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.\"     From: @(#)rcmd.3	8.1 (Berkeley) 6/4/93
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.Dd $Mdocdate: May 31 2007 $
.Dt IF_NAMETOINDEX 3
.Os
.Sh NAME
.Nm if_nametoindex ,
.Nm if_indextoname ,
.Nm if_nameindex ,
.Nm if_freenameindex
.Nd convert interface index to name, and vice versa
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Fd #include <net/if.h>
.Ft "unsigned int"
.Fn if_nametoindex "const char *ifname"
.Ft "char *"
.Fn if_indextoname "unsigned int ifindex" "char *ifname"
.Ft "struct if_nameindex *"
.Fn if_nameindex "void"
.Ft "void"
.Fn if_freenameindex "struct if_nameindex *ptr"
.Sh DESCRIPTION
These functions map interface indexes to interface names (such as
.Dq lo0 ) ,
and vice versa.
.Pp
The
.Fn if_nametoindex
function converts an interface name specified by the
.Fa ifname
argument to an interface index (positive integer value).
If the specified interface does not exist, 0 will be returned.
.Pp
.Fn if_indextoname
converts an interface index specified by the
.Fa ifindex
argument to an interface name.
The
.Fa ifname
argument must point to a buffer of at least
.Dv IF_NAMESIZE
bytes into which the interface name corresponding to the specified index is
returned.
.Pf ( Dv IF_NAMESIZE
is also defined in
.Aq Pa net/if.h
and its value includes a terminating NUL byte at the end of the
interface name.)
This pointer is also the return value of the function.
If there is no interface corresponding to the specified index,
.Dv NULL
is returned.
.Pp
.Fn if_nameindex
returns an array of
.Vt if_nameindex
structures.
.Vt if_nameindex
is also defined in
.Aq Pa net/if.h ,
and is as follows:
.Bd -literal -offset indent
struct if_nameindex {
	unsigned int	  if_index;	/* 1, 2, ... */
	char		 *if_name;	/* NUL-terminated name */
};
.Ed
.Pp
The end of the array of structures is indicated by a structure with
an
.Fa if_index
of 0 and an
.Fa if_name
of
.Dv NULL .
The function returns a null pointer on error.
The memory used for this array of structures along with the interface
names pointed to by the
.Fa if_name
members is obtained dynamically.
This memory is freed by the
.Fn if_freenameindex
function.
.Pp
.Fn if_freenameindex
takes a pointer that was returned by
.Fn if_nameindex
as argument
.Pq Fa ptr ,
and it reclaims the region allocated.
.Sh DIAGNOSTICS
.Fn if_nametoindex
returns 0 on error, positive integer on success.
.Fn if_indextoname
and
.Fn if_nameindex
return
.Dv NULL
on errors.
.Sh SEE ALSO
.Xr getifaddrs 3 ,
.Xr networking 4
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A W. Stevens
.%T Basic Socket Interface Extensions for IPv6
.%R RFC 2553
.%D March 1999
.Re
.Sh STANDARDS
These functions are defined in ``Basic Socket Interface Extensions for IPv6''
.Pq RFC 2533 .