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Re: Are designated initializer supposed to zero padding?

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

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>
> [...]
>
>>> In my example in the other post, I have an outer struct with an
>>> inner struct as one of its members, and an object defined with an
>>> initializer that doesn't explicitly initialize all its members.
>>> I can derive from the wording in N3220 that any padding between
>>> members of the inner struct are set to zero bits, but not that this
>>> is done for padding between members in the outer struct.
>>
>> Okay, I understand now what you're asking about.  Just a couple of
>> small detours before getting to your question.
>>
>> First, I have confirmed that the change to the C standard about
>> zeroing padding bits was made in C11, and was not present in C99.
>>
>> Second, I agree with your conclusion that a contained struct or
>> union that has not been initialized explicitly must have any
>> padding set to all zero bits.
>>
>> Third, as best I can determine, the rules under N3220 have the same
>> meaning as the corresponding rules in C11, of course modulo some
>> parts of C23 that were added after C11 and are not part of N1570.
>>
>> Now for the question.  Given this type definition:
>>
>>   typedef struct { struct { char c; short s; } inner; long k; } Outer;
>>
>> and a subsequent declaration
>>
>>   Outer outer = { .k = 1 };
>>
>> we agree that any padding in outer.inner will have been initialized
>> to all zero bits, but what about padding in outer (and outside of
>> outer.inner)?
>>
>> My reading of N3220, which coincides with my reading of N1570, is
>> that the bytes of the object representation of outer that correspond
>> to padding bytes (of outer) take unspecified values.  That result is
>> consistent with 6.2.6.1 paragraph 6, and I don't seen anything in
>> N3220's 6.7.11 (which is 6.7.9 in N1570) that overrides that or that
>> contradicts it.
>>
>> I don't see anything in either standard that talks about padding
>> bits (as opposed to padding bytes) that occur as the result of
>> bitfields.  Presumbly padding bits (in structs and unions) get the
>> same treatment as padding bytes:  set to zero if their containing
>> struct or union is not initialized, and given unspecified values if
>> any member is initialized.
>
> The standard uses the term "padding bits" to mostly refer to bits
> within an integer representation that do not contribute to its value.
> I speculate that that section did not intend to talk about how those
> bits are initialized.  But the term could also apply to extra bits
> between bit-fields.

The phrase "padding bits" is used in connection both with struct and
union types and with integer types.  Where the C standard says,
about struct and union types, that "any padding is initialized to
zero bits", presumably that is meant to apply both to padding caused
by ordinary members and to padding caused by bit-field members.  I
see no reason to suppose it applies only to one and not the other.

> I have mixed feelings about the fact that you agree with my
> interpretation, that padding (bytes) is set to zero in nested
> structs but not necessarily in an outermost struct.  I speculate
> that that was unintentional.  I can't think of any good reason
> to have such a rule intentionally.

For the same reason that assigning to a struct or union member gives
padding unspecified values.  Initializing individual members is very
much like a series of assignments to those members;  the semantics
are more symmetric if assignment and initialization are treated the
same way in the two situations.  Conversely, if the rules were not
the same in the two cases, the language would be less consistent.
Obviously, unless there is some specific reason not to be, being
more consistent it better.  Initializing individual members is like
assignment;  default initialization is not like assignment.  It is
because of this difference that it makes sense to treat the two
cases differently.

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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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