* Steven Rostedt <rost...@goodmis.org> wrote:
> On Wed, 27 Nov 2013 16:46:00 +0100
> Ingo Molnar <mi...@kernel.org> wrote:
>
> >
> > * Peter Zijlstra <pet...@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 ยง6.7.8 (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'.
That's what compiler legalese brings you ;-)
> 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?
Thanks,
Ingo
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