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Re: UB or not UB? was: On Undefined Behavior

From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: UB or not UB? was: On Undefined Behavior
Date 2026-02-03 05:34 -0800
Organization A noiseless patient Spider
Message-ID <86v7gehrxq.fsf@linuxsc.com> (permalink)
References (4 earlier) <10k71vr$15aeb$11@dont-email.me> <10k7kga$3q0mm$2@dont-email.me> <87y0lzolxf.fsf@example.invalid> <10kagfd$n920$1@dont-email.me> <87ikd3nkui.fsf@example.invalid>

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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> 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.

In fact, the rules in the C standard imply that any two struct
types that have members of the same types and in the same order
have the same layout (conceivably with different amounts of
padding at the end), regardless of whether the two struct types
are compatible or not.

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Thread

UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 16:28 +0200
  Re: UB or not UB? was: On Undefined Behavior bart <bc@freeuk.com> - 2026-01-12 15:58 +0000
    Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 20:08 +0200
      Re: UB or not UB? was: On Undefined Behavior scott@slp53.sl.home (Scott Lurndal) - 2026-01-12 20:02 +0000
        Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 21:09 -0500
          Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-13 11:31 +0200
            Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:21 -0500
      Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:19 -0500
        Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-14 09:35 +0100
          Re: UB or not UB? was: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-14 17:23 +0000
            Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-14 12:53 -0800
          Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-14 14:43 -0800
            Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 11:45 +0100
              Re: UB or not UB? was: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-15 06:16 -0500
              Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-15 04:04 -0800
                Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 13:56 +0100
                Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 05:34 -0800
              Re: UB or not UB? was: On Undefined Behavior scott@slp53.sl.home (Scott Lurndal) - 2026-01-15 15:10 +0000
                Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 16:23 +0100
    Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 21:54 +0000
      Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 21:58 -0500
        Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-13 22:02 -0800
          Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 14:24 +0000
            Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-14 16:48 +0200
  Re: UB or not UB? was: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-12 08:03 -0800
    Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 19:36 +0200
      Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-12 12:03 -0800
        Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 22:41 +0200
          Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-13 09:12 +0100
          Re: UB or not UB? was: On Undefined Behavior pa@see.signature.invalid (Pierre Asselin) - 2026-01-13 20:19 +0000
          Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:20 -0500
          Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 21:53 -0800
        Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 23:53 +0000
          Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 08:06 +0000
      Re: UB or not UB? was: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-13 08:11 -0800
        Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:10 -0500
        Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-01 22:53 -0800
      Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:20 -0500
  Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 20:35 -0500
    Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-13 11:07 +0200

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