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

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

On 04/06/17 19:47, Florian Weimer wrote:
> 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.
>

exactly *that* is what I want to make valid with that attribute, which
would be also useful in C and kernel code, IMHO.

But isn't the effective type changed by the assignment b[1] = 0;
as described in 6.5(6):
"If a value is stored into an object having no declared type through an
lvalue having a type that is not a character type, then the type of the
lvalue becomes the effective type of the object for that access and for
subsequent accesses that do not modify the stored value."



Bernd.