get broadcast address for an interface

getbroadaddr.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <net/if.h>
#include <netdb.h>
 
static void show_interface_broadaddr(int fd, const char *name) {
    int family;
    struct ifreq ifreq;
    char host[128];
    memset(&ifreq, 0, sizeof ifreq);
    strncpy(ifreq.ifr_name, name, IFNAMSIZ);

    if(ioctl(fd, SIOCGIFBRDADDR, &ifreq) != 0)
    {
        fprintf(stderr, "Could not find interface named %s", name);
        return; /* ignore */
    }

    getnameinfo(&ifreq.ifr_broadaddr, sizeof(ifreq.ifr_broadaddr), host, sizeof(host), 0, 0, NI_NUMERICHOST);
    printf("%-24s%s\n", name, host);
}
 
static void list_interfaces(int fd) {

    struct ifreq *ifreq;
    struct ifconf ifconf;
    char buf[16384];
    unsigned i;
    size_t len;
 
    ifconf.ifc_len = sizeof(buf);
    ifconf.ifc_buf = buf;

    if (ioctl(fd, SIOCGIFCONF, &ifconf) != 0)
    {
        perror("ioctl(SIOCGIFCONF)");
        exit(EXIT_FAILURE);
    }
 
    printf("Listing all interfaces:\n");
    ifreq = ifconf.ifc_req;
    i = 0;
    while (i < ifconf.ifc_len)
    {
#ifndef linux
        len = IFNAMSIZ + ifreq->ifr_addr.sa_len;
#else
        len = sizeof(struct ifreq);
#endif

        printf("%s\n", ifreq->ifr_name);

        ifreq = (struct ifreq*)((char*)ifreq + len);

        i += len;
    }
}
 
int main(int argc, char** argv) {

    int sock;
    struct ifreq ifreq;
    
    sock = socket(AF_INET, SOCK_DGRAM, 0);

    if (sock < 0)
    {
        perror("socket()");
        exit(EXIT_FAILURE);
    }

    if (argc == 1)
    {
        list_interfaces(sock);
    }
    else if (argc == 2)
    {
        show_interface_broadaddr(sock, argv[1]);
    }

    close(sock);

    return EXIT_SUCCESS;
}