Question

I developing a network application and using socket APIs.

I want to set sin6_addr byte order of sockaddr_in6 structure.

For 16 bits or 32 bits variables, it is simple: Using htons or htonl:

// IPv4
sockaddr_in addr;
addr.sin_port = htons(123);
addr.sin_addr.s_addr = htonl(123456);

But for 128 bits variables, I dont know how to set byte order to network byte order:

// IPv6
sockaddr_in6 addr;
addr.sin6_port = htons(123);
addr.sin6_addr.s6_addr = ??? // 16 bytes with network byte order but how to set?

Some answers may be using htons for 8 times (2 * 8 = 16 bytes), or using htonl for 4 times (4 * 4 = 16 bytes), but I don’t know which way is correct.

Thanks.

Answer 1

The s6_addr member of struct in6_addr is defined as:

uint8_t s6_addr[16];

Since it is an array of uint8_t, rather than being a single 128-bit integer type, the issue of endianness does not arise: you simply copy from your source uint8_t [16] array to the destination. For example, to copy in the address 2001:888:0:2:0:0:0:2 you would use:

static const uint8_t myaddr[16] = { 0x20, 0x01, 0x08, 0x88, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02 };

memcpy(addr.sin6_addr.s6_addr, myaddr, sizeof myaddr);