------- Comment #2 from edwintorok at gmail dot com 2009-04-25 13:49 -------
(In reply to comment #1)
> Casting through a union (2)
>
> describes an invalid way of doing type-punning.
There is also a citation from C99 on that page:
"An object shall have its stored value accessed only by an lvalue expression
that has one of the following types:
* a type compatible with the effective type of the object,
* a qualified version of a type compatible with the effective type of the
object,
* a type that is the signed or unsigned type corresponding to the effective
type of the object,
* a type that is the signed or unsigned type corresponding to a qualified
version of the effective type of the object,
* an aggregate or union type that includes one of the aforementioned types
among its members (including, recursively, a member of a subaggregate or
contained union), or
* a character type."
I'm casting to a union that has both types as members, why doesn't that fit
under the 5th case in the above quote?
Also there is a warning for foo(), but there is no warning for bar(), but I
think they are exactly the same things wrt to violating or not the aliasing
rules.
>
> The only conforming and portable way is
>
> unsigned bar(char *x)
> {
> unsigned un;
> memcpy (&un, x, sizeof (un));
> return un;
> }
That may be too slow for me, I'll go with a static inline function that takes a
void* instead of a macro that does the cast.
>
> I have no opinion on the different levels of warnings (I think this case
> should be unconditionally).
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39895
