Re: AW: A: a new bug to old plain C

On 21/11/2025 14:16, [email protected] wrote:
>> -----Ursprüngliche Nachricht-----
>> Von: Gcc-help <[email protected]> Im Auftrag von
>> Jonathan Wakely via Gcc-help
>> Gesendet: Freitag, 21. November 2025 12:47
>> An: Chris S <[email protected]>
>> Cc: Александр Поваляев <[email protected]>; gcc-help <gcc-
>> [email protected]>
>> Betreff: Re: A: a new bug to old plain C
>>
>> On Fri, 21 Nov 2025 at 03:25, Chris S via Gcc-help <[email protected]>
>> wrote:
>>>
>>> You're applying two consts to the same thing, so it's an error.
>>
>> No, that's not happening here.
>>
>> The explanation is
>> https://isocpp.org/wiki/faq/const-correctness#constptrptr-conversion
>>
>>>
>>> "Const int" and "int const" are the same (look up East vs West const),
>>> but "const int const" is an error.  Declaring a pointer to that
>>> invalid type declaration is still invalid.
>>>
>>> On Thu, Nov 20, 2025, 5:27 PM Александр Поваляев via Gcc-help <
>>> [email protected]> wrote:
>>>
>>>> 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
>>>> <http://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!!!*
>>>>
>>>> }
>>>>
>>>> Respectfully,
>>>> Aleksandr G Povaliaev.
>>>>
> 
> Consider:
> 
> char const * foo1(char * x) {
>      char const * y = x;
>      *x = 'a';
>      return y;
> }
> 
> char const ** foo2(char ** x) {
>      char const ** y = x;
>      **x = 'a';
>      return y;
> }
> 
> typedef char * cp;
> cp const * foo3(cp * x) {
>      cp const * y = x;
>      **x = 'a';
>      return y;
> }
> 
> All three should compile fine, but one does not.
> It is not consistent.
> 
> 

foo2 does not compile - it fails at the initialisation of "y".  That is 
because "x" is a pointer to a "char *", while "y" is a pointer to a 
"const char *" - a pointer to a pointer to a const char.  These are 
different types, and are not compatible (see §6.7.3p10 in the C 
standards if you want read the law).  You can implicitly convert a 
"thing" to a "const thing", but that is not what is happening here.

With foo3, the "const" in y refers to the "char *" pointer.  Thus "y" is 
now a pointer to a constant pointer to a char.  This is more akin to 
foo1.  You can add a "const" to the thing you are point to, but not to a 
thing you are pointing to indirectly (via one or more extra pointers).

You might find it illustrative to change the "**x = 'a'" lines to "**y = 
'a'", and put the code into godbolt.org to see what compiler errors you get.


It is all a bit subtle, and it's easy to get mixup up if you don't use 
double pointers often.  I rarely see them in my own code, and have to be 
careful when dealing with them - I will often use typedefs to keep 
things simpler.  (Opinions and styles will vary a lot as to whether 
typedefs help there - probably if you work with code where multiple 
indirections are common, it is better to use them "raw" until you are so 
familiar that you get an instinctive understanding.)

Lots of types in C might appear to be basically the same thing and yet 
are not considered compatible by the language - usually it is for good 
reasons, such as the faq entry Jonathan gave above.  (The faq was for 
C++, but applies equally to C.)