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Date: Wed, 21 Nov 2001 14:52:37 +0100 (CET)
From: Mathijs Mohlmann <mathijs@knoware.nl>
To: Jan Hudec <bulb@ucw.cz>
Subject: Re: [BUG] Bad #define, nonportable C, missing {}
Cc: linux-kernel@vger.kernel.org
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On 21-Nov-2001 Jan Hudec wrote:
>> 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)
> 
> *NO* 
> MODINC(x,y) (x = (x+1) % y)
> is correct and beaves as expected. Unfortunately:
> MODINC(x,y) (x = x++ % y)
> is a nonsence, because the evaluation is something like this
> x++ returns x
> x++ % y returns x % y
> x is assigned the result and it's incremented IN UNDEFINED ORDER!!!
> AFAIK the ANSI C spec explicitly undefines the order.

in fact, gcc does (according to my tests):
MODINC(x,y) (x = (x % y) + 1)



-- 
        me
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