On April 6, 2017 8:12:29 PM GMT+02:00, Bernd Edlinger
<bernd.edlin...@hotmail.de> wrote:
>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."
Yes. I think the example is valid. At least GCCs memory model makes it so.
Richard.
>
>
>Bernd.
