Re: [PATCH v2 1/2][next] UAPI: ethtool: Use __struct_group() in struct ethtool_link_settings

[Gustavo A. R. Silva]

On 09/11/24 12:02, Jakub Kicinski wrote:
> On Tue, 29 Oct 2024 15:55:35 -0600 Gustavo A. R. Silva wrote:
> > Use the `__struct_group()` helper to create a new tagged
> > `struct ethtool_link_settings_hdr`. This structure groups together
> > all the members of the flexible `struct ethtool_link_settings`
> > except the flexible array. As a result, the array is effectively
> > separated from the rest of the members without modifying the memory
> > layout of the flexible structure.
> >
> > This new tagged struct will be used to fix problematic declarations
> > of middle-flex-arrays in composite structs[1].
>
> Possibly a very noob question, but I'm updating a C++ library with
> new headers and I think this makes it no longer compile.
>
> $ cat > /tmp/t.cpp<<EOF
> extern "C" {
> #include "include/uapi/linux/ethtool.h"
> }
> int func() { return 0; }
> EOF
>
> $ g++ /tmp/t.cpp -I../linux -o /dev/null -c -W -Wall -O2
> In file included from /usr/include/linux/posix_types.h:5,
>                   from /usr/include/linux/types.h:9,
>                   from ../linux/include/uapi/linux/ethtool.h:18,
>                   from /tmp/t.cpp:2:
> ../linux/include/uapi/linux/ethtool.h:2515:24: error: ‘struct ethtool_link_settings::<unnamed union>::ethtool_link_settings_hdr’ invalid; an anonymous union may only have public non-static data members [-fpermissive]
>   2515 |         __struct_group(ethtool_link_settings_hdr, hdr, /* no attrs */,
>        |                        ^~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> I don't know much about C++, tho, so quite possibly missing something
> obvious.

We are in the same situation here.

It seems C++ considers it ambiguous to define a struct with a tag such
as `struct TAG { MEMBERS } ATTRS NAME;` within an anonymous union.

Let me look into this further...
--
Gustavo