Re: Invalid compilation without -fno-strict-aliasing

[Jean Tourrilhes]

On Wed, Feb 26, 2003 at 04:38:10PM +0100, Horst von Brand wrote:
> Jean Tourrilhes <[e-mail]> said:
> > 	It looks like a compiler bug to me...
> > 	Some users have complained that when the following code is
> > compiled without the -fno-strict-aliasing, the order of the write and
> > memcpy is inverted (which mean a bogus len is mem-copied into the
> > stream).
> > 	Code (from linux/include/net/iw_handler.h) :
> > --------------------------------------------
> > static inline char *
> > iwe_stream_add_event(char *	stream,		/* Stream of events */
> > 		     char *	ends,		/* End of stream */
> > 		     struct iw_event *iwe,	/* Payload */
> > 		     int	event_len)	/* Real size of payload */
> > {
> > 	/* Check if it's possible */
> > 	if((stream + event_len) < ends) {
> > 		iwe->len = event_len;
> > 		memcpy(stream, (char *) iwe, event_len);
> > 		stream += event_len;
> > 	}
> > 	return stream;
> > }
> > --------------------------------------------
> > 	IMHO, the compiler should have enough context to know that the
> > reordering is dangerous. Any suggestion to make this simple code more
> > bullet proof is welcomed.
> 
> The compiler is free to assume char *stream and struct iw_event *iwe point
> to separate areas of memory, due to strict aliasing.

	Which is true and which is not the problem I'm complaining about.
	Have fun...

	Jean