On Wed, 2 Feb 2011, Richard Guenther wrote: > The pointer conversions already invoke undefined behavior as specified by the > C standard (6.3.2.3/7). I would say: the conversions are undefined if the pointer is insufficiently aligned for any of the pointer types involved (source, destination or intermediate), where the appropriate alignment for a packed type is 1. Thus, the conversion from packed to non-packed is OK iff the pointer target is sufficiently aligned for the non-packed type. In general from a sequence of casts the compiler is permitted to deduce that the pointer is sufficiently aligned for whatever type in the sequence has the greatest alignment requirement (the middle-end may not have that information at present, but the front end could insert some form of alignment assertion if useful for optimization). *But* that is what is permitted in standards terms; it is not necessarily safe in practice. In particular, on non-strict-alignment targets such as x86 people do in practice assume that unaligned accesses are OK at the C level, not just the assembly level (glibc does so, for example), so it might be a bad idea to assume alignment in a way that would cause that to break. -- Joseph S. Myers joseph@xxxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
