GETIFADDRS(3) Linux Programmer's Manual GETIFADDRS(3)
getifaddrs, freeifaddrs - get interface addresses
SYNOPSIS top
#include <sys/types.h>
#include <ifaddrs.h>
int getifaddrs(struct ifaddrs **ifap);
void freeifaddrs(struct ifaddrs *ifa);
DESCRIPTION top
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.)
The ifa_netmask field points to a structure containing the netmask associated
with ifa_addr, if applicable for the address family.
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 VALUES top
On success, getifaddrs() returns zero; on error, -1 is returned, and errno is
set appropriately.
ERRORS top
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 top
The getifaddrs() function first appeared in glibc 2.3, but before glibc 2.3.3,
the implementation only supported IPv4 addresses; IPv6 support was added in
glibc 2.3.3. Support of address families other than IPv4 is only available on
kernels that support netlink.
CONFORMING TO top
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.
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
net_device_stats, defined in <linux/netdevice.h>, which contains various
interface attributes and statistics.
EXAMPLE top
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 top
bind(2), getsockname(2), socket(2), packet(7), ifconfig(8)
COLOPHON top
This page is part of release 3.32 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/.
GNU 2010-10-06 GETIFADDRS(3)
