Is this a bug?

[David Daney]

Honggang Xu wrote:
> Or my understand of keyword "volatile" is wrong, following code outputs
> compiled by gcc 4.1.1:  x=22 ,y=59
>  
> main()
> {
> volatile int  x=20,y=35;
> x=y++ + x++;
> y= ++y + ++x;
> printf("x=%d  y=%d\n" ,x,y);
>  
> }
> 

Your program has undefined behavior, the volatile may change the output, 
but it doesn't change the fact that its behavior is undefined.

The problem is that the affect of the increment operator can take place 
either before or after the affect of the assignment.  The compiler can 
order the affects any way that it desires between sequence points.

You might want to read up on sequence points: 
http://en.wikipedia.org/wiki/Sequence_point

David Daney