Messages in this thread | | | Subject | Re: [BUG] Bad #define, nonportable C, missing {} | Date | Thu, 22 Nov 2001 08:46:08 -0300 | From | Horst von Brand <> |
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Stevie O <stevie@qrpff.net> 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 Departamento de Informatica Fono: +56 32 654431 Universidad Tecnica Federico Santa Maria +56 32 654239 Casilla 110-V, Valparaiso, Chile Fax: +56 32 797513 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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