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

[Mathijs Mohlmann]

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