bug or specification ? (gcc 3.0)

[Geoff Keating]

saito@densan.co.jp (Hideo Saito) writes:

> To  : bug-gcc@gnu.org
> From: saito@densan.co.jp (Hideo Saito)
> Title: bug or specification ? (gcc 3.0)
> 
> Hello. I am H.Saito, from Japan. I use NetBSD, mips3 and ELF.
> 
> This sss.c program can't execute similarly with gcc-2.95.2.
> 
> result of gcc 2.95.2
> 0
> 10
> 
> result of gcc 3.0
> 10
> 0
> 
> I hope that gcc 3.0 generates same code at NOTICE with gcc 2.95.2.
> Thank you. Good bye.

Unfortunately, it is your program that is in error.

There is no sequence point between the expression 'p' at NOTICE and
the assignment to p in xxx().  Thus, by section 6.5 paragraphs 2 and 3
in the ISO C standard, your program invokes undefined behaviour.

If there was a sequence point there, arguably GCC 3.0's behaviour
would be more correct, as it delays evaluating the left-hand side of
the '=' until the right-hand side is completely evaluated.


> ---sss.c------------------------------------------------------------
> #include <stdio.h>
> 
> struct sss {
> 	int x, y, z;
> } sss0, sss1, *p;
> 
> main()
> {
> 	p = &sss0;
> 	p->x = xxx();               /* NOTICE */
> 	printf("%d\n", sss0.x);
> 	printf("%d\n", sss1.x);
> }
> 
> xxx()
> {
> 	p = &sss1;
> 	return (10);
> }

-- 
- Geoffrey Keating <geoffk@geoffk.org>