IPv6 address get function?

Question

unsigned char networkMask [sizeof (struct in6_addr)] = { [0 ... (sizeof (struct in6_addr) - 1)] = 0xff };

what does (0...(sizeof)) is representing here. How this array is allocated.

Answer 1

This particular syntax is a GCC extension of the designated initializer . With it, you can initialize an array like this:

unsigned char foo[<n>] = { [0 ... <n> - 1] = <k> };

Whereby <n> is the number of members and <k> is any given member value.

In the code you have shown, it initializes the networkMask array with 0xff for elements from index 0 through sizeof(struct in6_addr) - 1 . In other words, it initializes an array with the size of struct in6_addr and sets all bits to 1 .

It would be equivalent to this, given that an IPv6 address occupies 16 bytes:

unsigned char networkMask[sizeof(struct in6_addr)] = { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255 };