extbuf
    Points to a buffer that contains an extension header. This is
    either a valid pointer or a NULL pointer.
 extlen
    Specifies the length of the extension header to initialize. Valid
    values are 0 if extbuf equals 0, a value returned by inet6_opt_
    finish(), or any number that is a multiple of 8.
 offset
    Specifies the length of the existing extension header. Obtain
    this value from a prior call to inet6_opt_init() or inet6_opt_
    append().
 type
    Specifies the type of option. Specify a value from 2 to 255,
    inclusive, excluding 194.
 len
    Specifies the length of the option data, excluding the option
    type and option length fields. Specify a value from 0 to 255,
    inclusive.
 align
    Specifies the alignment of the option. Specify one of the
    following values: 1, 2, 4, or 8.
 databufp
    Points to a buffer that contains the option data.