Re: A: a new bug to old plain C

[[Below]]...

вт, 25 нояб. 2025 г. в 19:42, Jonathan Wakely <[email protected]>:

> On Tue, 25 Nov 2025 at 16:09, Александр Поваляев <[email protected]>
> wrote:
> >
> > Hi there!
> >
> > My code snippet which we're discussing was written in accordance with
> the code safety recommendations provided by your colleagues :)
> > So, the code snipped is naturally doing what was recommended in that C
> FAQ:
> > we're converting "Foo**" to "const Foo * const *" to avoid the possible
> const-violations.
>
> That's not the C FAQ, that's the C++ FAQ, you are confused.
>
[[Aleksandr]] It doesn't matter since "Foo**" to "const Foo * const *
const" conversion is in accordance with both C and C++. That is what I am
trying to say :)

>
> >
> > Here is a note from some recommendation which has been provided by your
> colleagues (
> https://isocpp.org/wiki/faq/const-correctness#constptrptr-conversion) :
>
> What "colleagues" are you talking about?
>
[[Aleksandr]] It looks like you know them very good, so I proposed they are
a kind of colleagues/may be friends of you.

>
> > "The rationale for why that error is a good thing is given below. But
> first, here is the most common solution: simply change const Foo** to const
> Foo* const*"
> > And we are NOT converting "Foo**" to "const Foo**", we ARE converting
> "Foo**" to "const Foo * const * const".
> > Please pay some more attention to what I am writing here.
>
> We could say the same to you.
>
> > Then one more "const" is added since it is going to change neither the
> value which the variable is pointing to nor the value of the variable
> itself, since it denotes "end-of-loop iteration".
> > Please, read carefully the use-case which I provided at the very
> beginning of this e-mail thread. Here it is:
> > "
> > struct zzz const * const * const end_of_arr = arr_of_ptr + count;
> > "
> > And it should work fine. Since it is written in all the recommendations
> and in accordance to C standard.
>
> No it isn't.
>
> > Nobody and no place and no recommendation forbid us from converting
> "Foo**" to "const Foo * const * const".
>
> The C standard forbids it.
>
[[Aleksandr]] Where is that place in either C or C++ standard which forbids
"Foo**" to "const Foo * const * const" conversion?

>
> > So, since it is allowed by C standard, it should work fine with all the
> GCC toolchains.
>
> It isn't allowed by the C standard. Maybe by a future version of the C
> standard, but not the current one.
>
> This should not be hard to understand, please try to read more carefully.
>
> > But with some of them, some of GCC toolchains (like GCC ARM trunk linux)
> it doesn't.
> > So, it should be fixed.
> >
> > Respectfully,
> > Aleksandr G Povaliaev.
> >
> >
> >
> > вт, 25 нояб. 2025 г. в 14:58, Matthias Pfaller <[email protected]>:
> >>
> >> As several people already pointed out to you, this is not a bug in the
> compiler. Your code is valid with C++, but invalid with C.  Is this so hard
> to understand? What you do is just like insisting on
> >>
> >> void f(int *);
> >> void nonsense(void)
> >> {
> >>   register int x;
> >>   f(&x);
> >> }
> >>
> >> to be valid C code. It might look ok, but it's still invalid code.
> >>
> >> On 2025-11-25 12:28, Александр Поваляев wrote:
> >>
> >> Hi there!
> >>
> >> Yes.
> >> Here is the original message from myself: "
> >> struct zzz {
> >> unsigned x, y;
> >> };
> >> void square(struct zzz ** arr_of_ptr, unsigned count) {
> >> // The first "const" produce an error (not a warning) within some GCC
> toolchains, for example ARM GCC trunk (linux), see goldbolt.org
> >> struct zzz const * const * const end_of_arr = arr_of_ptr + count;
> >> // Without the first const "struct zzz * const * const end_of_arr =
> arr_of_ptr + count;" it works fine!!!
