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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.std.c
Subject: Re: Can the new generic string functions accept void* arguments?
Date: Sun, 04 Jun 2023 21:37:00 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>>> The latest draft of the upcoming C23 standard is:
>>>     https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf
>>> It introduces several type-generic functions in <string.h>, replacing
>>> normal functions of the same names: memchr, strchr, strpbrk, strrchr,
>>> strstr.
>>>
>>> I'll use strchr() as an example; the same applies to the other str*()
>>> generic functions (but not to memchr()).
>> [...]
>>
>> Just after I posted the above, I thought of a potential issue with
>> memchr() that just might affect real code.
>>
>> In C17 and earlier, memchr() has this declaration:
>>
>>     void *memchr(const void *s, int c, size_t n);
>>
>> Given the implicit conversions between void* and other object pointer
>> types, the first argument can be a pointer to any const object type.
>> This is something that might plausibly be used in practice, unlike
>> (I think) passing a void pointer to the str*() functions.
>>
>> It's probably impractical to fix this, since it would require
>> the generic selection to cover all possible object pointer types.
>
> There may be a way round that...  This trick converts any object pointer
> to a const void * or a void * depending on the qualifiers of the object
> pointer:
>
> #include <stdio.h>
>
> #ifndef T
> #define T const int
> #endif
>
> int main(void)
> {
>      T i;
>      puts(_Generic((1 ? &i : (void *)&(int){0}),
>                    void *:       "void *",
>                    const void *: "const void *",
>                    default:      "other"));
> }
>
> (Compile with -DT=int for example to test the other case.)
>
> Taking the address of (int){0} is simply a way to get a void * that is
> not a null pointer constant.  One could, in a macro taking pointer, just
> use
>
>   (1 ? (p) (void *)(p))
>
> but some compilers will warn that the cast discards the const even
> though the overall effect of the expression is to keep it.

I think you're right.  I'll pass it on to the editors.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */