Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Florian Weimer <fweimer at redhat dot com>]

On 04/06/2017 07:39 PM, Bernd Edlinger wrote:
> On 04/06/17 16:17, Florian Weimer wrote:
> > > Here is what I want to write in the doc:
> > >
> > > @item typeless_storage
> > > @cindex @code{typeless_storage} type attribute
> > > A type declared with this attribute behaves like a character type
> > > with respect to aliasing semantics.
> > > This is attribute is similar to the @code{may_alias} attribute,
> > > except that it is not restricted to pointers.
> >
> > As Jakub pointed out, this is not what we need here.  An object of type
> > char does *not* have untyped storage.  Accessing it as a different type
> > is still undefined.
>
> but, do you agree that this is valid in C11?
>
> typedef char char_a[4];
>
> int
> main (void)
> {
>    char_a a = {1,2,3,4};
>    short *b = (short *) &a;
>
>    b[1] = 0;
>
>    if (a[0] == 1 && a[1] == 2 && a[2] == 3 && a[3] == 4)
>      abort();
>
>    exit(0);
> }
>
>
> all I want to do is replace "char" with a different type.

Thanks a lot for posting a concrete example.

The effective type of a[2] and [3] is char.  The character type wildcard 
in 6.5(7) only applies to the type of the lvalue expression ysed for the 
access, not the effective type of the object being accessed.  The type 
of the LHS of the assignment expression is short.  So the access is 
undefined.

Florian