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From: Bruno Haible <bruno@clisp.org>
To: bug-gnulib@gnu.org, Khem Raj <raj.khem@gmail.com>
Subject: Re: [PATCH] Define alignof_slot using _Alignof when using C11 or newer
Date: Sun, 15 Jan 2023 01:17:10 +0100	[thread overview]
Message-ID: <4385269.dumfJixkPq@nimes> (raw)
In-Reply-To: <20230114232744.215167-1-raj.khem@gmail.com>

Khem Raj wrote:
> [2] https://reviews.llvm.org/D133574

Thanks for the heads-up.

> WG14 N2350 made very clear that it is an UB having type definitions
> within "offsetof" [1].
> [1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm

This document is not normative; it is merely a discussion document.

What's (quasi) normative is

  * n3047.pdf = C23, which says in 7.21.(3)
    "If the specified type defines a new type or if the specified member is
     a bit-field, the behavior is undefined."

  * n2176.pdf = C18, which in 7.19.(3) merely says
    "If the specified member is a bit-field, the behavior is undefined."

So, only C23 and higher require to avoid offsetof with a new type.

Since gnulib/lib/stdalign.h mentions an _Alignof bug in older versions of
GCC and clang and some of these older versions (e.g. GCC 4.8.5) support C11,
it is better to enable the workaround only for C23 and higher, not for
C11 and higher.

> +#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && !defined(__cplusplus)

The '!defined(__cplusplus)' part is redundant here.

Bruno





  reply	other threads:[~2023-01-15  0:17 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-14 23:27 [PATCH] Define alignof_slot using _Alignof when using C11 or newer Khem Raj
2023-01-15  0:17 ` Bruno Haible [this message]
2023-01-15  1:20   ` Khem Raj
2023-01-15  1:59   ` Paul Eggert

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