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

LIU Hao via Gcc-help <[email protected]> Fri, 5 Dec 2025 16:29:30 +0800

Newsgroups	gmane.comp.gcc.help
Message-ID	<[email protected]>

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

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