### 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 specified with ifname.

  The return value is the interface index on success. On failure, the function’s return value is zero and `errno` is set accordingly. The following `errno` values are specific to this function:

  `ENODEV`

    There is no interface by the name requested.

  Additionally, since `if_nametoindex` invokes `socket` internally, `errno` may also be set to a value listed for the `socket` function (see Creating a Socket).

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 ifindex 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.

  The return value is ifname on success. On failure, the function’s return value is a null pointer and `errno` is set accordingly. The following `errno` values are specific to this function:

  `ENXIO`

    There is no interface at the index requested.

  Additionally, since `if_indextoname` invokes `socket` internally, `errno` may also be set to a value listed for the `socket` function (see Creating a Socket).

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`.