C - Casting “h_addr_list” array to type “struct in_addr **”

Question

I'm reading this tutorial: here But I can't understand how this part works. Casting from char** to a struct in_addr ** that contains an unsigned long seems weird.

// Cast the h_addr_list to in_addr 
// since h_addr_list also has the ip address in long format only
addr_list = (struct in_addr **) he->h_addr_list;

I wish someone could explain what's happening and how the code works.

EDIT: To be more specific: I want to know how can the compiler understand what we are trying to do here ? why should this code be correct ?

Answer 1

When you call gethostbyname , it returns a pointer to a struct hostent . One of the fields in this struct, h_addr_list , is an array of pointers to network addresses.

Since a network address can be either a struct in_addr for an IPv4 address or a struct in6_addr for an IPv6 address, h_addr_list is defined as a char ** as a generic type that can point to either one. The h_addrtype field tells you which one of these is valid.

Once you know that, you cast h_addr_list to either struct in_addr ** or struct in6_addr ** as appropriate, then access each element in the array.

EDIT:

Pointers between either void or char types and any other non-function type can freely be casted to or from each other.

In this particular case, it's likely that gethostbyname has a static char buffer that it casts to a struct in_addr ** or a struct in6_addr ** as appropriate in order to create the array. Then the calling function performs the same cast in order to get the values out.