https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107952
--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to qinzhao from comment #7)
> (In reply to Richard Biener from comment #1)
> > GCC considered this as a flex-array.
>
> do you mean for the following example:
>
> typedef struct {
> char pad;
> char data[];
> } F2;
>
> typedef struct {
> unsigned pad;
> F2 flex;
> } S2;
>
> although C standard disallow the above, GCC extension treats S2.flex.data as
> a flex-array?
>
> How about:
>
> typedef struct {
> char pad;
> char data[];
> } F2;
>
> typedef struct {
> F2 flex;
> unsigned pad;
> } S2;
>
> do we have any documentation on this Gcc extension?
GCC handles for example
struct A { char data[1]; };
struct B { int n; struct A a; };
as if the a.data[] array is a flex-array. It also handles
struct C { int n; struct A a; int x; };
as if a.data[] can be up to 4 elements large (we allow an array to extend
flexibly to padding - but only if it is trailing). I see that's not
consistently handled though.
I think the [] syntax should follow the C standard as what testcases are
accepted/rejected by the frontend and any extensions there should be
documented (those are separate from what the former array_at_struct_end_p
allowed semantically and where GCC is careful with optimization because
of code out in the wild).
