Re: Are designated initializer supposed to zero padding?

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Are designated initializer supposed to zero padding?
Date	2026-05-13 15:51 -0700
Organization	A noiseless patient Spider
Message-ID	<865x4qkjnu.fsf@linuxsc.com> (permalink)
References	<10tqqso$kn23$1@dont-email.me> <10tr3ia$nak8$1@dont-email.me> <10trver$topc$2@dont-email.me> <10u010b$12a0c$1@dont-email.me> <10u0tiv$2egqj$1@kst.eternal-september.org>

Show all headers | View raw

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

> James Kuyper <jameskuyper@alumni.caltech.edu> writes:
>
>> On 2026-05-11 03:11, highcrew wrote:
>
> [...]
>
>>> I've seen this section in the standard, and I thought I found what
>>> I was seeking for, but isn't it just referring to static and
>>> thread-local variables?
>>
>> No, earlier in the same section it says quite explicitly: "... If an
>> object that has static or thread storage duration is not initialized
>> explicitly, or any object is initialized with an empty initializer,
>> ...".  Objects with automatic storage duration are still covered by the
>> second clause of that sentence.
>
> Yes, but only automatic storage duration objects with an empty
> initializer.  An "empty initializer" is `{}`, which wasn't even
> legal pre-C23.  `{0}` is often used to (try to) achieve the same
> effect;  it specifies a value of 0 for the (recursively) first scalar
> subobject, and leaves everything else to be default-initialized.
>
> I have a feeling that there's something inconsistent about the
> C23 rules.  I'm working on nailing down the specifics.
>
> As a programmer, perhaps the best approach is to arrange things so
> you don't have to care about what's stored in padding.  That might
> mean that you can't use memcmp() to confirm that a struct has the
> right value;  you have to compare member by member.

My preference would be for a compiler option (eg -fzero-padding) or
a pragma (eg #pragma ZERO_PADDING=ON) or perhaps both.  Either of
those has advantages over changing the language definition, not the
least of which is they can be retrofitted to earlier versions of C
without needing to break conformance.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

Are designated initializer supposed to zero padding? highcrew <high.crew3868@fastmail.com> - 2026-05-10 22:47 +0200
  Re: Are designated initializer supposed to zero padding? James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-10 19:15 -0400
    Re: Are designated initializer supposed to zero padding? highcrew <high.crew3868@fastmail.com> - 2026-05-11 09:11 +0200
      Re: Are designated initializer supposed to zero padding? James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-12 16:02 -0400
        Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-12 21:10 -0700
          Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-13 15:51 -0700
            Re: Are designated initializer supposed to zero padding? David Brown <david.brown@hesbynett.no> - 2026-05-14 11:36 +0200
              Re: Are designated initializer supposed to zero padding? Richard Harnden <richard.nospam@gmail.invalid> - 2026-05-14 11:47 +0100
                Re: Are designated initializer supposed to zero padding? David Brown <david.brown@hesbynett.no> - 2026-05-14 14:11 +0200
              Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-14 15:55 -0700
                Re: Are designated initializer supposed to zero padding? David Brown <david.brown@hesbynett.no> - 2026-05-15 10:20 +0200
  Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-10 20:01 -0700
    Re: Are designated initializer supposed to zero padding? highcrew <high.crew3868@fastmail.com> - 2026-05-11 09:10 +0200
      Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-12 09:36 -0700
    Re: Are designated initializer supposed to zero padding? scott@slp53.sl.home (Scott Lurndal) - 2026-05-11 15:34 +0000
      Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-11 18:23 -0700
    Re: Are designated initializer supposed to zero padding? Michael S <already5chosen@yahoo.com> - 2026-05-11 23:22 +0300
      Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-11 14:34 -0700
        Re: Are designated initializer supposed to zero padding? Michael S <already5chosen@yahoo.com> - 2026-05-12 00:55 +0300
          Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-11 15:27 -0700
          Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-11 16:07 -0700
      Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-11 15:19 -0700
        Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-11 16:10 -0700
          Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-11 18:13 -0700
            Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-11 18:28 -0700
              Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-11 21:59 -0700
                Re: Are designated initializer supposed to zero padding? David Brown <david.brown@hesbynett.no> - 2026-05-12 09:15 +0200
                Re: Are designated initializer supposed to zero padding? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-12 00:27 -0700
                Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-12 06:44 -0700
      Re: Are designated initializer supposed to zero padding? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-11 18:15 -0700
  Re: Are designated initializer supposed to zero padding? Michael S <already5chosen@yahoo.com> - 2026-05-12 01:00 +0300

csiph-web