getifaddrs, freeifaddrs - get interface addresses
Standard C library (
libc,
-lc)
#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.
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).
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.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
getifaddrs (), freeifaddrs () |
Thread safety |
MT-Safe |
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.
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.
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>
#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)