Messages in this thread | | | From | vda <> | 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 - 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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