> >> }
> >> ".
> >>
> >> So, somebody made this BUG to some GCC toolchains (ARM GCC trunk(linux)
> and some others). We need to have it nailed down and fixed somehow.
> >>
> >> Respectfully,
> >> Aleksandr G Povaliev.
> >>
> >>
> >> вт, 25 нояб. 2025 г. в 13:32, Matthias Pfaller <[email protected]>:
> >>>
> >>> You are aware, that you are quoting the C++-FAQ? Aren't we talking
> about C?
> >>>
> >>> Congratulations btw, you are (I think) the first person managing to
> exhaust
> >>> Jonathan's patience. I always thought he had a limitless source of
> patience hidden
> >>> somewhere and envied him for it...
> >>>
> >>> On 2025-11-25 11:24, Александр Поваляев via Gcc-help wrote:
> >>> > We're not converting "Foo** -> const Foo**". We are doing "Foo** ->
> const
> >>> > Foo * const * const" conversion and this works fine (according to
> the right
> >>> > the same
> >>> > https://isocpp.org/wiki/faq/const-correctness#constptrptr-conversion
> which
> >>> > was mentioned).
> >>> >
> >>> > So, what we're doing is correct from both language syntax and coding
> >>> > practices (safe coding) points of view.
> >>> > This bug should be reported and fixed!!!
> >>> >
> >>> > Respectfully,
> >>> > Aleksandr G Povaliaev.
> >>> >
> >>> > пн, 24 нояб. 2025 г. в 20:49, Henrik Holst <
> [email protected]>:
> >>> >
> >>> >> no they are not, they are a WIKIfication of the C specification, in
> >>> >> particular the heading "Why am I getting an error converting a
> Foo** →
> >>> >> const Foo**?" in an official FAQ from the C Standards Committee
> should make
> >>> >> you realize this.
> >>> >>
> >>> >> /HH
> >>> >>
> >>> >> Den mån 24 nov. 2025 kl 18:45 skrev Александр Поваляев <
> >>> >> [email protected]>:
> >>> >>
> >>> >>> The articles mentioned are all about coding practices ("safe" and
> >>> >>> "unsafe"). They are not about bugs in compilers and hard
> restrictions being
> >>> >>> placed on where I can use "const" in my code and where I can't. :)
> >>> >>>
> >>> >>> The links mentioned earlier just give a recommendation to use
> "Foo** ->
> >>> >>> const Foo * const *" conversion instead of "Foo** -> const Foo**".
> >>> >>> Literally, what I am saying is that: "if you can use const, you
> should
> >>> >>> use it". And this is the case which just confirms my attitude.
> >>> >>> But again, this is about coding practices and recommendations.
> There is
> >>> >>> no place in C and C++ standards saying "DON"T USE CONST EVEN IF
> YOU CAN DO
> >>> >>> IT. WE'RE PROHIBITING THIS".
> >>> >>>
> >>> >>> And so, what GCC compiler (+ some SDKs) is doing now is just the
> opposite
> >>> >>> to the recommendation mentioned. It restricts the places where
> "const"
> >>> >>> might be used.
> >>> >>> Anyway, it is a BUG (BIG BUG!!!) in the GCC + SDK. And the BUG was
> not
> >>> >>> made intentionally to protect software engineers from using
> "unsafe coding
> >>> >>> practices" :)))))
> >>> >>> For such kind of practices, there are warnings with information
> messages.
> >>> >>> And the bug in GCC should be (1) reported; (2) nailed down and (3)
> fixed.
> >>> >>>
> >>> >>> Respectfully,
> >>> >>> Aleksandr G Povaliaev.
> >>> >>>
> >>> >>>
> >>> >>> пн, 24 нояб. 2025 г. в 16:10, Henrik Holst <
> [email protected]
> >>> >>>> :
> >>> >>>>
> >>> >>>> Den mån 24 nov. 2025 kl 13:40 skrev Александр Поваляев <
> >>> >>>> [email protected]>:
> >>> >>>>
> >>> >>>>> Hi there!
