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Date: Wed, 21 Nov 2001 23:24:02 -0500
To: Vincent Sweeney <v.sweeney@dexterus.com>,
        vda <vda@port.imtp.ilyichevsk.odessa.ua>
From: Stevie O <stevie@qrpff.net>
Subject: Re: [BUG] Bad #define, nonportable C, missing {}
Cc: linux-kernel@vger.kernel.org
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At 12:35 PM 11/21/2001 +0000, Vincent Sweeney wrote:
> > Bad code style. Bad name (sounds like 'module inc').
> > I can't even tell from this define what the hell it is trying to do:
> > x++ will return unchanged x, then we obtain (x mod y),
> > then we store it into x... and why x++ then??!
> > Alan, seems like you can help here...
>
>Go read up on C operator precedence. Unary ++ comes before %, so if we
>rewrite the #define to make it more "readable" it would be #define
>MODINC(x,y) (x = (x+1) % y)

But x++ is postincrement though. That means the value of 'x' is inserted, 
and after the expression is evaluated, x is incremented. Right?

If we were going to be semiobscure, wouldn't the correct code be

#define MODINC(x,y) (x = ++x % y)

Btw, to the original guy: this loops 'x' around between 0 and (y-1) -- 
i.e., if y=5, and x=0, successive "calls" to this #define would do

MODINC(x,y);  // x=1
MODINC(x,y);  // x=2
MODINC(x,y);  // x=3
MODINC(x,y);  // x=4
MODINC(x,y);  // x=0
MODINC(x,y);  // x=1
MODINC(x,y);  // x=2
MODINC(x,y);  // x=3
...




--
Stevie-O

Real programmers use COPY CON PROGRAM.EXE

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