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GETIFADDRS

Section: Linux Programmer's Manual (3)
Updated: 2012-11-11
Index Return to Main Contents
 

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.  

CONFORMING TO

Not in POSIX.1-2001. 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.  

EXAMPLE

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      address family: 17 (AF_PACKET)
eth0    address family: 17 (AF_PACKET)
lo      address family: 2 (AF_INET)
        address: <127.0.0.1>
eth0    address family: 2 (AF_INET)
        address: <10.1.1.4>
lo      address family: 10 (AF_INET6)
        address: <::1>
eth0    address family: 10 (AF_INET6)
        address: <fe80::2d0:59ff:feda:eb51%eth0>
 

Program source

#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    struct ifaddrs *ifaddr, *ifa;
    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 (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("%s	  address family: %d%s\n",
                ifa->ifa_name, family,
                (family == AF_PACKET) ? " (AF_PACKET)" :
                (family == AF_INET) ?   " (AF_INET)" :
                (family == AF_INET6) ?  " (AF_INET6)" : "");

        /* 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("\taddress: <%s>\n", host);
        }
    }

    freeifaddrs(ifaddr);
    exit(EXIT_SUCCESS);
}
 

SEE ALSO

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

COLOPHON

This page is part of release 3.51 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
EXAMPLE
Program source
SEE ALSO
COLOPHON


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