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Re: Are designated initializer supposed to zero padding?

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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.

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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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