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IF_NAMETOINDEX(3) Library Functions Manual IF_NAMETOINDEX(3)


if_nametoindex, if_indextoname, if_nameindex, if_freenameindexconvert interface index to name, and vice versa


#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>
unsigned int
if_nametoindex(const char *ifname);
char *
if_indextoname(unsigned int ifindex, char *ifname);
struct if_nameindex *
if_freenameindex(struct if_nameindex *ptr);


These functions map interface indexes to interface names (such as “lo0”), and vice versa.
The if_nametoindex() function converts an interface name specified by the ifname argument to an interface index (positive integer value). If the specified interface does not exist, 0 will be returned.
if_indextoname() converts an interface index specified by the ifindex argument to an interface name. The ifname argument must point to a buffer of at least IF_NAMESIZE bytes into which the interface name corresponding to the specified index is returned. (IF_NAMESIZE is also defined in <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, NULL is returned.
if_nameindex() returns an array of if_nameindex structures. if_nameindex is also defined in <net/if.h>, and is as follows:
struct if_nameindex { 
	unsigned int	  if_index;	/* 1, 2, ... */ 
	char		 *if_name;	/* NUL-terminated name */ 
The end of the array of structures is indicated by a structure with an if_index of 0 and an if_name of 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 if_name members is obtained dynamically. This memory is freed by the if_freenameindex() function.
if_freenameindex() takes a pointer that was returned by if_nameindex() as argument (ptr), and it reclaims the region allocated.


if_nametoindex() returns 0 on error, positive integer on success. if_indextoname() and if_nameindex() return NULL on errors.


getifaddrs(3), netintro(4)


R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 3493, February 2003.
November 21, 2015 OpenBSD-6.0