Re: Question about netinet6/in6.h

From: Bakul Shah <bakul_at_iitbombay.org>
Date: Sat, 27 Apr 2024 03:33:03 UTC

> 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]