Re: [musl] Potential bug in __res_msend_rc() wrt to union initialization.

[Rich Felker <dalias@libc.org>]

On Tue, Mar 19, 2024 at 08:04:31AM -0700, Mike Cui wrote:
> On Tue, Mar 19, 2024 at 6:18 AM Rich Felker <dalias@libc.org> wrote:
> 
> > On Mon, Mar 18, 2024 at 05:01:41PM -0700, Mike Cui wrote:
> > > Yeah I also just went over the C99 spec as well, section 6.7.8, and I
> > have
> > > to agree with clang developer's interpretation, that "{ 0 }"
> > > only initializes the first member of the union.
> >
> > There is no such thing as "only initializes [part]" in the C language.
> > The { 0 } *only provides a value for* the first member. The question
> > is about what happens to parts of the object for which the initializer
> > did not "provide a value". However, the C99 standard does not clearly
> > describe how the bits of a union that are not part of the member for
> > which a value is provided (usually the first, unless a designated
> > initializer is used) are filled on initialization.
> >
> > You are referring to this paragraph?
> 
> 6.7.9 ¶21
> If there are fewer initializers in a brace-enclosed list than there are
> elements or members of an aggregate, or fewer characters in a string
> literal used to initialize an array of known size than there are elements
> in the array, the remainder of the aggregate shall be initialized
> implicitly the same as objects that have static storage duration.
> 
> Folks on the LLVM discourse pointed out this paragraph does not apply to
> unions, since unions are not "aggegates" according to the definition in
> 6.2.5p21:
> 21. Arithmetic types and pointer types are collectively called scalar
> types. Array and structure types are collectively called *aggregate* types.

No, the part below that you didn't reply to covers unions:

> > C11 adds (in 6.7.9 ¶10):
> >
> >     "if it is a union, the first named member is initialized
> >     (recursively) according to these rules, and any padding is
> >     initialized to zero bits;"
> >
> > where C99 just had (6.7.8):
> >
> >     "if it is a union, the first named member is initialized
> >     (recursively) according to these rules."
> >
> > So I think C11 and later actually require the full zero
> > initialization of all bits, and clang is just wrong.
> >
> > > "{ }" apparently is added in C23 as the "universal zero initializer". So
> > > changing the order moving sin6 up is the only way to be C99 conformant.
> >
> > Indeed since at the source level we just depend on C99 not C11, this
> > should be changed. But clang needs to be fixed too.
> >
> > Rich
> >