How to initialize struct in6_addr?

Question

I do know one method to do this,

const struct in6_addr naddr6 = { { 
    0x3f, 0xfe, 0x05, 0x01,
    0x00, 0x08, 0x00, 0x00, 
    0x02, 0x60, 0x97, 0xff,
    0xfe, 0x40, 0xef, 0xab
}};

but could not with this,

const struct in6_addr naddr6 = 
                { { { 0x3ffe0501, 0x00080000, 0x026097ff, 0xfe40efab } } };

and it seems that I could either 1/2/3 paris of brackets.Why?

thanks.

Answer 1

Because one need to indicate which form of the address it is addressing (shown using C99):

const struct in6_addr naddr6 = 
    { { .u6_addr32 = { 0x3ffe0501, 0x00080000, 0x026097ff, 0xfe40efab } } };

The first bracket pair is for the in6_addr struct, the second for the union:

struct in6_addr
  {
    union
      {
 uint8_t u6_addr8[16];
 uint16_t u6_addr16[8];
 uint32_t u6_addr32[4];
      } in6_u;
  };

Answer 2

The portable way to do this is like so:

struct in6_addr s6 = { };
if (!IN6_IS_ADDR_UNSPECIFIED(&s6))
  inet_pton(AF_INET6, "2001:db8::1", &s6);