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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
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