Getting IPV4 address from a sockaddr structure

inet_ntoa() works for IPv4; inet_ntop() works for both IPv4 and IPv6.

Given an input struct sockaddr *res, here are two snippets of code (tested on macOS):

Using inet_ntoa()

#include <arpa/inet.h>

struct sockaddr_in *addr_in = (struct sockaddr_in *)res;
char *s = inet_ntoa(addr_in->sin_addr);
printf("IP address: %s\n", s);

Using inet_ntop()

#include <arpa/inet.h>
#include <stdlib.h>

// obviously INET6_ADDRSTRLEN is expected to be larger
// than INET_ADDRSTRLEN, but this may be required in case
// if for some unexpected reason IPv6 is not supported, and
// INET6_ADDRSTRLEN is defined as 0
// but this is not very likely and I am aware of no cases of
// this in practice (editor)
char s[INET6_ADDRSTRLEN > INET_ADDRSTRLEN ? INET6_ADDRSTRLEN : INET_ADDRSTRLEN]
    = '\0';

switch(res->sa_family) {
    case AF_INET: {
        struct sockaddr_in *addr_in = (struct sockaddr_in *)res;

        ////char s[INET_ADDRSTRLEN] = '\0';
            // this is large enough to include terminating null

        inet_ntop(AF_INET, &(addr_in->sin_addr), s, INET_ADDRSTRLEN);
        break;
    }
    case AF_INET6: {
        struct sockaddr_in6 *addr_in6 = (struct sockaddr_in6 *)res;
        
        ////char s[INET6_ADDRSTRLEN] = '\0';
            // not sure if large enough to include terminating null?

        inet_ntop(AF_INET6, &(addr_in6->sin6_addr), s, INET6_ADDRSTRLEN);
        break;
    }
    default:
        break;
}
printf("IP address: %s\n", s);

Just cast the entire sockaddr structure to a sockaddr_in. Then you can use:

char *ip = inet_ntoa(their_addr.sin_addr)

To retrieve the standard ip representation.

Emil's answer is correct, but it's my understanding that inet_ntoa is deprecated and that instead you should use inet_ntop. If you are using IPv4, cast your struct sockaddr to sockaddr_in. Your code will look something like this:

struct addrinfo *res;   // populated elsewhere in your code
struct sockaddr_in *ipv4 = (struct sockaddr_in *)res->ai_addr;
char ipAddress[INET_ADDRSTRLEN];
inet_ntop(AF_INET, &(ipv4->sin_addr), ipAddress, INET_ADDRSTRLEN);

printf("The IP address is: %s\n", ipAddress);

Take a look at this great resource for more explanation, including how to do this for IPv6 addresses.

Once sockaddr cast to sockaddr_in, it becomes this:

struct sockaddr_in {
    u_short     sin_family;
    u_short     sin_port;
    struct      in_addr sin_addr;
    char        sin_zero[8];
};