XSI was supposed to be available to have a posix-like portable shared
memory semantics for systems with only system v shared memory. However
lack of time and portability issues froze the development. Maybe some
class (xsi_shared_memory) is ready to be public, just like we have
windows_shared_memory. It shouldn't be hard to implement
managed_xsi_shared_memory. If there is interest in them I could add them
and try posix emulation on top of XSI later.