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Re: Are designated initializer supposed to zero padding?

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Michael S <already5chosen@yahoo.com> writes:

> On Mon, 11 May 2026 14:34:34 -0700
> Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
>
>> Michael S <already5chosen@yahoo.com> writes:
>>
>>> On Sun, 10 May 2026 20:01:53 -0700
>>> Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
>>>
>>>> Point 1:  initializers are not required to set padding (either
>>>> padding bits or padding bytes).  Don't expect padding to be
>>>> zeroed.  This statement applies to initializers in all forms -
>>>> regular initializers, designated initializers, and compound
>>>> literals.
>>>
>>> James Kuyper says that zeroing of padding is required by that
>>> standard.  I am not an expert in lawyer-style reading of the
>>> standard, but at my level it looks that he is correct and the
>>> wording in unequivocal.  For example, n3220, 6.7.11:
>>>
>>> 11
>>> If an object that has automatic storage duration is not initialized
>>> explicitly, its representation is indeterminate.  If an object that
>>> has static or thread storage duration is not initialized
>>> explicitly, or any object is initialized with an empty initializer,
>>> then it is subject to default initialization, which initializes an
>>> object as follows: ? if it has pointer type, it is initialized to a
>>> null pointer; ? if it has decimal floating type, it is initialized
>>> to positive zero, and the quantum exponent is
>>> implementation-defined; ? if it has arithmetic type, and it does
>>> not have decimal floating type, it is initialized to (positive or
>>> unsigned) zero; ? if it is an aggregate, every member is
>>> initialized (recursively) according to these rules, and any padding
>>> is initialized to zero bits;
>>
>> That applies only to objects with static storage duration *without*
>> an initializer (or with an empty initializer {}, a new feature
>> in C23).  It doesn't imply that if there is an initializer, padding
>> is zeroed.
>>
>> If you want to set *some* members to non-zero values and guarantee
>> zero for other members and all-bits-zero for padding, there's
>> no direct way to do it.  You could initialize an object with {}
>> (or {0} pre-C23) and then assign values to the desired members.
>
> In this draft it is less clear.  Let's look at N2310,6.7.9:
>
> On one hand, we have 10+19:
>
> If an object that has automatic storage duration is not initialized
> explicitly, its value is indeterminate.
> If an object that has static or thread storage duration is not
> initialized explicitly, then:
> ? if it has pointer type, it is initialized to a null pointer;
> ? if it has arithmetic type, it is initialized to (positive or
> unsigned) zero;
> ? if it is an aggregate, every member is initialized (recursively)
> according to these rules, and any padding is initialized to zero bits;
> ? if it is a union, the first named member is initialized (recursively)
> according to these rules, and any padding is initialized to zero bits;
>
>
> The initialization shall occur in initializer list order, each
> initializer provided for a particular subobject overriding any
> previously listed initializer for the same subobject) all subobjects
> that are not initialized explicitly shall be initialized implicitly the
> same as objects that have static storage duration.
>
> It sounds like zeroing of padding is required.
>
>
> On the other hand, we have 9:
> Except where explicitly stated otherwise, for the purposes of this
> subclause unnamed members of objects of structure and union type do not
> participate in initialization.  Unnamed members of structure objects
> have indeterminate value even after initialization.
>
> It sounds like zeroing of padding is not required.
>
> It's quite confusing.

Padding and unnamed members are different things.  All members,
including unnamed members, have a type.  Padding doesn't have
a type.

See also my other response.

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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

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