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Detail
    In net/if.h struct if_data was defined within struct ifnet, but also
    referred to later outside of struct ifnet.  This is fine in C since
    the scope of strutures persists to the end of the translation unit, but
    in C++ it persists until the end of the block.

    This problem is solved by moving the definition of struct if_data out
    of struct ifnet and into the outermost level, as it is in the current
    FreeBSD sources.

    In sys/mbuf.h there were two defintions of struct mbuf; one was for ANSI C
    and one was for K&R C, and the appropriate one was chosen by testing whether
    __STDC__ was defined or not.  The ANSI C version of the structure has
    const members in it, which C++ does not allow unless the structure/class
    has a constructor (since const members can only be initialised in a
    constructor's initialiser-list).  The C++ standard says that use of
    __STDC__ is implementation defined, which means we can't rely on picking up
    the non-ANSI C version of the structure in C++ code.

    A third definition of the structure has been added, which is used if
    __cplusplus is defined.  It retains the const members, but declares a
    private constructor, which keeps the compiler happy with respect to the
    const members.  Under the terms of the C++ standard, this is still a
    'P.O.D.' (Plain Old Data) structure, so it is guaranteed to have the same
    storage layout as the C one.  In addition, the m_sys* members have been
    made private, since the client should never access these.
Admin
    Tested with the ATM driver to verify the the changes work with C++.
    Tested with PPP to ensure that things still work for C.
    Version incremented to 5.10 and tagged.

Version 5.10. Tagged as 'TCPIPLibs-5_10'