getifaddrs, freeifaddrs - get interface addresses

#include <sys/types.h>
#include <ifaddrs.h>
int getifaddrs(struct ifaddrs **ifap);
void freeifaddrs(struct ifaddrs *ifa);

The getifaddrs() function creates a linked list of structures describing the network interfaces of the local system, and stores the address of the first item of the list in *ifap. The list consists of ifaddrs structures, defined as follows:

struct ifaddrs {
    struct ifaddrs  *ifa_next;    /* Next item in list */
    char            *ifa_name;    /* Name of interface */
    unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
    struct sockaddr *ifa_addr;    /* Address of interface */
    struct sockaddr *ifa_netmask; /* Netmask of interface */
    union {
        struct sockaddr *ifu_broadaddr;
                         /* Broadcast address of interface */
        struct sockaddr *ifu_dstaddr;
                         /* Point-to-point destination address */
    } ifa_ifu;
#define              ifa_broadaddr ifa_ifu.ifu_broadaddr
#define              ifa_dstaddr   ifa_ifu.ifu_dstaddr
    void            *ifa_data;    /* Address-specific data */
};

The ifa_next field contains a pointer to the next structure on the list, or NULL if this is the last item of the list.

The ifa_name points to the null-terminated interface name.

The ifa_flags field contains the interface flags, as returned by the SIOCGIFFLAGS ioctl(2) operation (see netdevice(7) for a list of these flags).

The ifa_addr field points to a structure containing the interface address. (The sa_family subfield should be consulted to determine the format of the address structure.) This field may contain a null pointer.

The ifa_netmask field points to a structure containing the netmask associated with ifa_addr, if applicable for the address family. This field may contain a null pointer.

Depending on whether the bit IFF_BROADCAST or IFF_POINTOPOINT is set in ifa_flags (only one can be set at a time), either ifa_broadaddr will contain the broadcast address associated with ifa_addr (if applicable for the address family) or ifa_dstaddr will contain the destination address of the point-to-point interface.

The ifa_data field points to a buffer containing address-family-specific data; this field may be NULL if there is no such data for this interface.

The data returned by getifaddrs() is dynamically allocated and should be freed using freeifaddrs() when no longer needed.

On success, getifaddrs() returns zero; on error, -1 is returned, and errno is set to indicate the error.

The program below demonstrates the use of getifaddrs(), freeifaddrs(), and getnameinfo(3). Here is what we see when running this program on one system:

$ ./a.out
lo       AF_PACKET (17)
                tx_packets =        524; rx_packets =        524
                tx_bytes   =      38788; rx_bytes   =      38788
wlp3s0   AF_PACKET (17)
                tx_packets =     108391; rx_packets =     130245
                tx_bytes   =   30420659; rx_bytes   =   94230014
em1      AF_PACKET (17)
                tx_packets =          0; rx_packets =          0
                tx_bytes   =          0; rx_bytes   =          0
lo       AF_INET (2)
                address: <127.0.0.1>
wlp3s0   AF_INET (2)
                address: <192.168.235.137>
lo       AF_INET6 (10)
                address: <::1>
wlp3s0   AF_INET6 (10)
                address: <fe80::7ee9:d3ff:fef5:1a91%wlp3s0>

Program source

#define _GNU_SOURCE     /* To get defns of NI_MAXSERV and NI_MAXHOST */
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/if_link.h>
int main(int argc, char *argv[])
{
    struct ifaddrs *ifaddr;
    int family, s;
    char host[NI_MAXHOST];
    if (getifaddrs(&ifaddr) == -1) {
        perror("getifaddrs");
        exit(EXIT_FAILURE);
    }
    /* Walk through linked list, maintaining head pointer so we
       can free list later. */
    for (struct ifaddrs *ifa = ifaddr; ifa != NULL;
             ifa = ifa->ifa_next) {
        if (ifa->ifa_addr == NULL)
            continue;
        family = ifa->ifa_addr->sa_family;
        /* Display interface name and family (including symbolic
           form of the latter for the common families). */
        printf("%-8s %s (%d)\n",
               ifa->ifa_name,
               (family == AF_PACKET) ? "AF_PACKET" :
               (family == AF_INET) ? "AF_INET" :
               (family == AF_INET6) ? "AF_INET6" : "???",
               family);
        /* For an AF_INET* interface address, display the address. */
        if (family == AF_INET || family == AF_INET6) {
            s = getnameinfo(ifa->ifa_addr,
                    (family == AF_INET) ? sizeof(struct sockaddr_in) :
                                          sizeof(struct sockaddr_in6),
                    host, NI_MAXHOST,
                    NULL, 0, NI_NUMERICHOST);
            if (s != 0) {
                printf("getnameinfo() failed: %s\n", gai_strerror(s));
                exit(EXIT_FAILURE);
            }
            printf("\t\taddress: <%s>\n", host);
        } else if (family == AF_PACKET && ifa->ifa_data != NULL) {
            struct rtnl_link_stats *stats = ifa->ifa_data;
            printf("\t\ttx_packets = %10u; rx_packets = %10u\n"
                   "\t\ttx_bytes   = %10u; rx_bytes   = %10u\n",
                   stats->tx_packets, stats->rx_packets,
                   stats->tx_bytes, stats->rx_bytes);
        }
    }
    freeifaddrs(ifaddr);
    exit(EXIT_SUCCESS);
}

bind(2), getsockname(2), socket(2), packet(7), ifconfig(8)