Re: A: a new bug to old plain C

On Sun, 23 Nov 2025, 03:19 Александр Поваляев, <[email protected]> wrote:

> Hi there!
>
> And again, we are not talking about warnings, some compilers show an error
> (so, it just stops compiling), some not.
>

Yes, I understand that. GCC gives an error because the code violates the
rules of the C standard:
https://cigix.me/c23#6.5.17.2

There is an example in the C standard explicitly saying that the pointer
conversion from char** to const char** violates a constraint:
https://cigix.me/c23#6.5.17.2.p6

The C standard requires a diagnostic for a constraint violation:
https://cigix.me/c23#5.1.1.3.p1


> I believe it should be fixed, because there is a quite evident use-case
> when using "const Foo * const * const"  is in the order.
>
> And it is also quite in accordance with the recommendations given (links
> mentioned says "don't use const Foo **, cause it might be dangerous").
> And what we're doing is not using "const Foo**". We tend to use "const Foo
> * const * const" instead.
>
> "const" is one of the cornerstones of C language, which prevents possible
> errors and makes programming safer.
>  So, it should be handled appropriately.
>

It is handled according to the C standard.

I'm sorry you are unable to accept this, but I'm not going to waste any
more time replying in this thread.




> Respectfully,
> Aleksandr G Povaliaev.
>
> сб, 22 нояб. 2025 г. в 21:37, Jonathan Wakely <[email protected]>:
>
>>
>>
>> On Sat, 22 Nov 2025, 15:18 Александр Поваляев, <[email protected]>
>> wrote:
>>
>>> Hi there!
>>>
>>> The links you posted are all about "Don't use Foo** → const Foo**, use const
>>> Foo** to const Foo* const* conversion instead"
>>> (https://isocpp.org/wiki/faq/const-correctness#constptrptr-conversion,
>>> https://c-faq.com/ansi/constmismatch.html).
>>> Also, it is being discussed if it is safe or not to use such kind of
>>> conversion.
>>>
>>> But we don't use "Foo** -> const Foo**" conversion anyhow.
>>> We use "Foo** -> const Foo * const * const" conversion. So, we just
>>> follow the recommendation given and not the opposite. :)
>>>
>>
>>
>> And so your code would compile in C++.
>>
>> But like it says in the C FAQ, the rules for C and C++ are different. C
>> does not allow that conversion, even though it would be safe (and C++ does
>> allow it).
>>
>> Here is the relevant part:
>>
>> "C++ has more complicated rules for assigning const-qualified pointers
>> which let you make more kinds of assignments without incurring warnings,
>> but still protect against inadvertent attempts to modify const values. C++
>> would still not allow assigning a char ** to a const char **, but it would
>> let you get away with assigning a char ** to a const char * const *.)"
>>
>>
>>
>>
>>
>>> Anyway some compilers return an error. And this is not the expected
>>> behaviour for use. It is A BUG!!!
>>>
>>
>> OK, maybe try a different compiler then.
>>
>>
>>
>>
>>>
>>>>>
>>>>>