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Date: Sun, 21 Apr 2024 15:50:31 +0000 (UTC)
From: Thorsten Glaser <tg@...bsd.de>
To: musl@...ts.openwall.com
Subject: Re: Alignment attribute in headers
Michael Forney dixit:
>Something like
>
>#if __STDC_VERSION__ >= 201112L
>/* use _Alignas */
>#elif defined(__cplusplus) && !defined(__GNUC__)
>/* use alignas */
>#else
>/* use __attribute__((__aligned__(N))) */
>#end
Something I noticed recently while doing m68k alignment work:
The C++ alignas is UB if the specified alignment is smaller
than what the structure would normally have, so adding cautious
alignments can explode in one’s face. ☹ Not only is this really
stupid, but makes it not generally usable, too.
GCC’s attribute, in contrast, (without __packed__) just gets
ignored for those cases.
I haven’t looked at the C11 one.
bye,
//mirabilos
--
“It is inappropriate to require that a time represented as
seconds since the Epoch precisely represent the number of
seconds between the referenced time and the Epoch.”
-- IEEE Std 1003.1b-1993 (POSIX) Section B.2.2.2
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