RE: Is this a gcc bug?

[David Korn <dkorn at pixelpower dot com>]

>From: Uros Bizjak [mailto:uros@kss-loka.si]
>Sent: 11 January 2001 10:11

>  This code produces a strange result on Solaris 2.5.1 with gcc 2.8.1:
>
>--cut here--
>
>#include <stdio.h>
>int main()
>{
>        int x = 0;
>        int y = 5;
>        int z = 15;
>
>        printf ("%i, %i, %i, %i\n", x,  x += y, x += 2, x += z); 
>        return 0;
>}
>
>--cut here--
>
>  It displays "22, 5, 7, 22".
>  I would expect the result to be "0, 5, 7, 22".

  The code isn't valid.  It is part of the language specification that you
cannot rely on the order of evaluation of arguments to a function call, nor
can you rely on when the side effects take place.  There's a technical
thing about these things called sequence points and how you're only allowed
to change the value of a variable once between them, but that's quite 
in-depth.  If you called 

   printf("%d %d %d\n", x++, x++, x++);

the compiler is free to pass the same value for all three occurrences of x
and add three to it after the function call, or to evaluate the arguments
from left to right and pass 0, 1, 2, or from right to left and pass 2, 1, 0,
or indeed (since the language spec says this is undefined behaviour) to 
output a program that plays space invaders when you run it.  Steer clear
of such ambiguous constructs and you'll be ok.

      DaveK
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