> >>> >>>>>
> >>> >>>>> "False sense of security". Const is a hint to any C compiler to
> make
> >>> >>>>> code more error-prone. So, it is always better to use as much
> "const" as
> >>> >>>>> possible.
> >>> >>>>> My point is that "if const might be used, it SHOULD be used".
> That is
> >>> >>>>> the cornerstone.
> >>> >>>>> I want to use it, so GCC compiler + SDK should make it possible.
> It is
> >>> >>>>> my code and my design decisions being undertaken.
> >>> >>>>>
> >>> >>>>> All discussions if it is possible to overcome or not "Const"
> >>> >>>>> restriction is irrelivant here.
> >>> >>>>>
> >>> >>>> no it is not irrelevant here because if I declare a variable as
> const in
> >>> >>>> my local code the value should not be changed by other code
> either, look at
> >>> >>>> the code in the link given to you earlier which shows just such a
> case as
> >>> >>>> an example on why this is not allowed in C.
> >>> >>>>
> >>> >>>> /HH
> >>> >>>>
> >>> >>>>> Respectfully,
> >>> >>>>> Aleksandr G Povaliaev.
> >>> >>>>>
> >>> >>>>> пн, 24 нояб. 2025 г. в 13:46, Henrik Holst <
> >>> >>>>> [email protected]>:
> >>> >>>>>
> >>> >>>>>> The things is that this is unsafe and invalid due to it
> creating a
> >>> >>>>>> false sense of security. Aka your "const Foo *" can change
> under your feet
> >>> >>>>>> by _other_ code leading to your const here giving you a false
> sense of
> >>> >>>>>> security. Yes it prevents you from modifying it locally but not
> some other
> >>> >>>>>> code or thread from modifying your const pointer to now point
> to something
> >>> >>>>>> else, hence the compiler issues an error here since you do not
> get what you
> >>> >>>>>> think you are getting.
> >>> >>>>>>
> >>> >>>>>> /HH
> >>> >>>>>>
> >>> >>>>>> Den mån 24 nov. 2025 kl 11:10 skrev Александр Поваляев via
> Gcc-help <
> >>> >>>>>> [email protected]>:
> >>> >>>>>>
> >>> >>>>>>> Hi there!
> >>> >>>>>>>
> >>> >>>>>>> The code is not invalid and neither of the links posted saying
> that
> >>> >>>>>>> "const
> >>> >>>>>>> Foo * const * const" is invalid construction for C.
> >>> >>>>>>> The links posted just are discussing that it might be UNSAFE
> to cast
> >>> >>>>>>> "Foo
> >>> >>>>>>> **" -> "const Foo **" and recommending usage of "Foo **" ->
> "const
> >>> >>>>>>> Foo *
> >>> >>>>>>> const *" instead.
> >>> >>>>>>> It's OK. If you feel that it is unsafe, please don't use it.
> It is
> >>> >>>>>>> your own
> >>> >>>>>>> decision.
> >>> >>>>>>>
> >>> >>>>>>> But the syntax itself "const Foo * const * const" is correct
> and the
> >>> >>>>>>> use-case I mentioned is quite simple and evident.
> >>> >>>>>>> And so I need this code snipped to be compiled transparently
> over all
> >>> >>>>>>> the
> >>> >>>>>>> GCC (and platform SDKs) available versions.
> >>> >>>>>>>
> >>> >>>>>>> Respectfully,
> >>> >>>>>>> Aleksandr G Povaliaev.
> >>> >>>>>>>
> >>> >>>>>>>
> >>> >>>>>>> пн, 24 нояб. 2025 г. в 01:48, David Brown <
> [email protected]>:
> >>> >>>>>>>
> >>> >>>>>>>> On 23/11/2025 21:45, Александр Поваляев via Gcc-help wrote:
> >>> >>>>>>>>> Hi there!
