Why is h_addr_list in struct hostent a char ** instead of struct in_addr **?

Question

I am new to network programming. The following structure definitions are quite confusing to me. Here h_addr_list is a defined as string array, but it is used to store array of in_addr structures. Why didn't it define as struct in_addr **h_addr_list rather than char **h_addr_list?

struct hostent 
{
    char *h_name;       /* Official domain name of host */
    char **h_aliases;   /* Null-terminated array of domain names */
    int h_addrtype;     /* Host address type (AF_INET) */
    int h_length;       /* Length of an address, in bytes */
    char **h_addr_list;     /* Null-terminated array of in_addr structs */
};


struct in_addr 
{
    unsigned int s_addr; /* Network byte order (big-endian) */
};

Answer 1

The structure definition dates back to the era before C supported void * (or void at all, or prototypes). In those days, char * was the 'universal pointer'. This accounts for some of the weirdnesses of the networking function interfaces.

It also dates back to the era when there were many different networking systems (IPX/SPX, SNA, TCP/IP, …). These days, TCP/IP is dominant, but even now, you could have an array of IPv4 or an array of IPv6 addresses being returned, so specifying either struct in_addr or struct in6_addr would cause problems.

The intention was that you'd have an array of pointers to appropriate structure types. Nowadays, it would be written void **h_addr_list — an array of void *. But this option was not available when the structures were first defined, and the rest is history (you don't change an interface after it is standardized if you can avoid it).

Answer 2

When the struct was created, the creator wasn't sure whether AF_INET was going to be the winning address type.

What if h_addrtype is something other than AF_INET? Then h_addr_list will contain addresses that are not struct in_addr.

Now, a couple of decades later, we find that IPV4 addresses are running out. Soon, struct inaddr will be replaced more and more by IPV6 addresses.