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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Are designated initializer supposed to zero padding?
Date: Wed, 13 May 2026 15:51:33 -0700
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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.