Re: Question about netinet6/in6.h

On Fri, Apr 26, 2024, 9:33 PM Bakul Shah <bakul@iitbombay.org> wrote:

>
>
> > On Apr 26, 2024, at 5:02 PM, Mike Karels <mike@karels.net> wrote:
> >
> > On 26 Apr 2024, at 18:06, Warner Losh wrote:
> >
> >> On Fri, Apr 26, 2024 at 4:21 PM Mike Karels <mike@karels.net> wrote:
> >>
> >>> On 26 Apr 2024, at 15:49, Mike Karels wrote:
> >>>
> >>>> On 26 Apr 2024, at 15:01, Warner Losh wrote:
> >>>>
> >>>>> This has to be a FAQ
> >>>>>
> >>>>> I'm porting a program from Linux, I often see an error like:
> >>>>> ./test/mock-ifaddrs.c:95:19: error: no member named 's6_addr32' in
> >>> 'struct
> >>>>> in6_addr'
> >>>>>   95 |                 ipv6->sin6_addr.s6_addr32[3] = 0;
> >>>>>      |                 ~~~~~~~~~~~~~~~ ^
> >>>>> but yet, we kinda define them, but only for the kernel and boot
> loader:
> >>>>> /*
> >>>>> * IPv6 address
> >>>>> */
> >>>>> struct in6_addr {
> >>>>>        union {
> >>>>>                uint8_t         __u6_addr8[16];
> >>>>>                uint16_t        __u6_addr16[8];
> >>>>>                uint32_t        __u6_addr32[4];
> >>>>>        } __u6_addr;                    /* 128-bit IP6 address */
> >>>>> };
> >>>>>
> >>>>> #define s6_addr   __u6_addr.__u6_addr8
> >>>>> #if defined(_KERNEL) || defined(_STANDALONE) /* XXX nonstandard */
> >>>>> #define s6_addr8  __u6_addr.__u6_addr8
> >>>>> #define s6_addr16 __u6_addr.__u6_addr16
> >>>>> #define s6_addr32 __u6_addr.__u6_addr32
> >>>>> #endif
> >>>>>
> >>>>> I'm wondering if anybody why it's like that? git blame suggests we
> >>> imported
> >>>>> that from kame, with
> >>>>> only tweaks by people that are now deceased*.*
> >>>>>
> >>>>> Why not just expose them?
> >>>>
> >>>> Looks like only s6_addr is specified in the RFCs (2553 and 3493).
> Oddly,
> >>>> though, the RFCs give an example implementation using that union with
> >>>> different element names (like _S6_u8), and show the one #define.
> >>>> Similarly, POSIX specifies only s6_addr, but it allows other members
> >>>> of the structure, so I don't see a problem with exposing them all even
> >>>> in a POSIX environment.
> >>>>
> >>>> I would have no objection to exposing all four definitions, especially
> >>>> if Linux apps use them.
> >>>
> >>> I put the change, along with an explanatory comment, in
> >>> https://reviews.freebsd.org/D44979.  Comments welcome.
> >>>
> >>
> >> Thanks! I was testing a similar change, but I like yours better...
> though
> >> maybe
> >> we should just make it visible when __BSD_VISIBLE is true.... I'll have
> to
> >> look
> >> closely at what Linux does here... I think they have it always visible,
> or
> >> at least
> >> musl does that (glibc is harder to track down due to the many layers of
> >> indirection).
> >
> > I thought briefly about __BSD_VISIBLE, but wasn't sure it was necessary.
> > Let me know what you find out.  I think it should work either way; in.h
> > includes cdefs.h, so it's guaranteed to have been included.
>
> If the -ms-extensions option is used with gcc or clang, this ugliness can
> go away as you can have nested anonymous unions or -structs and their
> fields
> can be referenced as if they're directly in the parent struct/union.
>
> [IIRC this was present in Plan9 C from very early on. Also in C11 or later]


True. In fact c11 and newer doesn't need anything on the command line here.
If it were only in the kernel then I'd chamge it like thay while I was
here... but lots of code in ports will specify c99 + POSIX 2001 and to
compile there your only hope is this construct....

Warner