Question

struct hostent *gethostbyname(const char *name)

Note that hostent.h_addr_list is a field with variant length.

How does the function gethostbyname have the implementation that returns a pointer pointing to a struct but doesn’t require the caller to release the resource?

All examples used in the famous book Unix Network Programming Vol 1 by R. Stevens do not contain code to release those returned pointers and I assume that these are not ignorance. Also one example from MSDN does the same thing example of usage

Answer 1

Supposing that an implementation wants to to handle arbitrarily large lists of addresses, it could do something like this:

struct hostent *gethostbyname(const char *name) {
    static struct hostent *results = 0;
    static size_t resultsize = 0;
    size_t count = get_count_of_addresses(name)
    if (count > resultsize) {
        struct hostent *tmp = realloc(results, N * count + M);
        if (tmp) {
            results = tmp;
            resultsize = count;
        } else {
            // handle error, I can't remember what the docs say
        }
    }
    fill_in_hostent(results, name);
    return results;
};

Optionally, the sockets library could do something to free results on exit (such as install an atexit handler), to avoid debugging tools reporting a memory leak.

I’ve ignored the possibility that the count of addresses could change in between sizeing the structure and filling it in — in practice you’d get the DNS result back and then do stuff with it, so that would not be possible. I’ve left it as two separate calls to avoid introducing a pseudo-code representation for the DNS result.