Re: [BUG] Bad #define, nonportable C, missing {}

[Horst von Brand]

Stevie O <[e-mail]> said:
> 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?

Nope. x++ increments x sometime (not defined when) after taking the value. 

   x = x++ % y

is wrong: There is just one sequence point at the end of the expression,
and x is modified twice in between (++ and =). If this gives the same as:

   x = (x + 1) % y

gcc is smoking potent stuff... but it could legally do anything at all.
-- 
Dr. Horst H. von Brand                   User #22616 counter.li.org
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