> >>> >>>>>>>>>
> >>> >>>>>>>> This is getting /extremely/ frustrating.  /Please/ find some
> way to
> >>> >>>>>>>> learn the C programming language - courses, books, good online
> >>> >>>>>>> resources.
> >>> >>>>>>>> And /please/ believe me when I say that people who have
> worked with
> >>> >>>>>>> C
> >>> >>>>>>>> and C++ for decades, studied the standards, who serve on the
> >>> >>>>>>> standards
> >>> >>>>>>>> committee, know better about these things than you do.  (I'm
> talking
> >>> >>>>>>>> about Jonathan here, not myself - he really is a world-class
> expert
> >>> >>>>>>> here.)
> >>> >>>>>>>>> Either the links you've provided or the links you provided
> earlier
> >>> >>>>>>>>> (yesterday) are irrelevant, cause they are just discussing
> if it
> >>> >>>>>>> is safer
> >>> >>>>>>>>> to convert "Foo_Type **" to "const Foo_Type **" or not.
> >>> >>>>>>>>>
> >>> >>>>>>>> The links are highly relevant.  They explain why you cannot
> convert
> >>> >>>>>>> from
> >>> >>>>>>>> "char **" to "const char **".  That also explains why you
> cannot
> >>> >>>>>>> convert
> >>> >>>>>>>> from "char **" to "const char * const * const".  Adding even
> more
> >>> >>>>>>>> "const" qualifiers to the invalid conversion does not somehow
> >>> >>>>>>> magically
> >>> >>>>>>>> make it work.
> >>> >>>>>>>>
> >>> >>>>>>>>> Once again, we are not converting "char** to const char**",
> we're
> >>> >>>>>>>>> converting char** to const char * const * const".
> >>> >>>>>>>>> And we're not discussing if it is safer or not, we are
> discussing
> >>> >>>>>>> GCC
> >>> >>>>>>>>> compiler error.
> >>> >>>>>>>>>
> >>> >>>>>>>> The compiler is generating an error message because you wrote
> >>> >>>>>>> invalid
> >>> >>>>>>>> code.  The error - or bug, if you like - is in /your/ code.
> Not the
> >>> >>>>>>>> compiler.  GCC is doing its job correctly here.
> >>> >>>>>>>>
> >>> >>>>>>>>> Respectfully,
> >>> >>>>>>>>> Aleksandr G Povaliaev.
> >>> >>>>>>>>>
> >>> >>>>>>>> I'm sorry, but signing the post with "respectfully" does not
> make it
> >>> >>>>>>>> respectful.  You started the thread disrespectfully -
> insisting that
> >>> >>>>>>>> there is a bug in GCC when you simply do not understand the
> relevant
> >>> >>>>>>>> aspects of the C programming language.  And you have gone
> downhill
> >>> >>>>>>> since
> >>> >>>>>>>> then, ignoring and rejecting the help and advice you have been
> >>> >>>>>>> given.
> >>> >>>>>>>> Getting things wrong once is entirely understandable - this
> is a
> >>> >>>>>>> subtle
> >>> >>>>>>>> point of the C language, and you are posting to a "help" list.
> >>> >>>>>>> Doubling
> >>> >>>>>>>> down on your ignorance when you have received the help and
> >>> >>>>>>> information
> >>> >>>>>>>> you need, is a different matter entirely.
> >>> >>>>>>>>
> >>> >>>>>>>> Your code is flawed.  You need to fix it.  Mindlessly
> repeating your
> >>> >>>>>>>> misunderstandings about how you think C works will not do
> anything
> >>> >>>>>>> but
> >>> >>>>>>>> annoy people.
> >>> >>>>>>>>
> >>> >>>>>>>> David
> >>> >>>>>>>>
> >>> >>>>>>>>
>