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Re: Are designated initializer supposed to zero padding?

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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;

The problem is padding is none of those things.  Padding

  (1) does not have pointer type;
  (2) does not have decimal floating type;
  (3) does not have arithmetic type;
  (4) is not a struct, a union, or an array;
  (5) is not a member of any struct or union;
  (6) is not an element of any array;  and
  (7) is not an object, as the C standard uses the term.

To see these are true, for 1 to 6, it's enough to note that
all of them have a type.  Padding does not have a type.

For number 7, the C standard defines object as a region of storage,
the contents of which can hold values.  To hold a value, an object
has to have a type.  Padding does not have a type.

We can see from other parts of the C standard that it doesn't
consider padding to be objects.  In 6.2.6.1 paragraph 6, the C
standard says

   When a value is stored in an object of struct or union type,
   including in a member object, the bytes of the object
   representation that correspond to any padding bytes take
   unspecified values

Note the wording used:  the text doesn't talk about padding objects,
it says "the bytes of the object representation that correspond to
any padding bytes".  The C standard doesn't consider the space
occupied by padding to be an object, or a subobject of the struct or
union surrounding it.

For that matter, initializing a struct or a union stores a value
in it.  By the paragraph quoted above, any padding bytes take
unspecified values.  So not necessarily zero.

Of course there is nothing stopping an implementation from setting
all the padding bytes (and padding bits, if there are any) to zero;
but the C standard does not require it.

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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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