On Fri, 1 Sept 2023 at 15:25, John Naylor <
john.naylor@enterprisedb.com> wrote:
> On Fri, Sep 1, 2023 at 7:48 PM Jelte Fennema <
postgres@jeltef.nl> wrote:
> > The C standard says:
> > > When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values.
> >
> > So if you set any of the fields after a MemSet, the values of the
> > padding bytes that were set to 0 are now unspecified. It seems much
> > safer to actually spell out the padding fields of a hash key.
>
> No, the standard is telling you why you need to memset if consistency of padding bytes matters.
Maybe I'm misunderstanding the sentence from the C standard I quoted. But under my interpretation it means that even an assignment to a field of a struct causes the padding bytes to take unspecified (but not undefined) values, because of the "including in a member object" part of the sentence. It's ofcourse possible that all compilers relevant to Postgres never actually change padding when assigning to a field.