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From: Mike Meyer <mwm@mired.org>
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Date: Wed, 20 Jun 2001 12:36:12 -0500
To: j mckitrick <jcm@FreeBSD-uk.eu.org>
Cc: questions@freebsd.org
Subject: Re: what does this define do?
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j mckitrick <jcm@FreeBSD-uk.eu.org> types:
> at the risk of making a fool of myself (one which i am never hesitant to
> take  ;-) could someone explain to me how this macro
> 
> #define n(flags) (~(flags) & (flags))
> 
> is different from this one?
> 
> #define n(flags) 0

The first one can evaluate flags twice. If flags changes between the
evaluations, the value can be something other than zero. Here's a QAD
example:

#include <stdio.h>

#define n(flags) (~(flags) & (flags))

int x ;
int
f() {

  return x++ ;
}

main() {
  int c ;

  for (c = 0; c < 64; c += 1) {
    x = c ;
    printf("n(%d) is %d\n", c, n(f())) ;
  }
}

This isn't a practice I'd recommend, though.

	<mike
--
Mike Meyer <mwm@mired.org>			http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

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