Returns the length of an IPv6 extension header with no options
    and initializes the header.
    Format
      #include  <in6.h>
      int inet6_opt_init  ( void *extbuf, size_t extlen );

1  –  Arguments

 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 and any number that is a multiple of 8.

2  –  Description

    This function, when called with extbuf as a NULL pointer and
    extlen as 0, returns the number of bytes in an extension header
    that has no options.
    If you specify extbuf as a valid pointer and extlen as a
    number that is a multiple of 8, the function returns the same
    information as in the previous case, initializes the extension
    header, and sets the length field.

3  –  Return Values

    x                  Upon successful completion, the inet6_opt_
                       init() function returns the number of bytes in
                       an extension header with no options.
    -1                 Failure