Re: `atomic_fetch_add` on pointer types?

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From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: `atomic_fetch_add` on pointer types?
Date	Tue, 31 Dec 2024 06:08:31 -0800
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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

[some whitespace in quoted text was modified]

> The code
>
>   #include <stdatomic.h>
>   #include <stdio.h>
>
>   int main()
>   {
>     int v[2], *_Atomic p = v;
>     atomic_fetch_add(&p, 1);
>     printf("%tu\n", (char *) p - (char *) v);
>   }
>
> GCC prints 1 (https://godbolt.org/z/55Pjcnd3Y), while Clang prints 4
> (https://godbolt.org/z/e4x6z85fe).  Obviously, Clang performed the
> "proper" pointer arithmetic, while GCC ignored the pointer type.
>
> Which one is correct here?

The behavior shown by clang is obviously the more sensible choice.
The C standard is not as clear on this point as I would like it to
be, but I believe the behavior shown by clang is what was intended.

> 7.17.7.5 says (https://port70.net/~nsz/c/c11/n1570.html#7.17.7.5)
>
>    1 The following operations perform arithmetic and bitwise
>      computations.  All of these operations are applicable to an
>      object of any atomic integer type.  None of these operations
>      is applicable to atomic_bool.
>
> Is this wording intended to restrict these operations to integer
> types only?

I believe it was not;  otherwise there is no point to the paragraph
you mention next.  See also below.

> But later 7.17.7.5 also says
>
>    3 For address types, the result may be an undefined address, but
>      the operations otherwise have no undefined behavior.
>
> However, I was unable to find any mention of "address types" anywhere
> else in the standard.  This is the only spot in the entire document
> mentioning "address types".  What types are "address types"?

Surely "address types" should be taken as being synonymous with
"pointer types".  More details to follow in comp.std.c.

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Thread

`atomic_fetch_add` on pointer types? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-12-27 14:28 -0800
  Re: `atomic_fetch_add` on pointer types? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-12-27 17:43 -0800
  Re: `atomic_fetch_add` on pointer types? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-12-27 20:12 -0800
  Re: `atomic_fetch_add` on pointer types? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-31 06:08 -0800

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