Path: csiph.com!eternal-september.org!feeder.eternal-september.org!nntp.eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: UB or not UB? was: On Undefined Behavior
Date: Thu, 15 Jan 2026 04:04:05 -0800
Organization: None to speak of
Lines: 28
Message-ID: <87ikd3nkui.fsf@example.invalid>
References: <10j6qdt$3q9n4$1@dont-email.me> <20260112162857.00003dd8@yahoo.com> <10k35mn$2ean4$1@dont-email.me> <20260112200821.000020a5@yahoo.com> <10k71vr$15aeb$11@dont-email.me> <10k7kga$3q0mm$2@dont-email.me> <87y0lzolxf.fsf@example.invalid> <10kagfd$n920$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain
Injection-Date: Thu, 15 Jan 2026 12:04:06 +0000 (UTC)
Injection-Info: dont-email.me; posting-host="4d7fe61be3b508b2d5a980eafedcef32"; logging-data="802666"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19zSfo5iponiWcrjjsmRBvs"
User-Agent: Gnus/5.13 (Gnus v5.13)
Cancel-Lock: sha1:1sUFrWOyojnAN/7hqKTCWWkpVsU= sha1:N2zRWoESG0Sbf3jLxXUjzUL1fso=
Xref: csiph.com comp.lang.c:396431

David Brown <david.brown@hesbynett.no> writes:
> On 14/01/2026 23:43, Keith Thompson wrote:
[...]
>> There can be arbitrary padding between struct members, or after the
>> last member.  Almost(?) all implementations add padding only to
>> satisfy alignment requirements, but the standard doesn't state any
>> restrictions.  There can be no padding before the first member, and
>> offsets of members must be increasing.
>
> On closer reading, I agree with you here.  I find it a little
> surprising that this is not implementation-defined.  If an
> implementation can arbitrarily add extra padding within a struct, it
> severely limits the use of structs in contexts outside the current
> translation unit.

In practice, struct layouts are (I think) typically specified by
a system's ABI, and ABIs generally permit/require only whatever
padding is necessary to meet alignment requirements.

And I think C has rules about type compatibility that are intended to
cover the same struct definition being used in different translation
units within a program, though I'm too lazy to look up the details.

[...]

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