Re: One question on the source code of tree-object-size.cc

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

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.

Sid