man3/getifaddrs.3

.\" Copyright (c) 2008 Petr Baudis <pasky@suse.cz>
.\" and copyright (c) 2009, Linux Foundation, written by Michael Kerrisk
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.\" 2008-12-08 Petr Baudis <pasky@suse.cz>
.\"    Rewrite the BSD manpage in the Linux man pages style and account
.\"    for glibc specificities, provide an example.
.\" 2009-01-14 mtk, many edits and changes, rewrote example program.
.\"
.TH GETIFADDRS 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
getifaddrs, freeifaddrs \- get interface addresses
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <ifaddrs.h>
.PP
.BI "int getifaddrs(struct ifaddrs **" "ifap" );
.PP
.BI "void freeifaddrs(struct ifaddrs *" "ifa" );
.fi
.SH DESCRIPTION
The
.BR 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
.IR *ifap .
The list consists of
.I ifaddrs
structures, defined as follows:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
The
.I ifa_next
field contains a pointer to the next structure on the list,
or NULL if this is the last item of the list.
.PP
The
.I ifa_name
points to the null-terminated interface name.
.\" The constant
.\" .B IF NAMESIZE
.\" indicates the maximum length of this field.
.PP
The
.I ifa_flags
field contains the interface flags, as returned by the
.B SIOCGIFFLAGS
.BR ioctl (2)
operation (see
.BR netdevice (7)
for a list of these flags).
.PP
The
.I ifa_addr
field points to a structure containing the interface address.
(The
.I sa_family
subfield should be consulted to determine the format of the
address structure.)
This field may contain a null pointer.
.PP
The
.I ifa_netmask
field points to a structure containing the netmask associated with
.IR ifa_addr ,
if applicable for the address family.
This field may contain a null pointer.
.PP
Depending on whether the bit
.B IFF_BROADCAST
or
.B IFF_POINTOPOINT
is set in
.I ifa_flags
(only one can be set at a time),
either
.I ifa_broadaddr
will contain the broadcast address associated with
.I ifa_addr
(if applicable for the address family) or
.I ifa_dstaddr
will contain the destination address of the point-to-point interface.
.PP
The
.I 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.
.PP
The data returned by
.BR getifaddrs ()
is dynamically allocated and should be freed using
.BR freeifaddrs ()
when no longer needed.
.SH RETURN VALUE
On success,
.BR getifaddrs ()
returns zero;
on error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.BR getifaddrs ()
may fail and set
.I errno
for any of the errors specified for
.BR socket (2),
.BR bind (2),
.BR getsockname (2),
.BR recvmsg (2),
.BR sendto (2),
.BR malloc (3),
or
.BR realloc (3).
.SH VERSIONS
The
.BR 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.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw27 lb lb
l l l.
Interface	Attribute	Value
T{
.BR getifaddrs (),
.BR freeifaddrs ()
T}	Thread safety	MT-Safe
.TE
.sp 1
.SH 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\(emreturning one entry per interface,
not per address.
This means
.I 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
.I ifa_broadaddr
or
.I ifa_dstaddr
differs on various systems.
.\" , but the BSD-derived documentation generally
.\" appears to be confused and obsolete on this point.
.\" i.e., commonly it still says one of them will be NULL, even if
.\" the ifa_ifu union is already present
.SH NOTES
The addresses returned on Linux will usually be the IPv4 and IPv6 addresses
assigned to the interface, but also one
.B AF_PACKET
address per interface containing lower-level details about the interface
and its physical layer.
In this case, the
.I ifa_data
field may contain a pointer to a
.IR "struct rtnl_link_stats" ,
defined in
.IR <linux/if_link.h>
(in Linux 2.4 and earlier,
.IR "struct net_device_stats" ,
defined in
.IR <linux/netdevice.h> ),
which contains various interface attributes and statistics.
.SH EXAMPLE
The program below demonstrates the use of
.BR getifaddrs (),
.BR freeifaddrs (),
and
.BR getnameinfo (3).
Here is what we see when running this program on one system:
.PP
.in +4n
.EX
$ \fB./a.out\fP
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>
.EE
.in
.SS Program source
\&
.EX
#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, *ifa;
    int family, s, n;
    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, n = 0; ifa != NULL; ifa = ifa\->ifa_next, n++) {
        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)\en",
               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\en", gai_strerror(s));
                exit(EXIT_FAILURE);
            }

            printf("\et\etaddress: <%s>\en", host);

        } else if (family == AF_PACKET && ifa\->ifa_data != NULL) {
            struct rtnl_link_stats *stats = ifa\->ifa_data;

            printf("\et\ettx_packets = %10u; rx_packets = %10u\en"
                   "\et\ettx_bytes   = %10u; rx_bytes   = %10u\en",
                   stats\->tx_packets, stats\->rx_packets,
                   stats\->tx_bytes, stats\->rx_bytes);
        }
    }

    freeifaddrs(ifaddr);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR bind (2),
.BR getsockname (2),
.BR socket (2),
.BR packet (7),
.BR ifconfig (8)