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From: vda <vda@port.imtp.ilyichevsk.odessa.ua>
To: Mathijs Mohlmann <mathijs@knoware.nl>, Jan Hudec <bulb@ucw.cz>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [BUG] Bad #define, nonportable C, missing {}
Date: Wed, 21 Nov 2001 17:12:27 +0000
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On Wednesday 21 November 2001 13:52, Mathijs Mohlmann wrote:
> 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)

drivers/message/i2o/i2o_config.c:#define MODINC(x,y) (x = x++ % y)

Alan, can you clarify what this macro is doing?
What about making it less confusing?
--
vda
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