This function dynamically allocates memory for an array of
    if_nameindex structures, one structure for each interface. A
    structure with an if_index value of 0 and a NULL if_name value
    indicates the end of the array.
    The following if_nameindex structure must also be defined
    by including the IF.H header file prior to the call to if_
    nameindex():
    struct if_nameindex {
       unsigned int   if_index;
       char           *if_name;
    };
    To free the memory allocated by this function, use the if_
    freenameindex() function. If an error occurs, the function
    returns a NULL pointer and sets errno to an appropriate value.