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 defined as a string array, but is used to store an array of in_addr structures. Why was it not defined as struct in_addr **h_addr_list and not 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 definition of structure dates back to the era before C supports void * (or void in general or prototypes). In those days, char * was a "universal pointer". This explains some of the oddities of network function interfaces.

This also refers to the era when there were many different network systems (IPX / SPX, SNA, TCP / IP, ...). TCP / IP is dominant these days, but even now you may have an IPv4 array or an array of returned IPv6 addresses, so specifying a struct in_addr or struct in6_addr can cause problems.

It was assumed that you would have an array of pointers to the corresponding structure types. Currently, void **h_addr_list would be written - an array of void * . But this parameter was not available when the structures were first defined, and the rest is history (you do not change the interface after standardization, if you can avoid it).

Answer 2

When the structure was created, the creator was not sure whether AF_INET would be the type of address to receive.

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

Now, after a couple of decades, we find that IPV4 addresses are ending. Soon, the struct inaddr will increasingly be replaced by IPV6 addresses.