### 16.4 Interface Naming

Each network interface has a name. This usually consists of a few letters that relate to the type of interface, which may be followed by a number if there is more than one interface of that type. Examples might be `lo` (the loopback interface) and `eth0` (the first Ethernet interface).

Although such names are convenient for humans, it would be clumsy to have to use them whenever a program needs to refer to an interface. In such situations an interface is referred to by its index, which is an arbitrarily-assigned small positive integer.

The following functions, constants and data types are declared in the header file `net/if.h`.

— Constant: size_t IFNAMSIZ

> This constant defines the maximum buffer size needed to hold an interface name, including its terminating zero byte.

— Function: unsigned int if_nametoindex (const char *ifname)

> Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd | See POSIX Safety Concepts.

> This function yields the interface index corresponding to a particular name. If no interface exists with the name given, it returns 0.

— Function: char * if_indextoname (unsigned int ifindex, char *ifname)

> Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd | See POSIX Safety Concepts.

> This function maps an interface index to its corresponding name. The returned name is placed in the buffer pointed to by `ifname`, which must be at least `IFNAMSIZ` bytes in length. If the index was invalid, the function's return value is a null pointer, otherwise it is `ifname`.

— Data Type: struct if_nameindex

> This data type is used to hold the information about a single interface. It has the following members:

> `unsigned int if_index;`
>   This is the interface index.
> `char *if_name`
>   This is the null-terminated index name.

— Function: struct if_nameindex * if_nameindex (void)

> Preliminary: | MT-Safe | AS-Unsafe heap lock/hurd | AC-Unsafe lock/hurd fd mem | See POSIX Safety Concepts.

> This function returns an array of `if_nameindex` structures, one for every interface that is present. The end of the list is indicated by a structure with an interface of 0 and a null name pointer. If an error occurs, this function returns a null pointer.

> The returned structure must be freed with `if_freenameindex` after use.

— Function: void if_freenameindex (struct if_nameindex *ptr)

> Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> This function frees the structure returned by an earlier call to `if_nameindex`.