Q:

Getting IPV4 address from a sockaddr structure

How can I extract an IP address into a string? I can't find a reference that tells me how char sa_data[14] is encoded.

A:

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.

A:

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>

char *s = NULL;
switch(res->sa_family) {
    case AF_INET: {
        struct sockaddr_in *addr_in = (struct sockaddr_in *)res;
        s = malloc(INET_ADDRSTRLEN);
        inet_ntop(AF_INET, &(addr_in->sin_addr), s, INET_ADDRSTRLEN);
        break;
    }
    case AF_INET6: {
        struct sockaddr_in6 *addr_in6 = (struct sockaddr_in6 *)res;
        s = malloc(INET6_ADDRSTRLEN);
        inet_ntop(AF_INET6, &(addr_in6->sin6_addr), s, INET6_ADDRSTRLEN);
        break;
    }
    default:
        break;
}
printf("IP address: %s\n", s);
free(s);

A:

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.

