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

[Richard Biener <rguenther@suse.de>]

On Thu, 6 Apr 2017, Bernd Edlinger wrote:

> On 04/06/17 21:05, Florian Weimer wrote:
> > On 04/06/2017 08:49 PM, Bernd Edlinger wrote:
> >
> >> For instance how do you "declare an object without a declared type"?
> >
> > malloc and other allocation functions return pointers to objects without
> > a declared type.
> >
> 
> Thanks Florian,
> 
> this discussion is very helpful.
> 
> How about this for the documentation:
> 
> @item typeless_storage
> @cindex @code{typeless_storage} type attribute
> In the context of section 6.5 paragraph 6 of the C11 standard,
> an object of this type behaves as if it has no declared type.
> In the context of section 6.5 paragraph 7 of the C11 standard,
> an object or a pointer if this type behaves as if it were a
> character type.
> This is attribute is similar to the @code{may_alias} attribute,
> except that it is not restricted to pointers.
> 
> Example of use:
> 
> @smallexample
> typedef int __attribute__((__typeless_storage__)) int_a;
> 
> int
> main (void)
> @{
>    int_a a = 0x12345678;
>    short *b = (short *) &a;
> 
>    b[1] = 0;
> 
>    if (a == 0x12345678)
>      abort();
> 
>    exit(0);
> @}
> @end smallexample

Seriously, do not suggest such broken case.  There's a union to
do this example portably.

Richard.