> On Aug 4, 2023, at 10:42 AM, Siddhesh Poyarekar <siddh...@gotplt.org> wrote:
>
> On 2023-08-04 10:40, Siddhesh Poyarekar wrote:
>> On 2023-08-03 13:34, Qing Zhao wrote:
>>> One thing I need to point out first is, currently, even for regular fixed
>>> size array in the structure,
>>> We have this same issue, for example:
>>>
>>> #define LENGTH 10
>>>
>>> struct fix {
>>> size_t foo;
>>> int array[LENGTH];
>>> };
>>>
>>> …
>>> int main ()
>>> {
>>> struct fix *p;
>>> p = alloc_buf_more ();
>>>
>>> expect(__builtin_object_size(p->array, 1), LENGTH * sizeof(int));
>>> expect(__builtin_object_size(p->array, 0), -1);
>>> }
>>>
>>> Currently, for __builtin_object_size(p->array, 0), GCC return UNKNOWN for
>>> it.
>>> This is not a special issue for flexible array member.
>> That's fine for fixed arrays at the end of a struct because the "whole
>> object" size could be anything; `p` could be pointing to the beginning of an
>> array for all we know. If however `array` is strictly a flex array, i.e.:
>> ```
>> struct A
>> {
>> size_t foo;
>> int array[];
>> };
>> ```
>> then there's no way in valid C to have an array of `struct fix`, so `q` must
>> be pointing to a single element. So you could deduce:
>> 1. the minimum size of the whole object that q points to.
>
> Actually for minimum size we'd also need a guarantee that `alloc_buf_more`
> returns a valid allocated object.
Why? Please explain a little bit here.
thanks.
Qing
>
> Sid
