freeifaddrs (3) - Linux Manuals

freeifaddrs: get interface addresses

Command to display freeifaddrs manual in Linux: $ man 3 freeifaddrs

NAME

getifaddrs, freeifaddrs - get interface addresses

SYNOPSIS

#include <sys/types.h>
#include <ifaddrs.h>

int getifaddrs(struct ifaddrs **ifap);

void freeifaddrs(struct ifaddrs *ifa);

DESCRIPTION

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.

RETURN VALUE

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

ERRORS

getifaddrs() may fail and set errno for any of the errors specified for socket(2), bind(2), getsockname(2), recvmsg(2), sendto(2), malloc(3), or realloc(3).

VERSIONS

The getifaddrs() function first appeared in glibc 2.3, but before glibc 2.3.3, the implementation supported only IPv4 addresses; IPv6 support was added in glibc 2.3.3. Support of address families other than IPv4 is available only on kernels that support netlink.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
getifaddrs(), freeifaddrs()	Thread safety	MT-Safe

CONFORMING TO

Not in POSIX.1. This function first appeared in BSDi and is present on the BSD systems, but with slightly different semantics documented---returning one entry per interface, not per address. This means ifa_addr and other fields can actually be NULL if the interface has no address, and no link-level address is returned if the interface has an IP address assigned. Also, the way of choosing either ifa_broadaddr or ifa_dstaddr differs on various systems.

NOTES

The addresses returned on Linux will usually be the IPv4 and IPv6 addresses assigned to the interface, but also one AF_PACKET address per interface containing lower-level details about the interface and its physical layer. In this case, the ifa_data field may contain a pointer to a struct rtnl_link_stats, defined in <linux/if_link.h> (in Linux 2.4 and earlier, struct net_device_stats, defined in <linux/netdevice.h>), which contains various interface attributes and statistics.

EXAMPLES

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); }

COLOPHON

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