Members of `struct sockaddr_un` are requesting the definitions of `sun_family` and `sun_path`.
https://pubs.opengroup.org/onlinepubs/009696699/basedefs/sys/un.h.html

But the other members are optional and environment dependent.

In fact, other members are defined in the BSD series.
from NetBSD-9.1 <https://github.com/NetBSD/src/blob/da504f75982b244b2288bc9970bbc203bd77a9c1/sys/sys/un.h#L49-L53>

```c
struct  sockaddr_un {
        unsigned char   sun_len;        /* sockaddr len excluding NUL */
        sa_family_t     sun_family;     /* AF_UNIX */
        char    sun_path[104];          /* path name (gag) */
};
```