With @chrisaycock answer, I got this working in FreeBSD 4.9 and 14.0 with additional headers.

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <ifaddrs.h>
#include <stdio.h>

int main()
{
    struct ifaddrs *ifap, *ifa;
    struct sockaddr_in *sa;
    char *addr;

    getifaddrs(&ifap);
    for (ifa = ifap; ifa; ifa = ifa->ifa_next) {
        if (ifa->ifa_addr && ifa->ifa_addr->sa_family == AF_INET) {
            sa = (struct sockaddr_in *) ifa->ifa_addr;
            addr = inet_ntoa(sa->sin_addr);
            printf("Interface: %s\tAddress: %s\n", ifa->ifa_name, addr);
        }
    }

    freeifaddrs(ifap);
    return 0;
}