Re: in6_addr struct union with uint64_t

[Florian Weimer <fweimer@redhat.com>]

* James Hanley:

> #if !__USE_KERNEL_IPV6_DEFS
> /* IPv6 address */
> struct in6_addr
>   {
>     union
>       {
>         uint8_t __u6_addr8[16];
>         uint16_t __u6_addr16[8];
>         uint32_t __u6_addr32[4];
>         uint64_t __u6_addr64[2]; // new
>         unsigned __int128 __u6_addr128; // new
>       } __in6_u;
> #define s6_addr                 __in6_u.__u6_addr8
> #ifdef __USE_MISC
> # define s6_addr16              __in6_u.__u6_addr16
> # define s6_addr32              __in6_u.__u6_addr32
> # define s6_addr64              __in6_u.__u6_addr64
> # define s6_addr128             __in6_u.__u6_addr128
> #endif
>   };
> #endif /* !__USE_KERNEL_IPV6_DEFS */ 
>
> First, is this useful, and is there a standard that prohibits defining
> additional union members?

It changes alignment from 4 bytes to 16 bytes on some architectures,
which is an ABI-breaking change.  For example, if a member of type
struct in6_addr is embedded in a struct, it may require additional
padding in front of it as a result of this change.

Compilers should be able to use wider instructions if beneficial.

Thanks,
Florian