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

I'm glad to see someone trying to champion this cause.... because the
usefulness of inheriting const ( maybe things like volatile too)
automatically, at any level offers protections, and posits intention to not
cause changes.

This looks like a good place to jump in....


    int
    main(void)
      {
        const char* s1 = "meow";
        void* p = &s1;
        char** pp = p;   // implicit conversion from `void*` drops qualifier
        **pp = 'b';
      }

if you deliberately want to mess yourself up, just cast some things.  If
you want to undermine the advertised promise not to change data passed to a
function, so be it - there's memcpy too.

There's been so many examples of 'well I could use such a behavior to do
this bad thing'.  It was my understanding of the C standard from K&R days,
was that pointers could inherit const, but not lose const... I was sad to
see that there was a bizarre behavior of '" char const *const * argv =
(char**)argv_"  didn't work.  I just wanted const so I didn't incidentally
change things... on a function interface, (contrary to consensus about
specific meaning of what const is and implies) is just a notation that a
function won't change the content passed.  Again, there's plenty of ways
that could be worked around.

I honestly haven't seen a good argument as to why you can't just inherit
const.  It doesn't mean the data underlaying it doesn't change...
voltatile const is a good example ... a fetch-only sort of variable and
read always, don't cache the 'const' in a register.




On Fri, Dec 5, 2025 at 12:29 AM LIU Hao via Gcc-help <[email protected]>
wrote:

> 在 2025-12-5 16:10, [email protected] 写道:
> > Refering to that example
> >
> > int compare (const void *a1, const void *b1) {
> >    char *const *a = a1;
> >     ...
> >
> > Here a and a1 have a different types.
> >
> This conversion is allowed due to the special rule about pointer-to-void,
> which can indeed be exploited
> to modify a const-qualified object (https://gcc.godbolt.org/z/qa7qM9W8Y):
>
>     int
>     main(void)
>       {
>         const char* s1 = "meow";
>         void* p = &s1;
>         char** pp = p;   // implicit conversion from `void*` drops
> qualifier
>         **pp = 'b';
>       }
>
> It can also be exploited in C++ to modify a const-qualified object without
> `const_cast`
> (https://gcc.godbolt.org/z/cKo3Ghfaq). It's nothing about safety; it's
> only about what is defined and
> what is not. Anything that is not defined is undefined. There are all
> kinds of holes in the standard.
>
>
>
> --
> Best regards,
> LIU Hao
>