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(libc.info)Interface Naming

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Interface Naming
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   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)
     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)
     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)
     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)
     This function frees the structure returned by an earlier call to
     `if_nameindex'.