Am Donnerstag, dem 13.11.2025 um 15:44 +0000 schrieb Qing Zhao:
>
> > On Nov 13, 2025, at 01:32, Martin Uecker <[email protected]> wrote:
> >
> > Am Mittwoch, dem 12.11.2025 um 22:07 +0000 schrieb Joseph Myers:
> > > On Wed, 12 Nov 2025, Qing Zhao wrote:
> > >
> > > > > There are the various extensions (-fms-extensions /
> > > > > -fplan9-extensions) to
> > > > > consider as well, but it might be reasonable to say that only
> > > > > genuinely
> > > > > anonymous struct / union members get the special counted_by handling,
> > > >
> > > > So, in C FE, what’s the best way to determine whether the anonymous
> > > > struct/union is
> > > > genuine or not? (The DECL_CONTEXT of this anonymous structure/union is
> > > > NULL_TREE?)
> > >
> > > You can't tell until some time after the definition is complete, because
> > > you need to see whether the type is used as the type of an unnamed field
> > > (possibly with qualifiers).
> > >
> > > Maybe this suggests that, as in other cases, an anonymous structure or
> > > union involving counted_by should actually be required to be valid
> > > without
> > > reference to the containing struct or union after all - that's how things
> > > generally work with anonymous structs and unions. That is, it would be
> > > OK
> > > for an outer struct or union to have counted_by pointing to a count in an
> > > inner anonymous struct or union, but not vice versa.
> >
> > Without -fms-extensions, anonymous structs are supported only
> > if they are defined inline. The following is rejected with
> > --pedantic-errors but even without it an access to .buf would
> > be rejected.
> >
> > typedef struct { char *buf; } x_t;
> >
> > struct foo {
> > int n;
> > x_t;
> > };
> >
> >
> > But if they are declined inline
> >
> > struct foo {
> > int n;
> > struct { char *buf; };
> > };
> >
>
> You mean the following case:
>
> struct foo {
> Int n;
> struct {
> char *buf __counted_by (n);
> }
> }
Yes, I think this could be supported.
>
>
> > In this, counted_by could be supported and you would need to check
> > for the field name to NULL_TREE, i.e. DECL_NAME (...) == NULL_TREE and
> > also c_type_tag (...) == NULL_TREE.
>
> i.e, both the name of the field type and the name of the field are NULL_TREE?
TYPE_NAME may also hold a typedef name. This may or may not be
what you want. If you want to resolved typedef names and get the tag,
you need to use c_type_tag.
>
> >
> > You would then check correctness of the attribute only when the parent
> > type is complete.
>
> Okay.
> >
> > If we want to support counted_by in even more generic cases,
>
> What kind of more generic cases? Any example?
I was thinking about
struct bar {
char *buf __counted_by(n);
};
struct foo {
int n;
struct bar;
};
which is allowed with -fms-extensions.
I don't think we should support this.
>
> How about the following? Shall we support it?
>
> struct nested_mixed {
> struct {
> union {
> int b;
> float f;
> };
> int n;
> };
> struct {
> int *pointer __counted_by(n);
> char c[] __counted_by(b);
> };
> };
I think this could be allowed because everythig is inline.
Martin
>
>
> Thanks a lot.
>
> Qing
> > I think
> > checking would need to deferred to declaration of variables, but
> > I am not sure this is a good idea.
> >
> > Martin
>
