Thank both of you, I didn't think the problem in right way -- the behavior is undefined, I have been thinking it was caused by volatile key word, your explanation makes me released. -Honggang -----Original Message----- From: Andrew Haley [mailto:aph@xxxxxxxxxx] Sent: Thursday, July 16, 2009 11:22 AM To: David Daney Cc: Honggang Xu; gcc-help@xxxxxxxxxxx Subject: Re: Is this a bug? David Daney wrote: > 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. Actually, that's not quite true: as this expression exhibits undefined behaviour, the compiler can return absolutely anything: it isn't limited to just where it does the increment. Andrew.
