resources Center
                      Clarkson University
                      Potsdam, NY  13676
                      (315) 268-2292

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