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.TH IF_NAMEINDEX 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
if_nameindex, if_freenameindex \- get network interface names and indexes
.SH SYNOPSIS
.nf
.B #include <net/if.h>
.PP
.BI "struct if_nameindex *if_nameindex(void);
.BI "void if_freenameindex(struct if_nameindex *" "ptr" );
.fi
.SH DESCRIPTION
The
.BR if_nameindex ()
function returns an array of
.I if_nameindex
structures, each containing information
about one of the network interfaces on the local system.
The
.I if_nameindex
structure contains at least the following entries:
.PP
.in +4n
.EX
unsigned int if_index; /* Index of interface (1, 2, ...) */
char        *if_name;  /* Null-terminated name ("eth0", etc.) */
.EE
.in
.PP
The
.I if_index
field contains the interface index.
The
.I if_name
field points to the null-terminated interface name.
The end of the array is indicated by entry with
.I if_index
set to zero and
.I if_name
set to NULL.
.PP
The data structure returned by
.BR if_nameindex ()
is dynamically allocated and should be freed using
.BR if_freenameindex ()
when no longer needed.
.SH RETURN VALUE
On success,
.BR if_nameindex ()
returns pointer to the array;
on error, NULL is returned, and
.I errno
is set appropriately.
.SH ERRORS
.BR if_nameindex ()
may fail and set
.I errno
if:
.TP
.B ENOBUFS
Insufficient resources available.
.PP
.BR if_nameindex ()
may also fail for any of the errors specified for
.BR socket (2),
.BR bind (2),
.BR ioctl (2),
.BR getsockname (2),
.BR recvmsg (2),
.BR sendto (2),
or
.BR malloc (3).
.SH VERSIONS
The
.BR if_nameindex ()
function first appeared in glibc 2.1, but before glibc 2.3.4,
the implementation supported only interfaces with IPv4 addresses.
Support of interfaces that don't have IPv4 addresses 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;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR if_nameindex (),
.br
.BR if_freenameindex ()
T}	Thread safety	MT-Safe
.TE
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, RFC\ 3493.
.PP
This function first appeared in BSDi.
.SH EXAMPLE
The program below demonstrates the use of the functions described
on this page.
An example of the output this program might produce is the following:
.PP
.in +4n
.EX
$ \fB./a.out\fI
1: lo
2: wlan0
3: em1
.EE
.in
.SS Program source
.EX
#include <net/if.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    struct if_nameindex *if_ni, *i;

    if_ni = if_nameindex();
    if (if_ni == NULL) {
        perror("if_nameindex");
        exit(EXIT_FAILURE);
    }

    for (i = if_ni; ! (i\->if_index == 0 && i\->if_name == NULL); i++)
        printf("%u: %s\en", i\->if_index, i\->if_name);

    if_freenameindex(if_ni);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR getsockopt (2),
.BR setsockopt (2),
.BR getifaddrs (3),
.BR if_indextoname (3),
.BR if_nametoindex (3),
.BR ifconfig (8)