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SubjectRe: [PATCH 08/14] sched: add latency tracing for -deadline tasks.
On Wed, 27 Nov 2013 17:13:08 +0100
Ingo Molnar <mingo@kernel.org> wrote:

>
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > On Wed, 27 Nov 2013 16:46:00 +0100
> > Ingo Molnar <mingo@kernel.org> wrote:
> >
> > >
> > > * Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > > On Wed, Nov 27, 2013 at 04:35:19PM +0100, Ingo Molnar wrote:
> > > > > So why does GCC then behave like this:
> > > >
> > > > I think because its a much saner behaviour; also it might still be the
> > > > spec actually says this, its a somewhat opaque text.
> > > >
> > > > Anyway, yes GCC seems to behave as we 'expect' it to; I just can't find
> > > > the language spec actually guaranteeing this.
> > >
> > > So from C99 standard §6.7.8 (Initialization)/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."
> > >
> > > static initialization == zeroing in this case.
> > >
> >
> > The confusion here is that the above looks to be talking about arrays.
> > But it really doesn't specify structures.
>
> It talks about neither 'arrays' nor 'structures', it talks about
> 'aggregates' - which is defined as _both_: 'structures and arrays'.

Yeah, I misread it. I was reading the array section for awhile, and got
confused.


>
> That's what compiler legalese brings you ;-)

Yep.

>
> > But searching the internet, it looks as though most people believe
> > it applies to structures, and any compiler that does otherwise will
> > most likely break applications.
> >
> > That is, this looks to be one of the gray areas that the compiler
> > writers just happen to do what's most sane. And they probably assume
> > it's talking about structures as well, hence the lack of warnings.
>
> I don't think it's grey, I think it's pretty well specified.
>
> > It gets confusing, as the doc also shows:
> >
> > struct { int a[3], b; } w[] = { { 1 }, 2 };
>
> I don't think this is valid syntax, I think this needs one more set of
> braces:
>
> struct { int a[3], b; } w[] = { { { 1 }, 2 } };
>
> > Then points out that w.a[0] is 1 and w.b[0] is 2, and all other
> > elements are zero.
>
> If by 'w.a[0]' you mean 'w[0].a[0]', and if by 'w.b[0]' you mean
> 'w[0].b' then yes, this comes from the definition and it's what I'd
> call 'obvious' initialization behavior.
>
> What makes it confusing to you?

Well, because it's mixing arrays and structures, and I was on the
misconception that the paragraph was talking about just arrays.

Can I just have my turkey now?

-- Steve
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