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

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

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


Bernd.