The purpose of the namespace parameter for cdb_subscribe( ) is to disambiguate between
equally-named toplevel nodes.
I.e. if you have /foo in both the bar and baz namespaces,
cdb_subscribe(s, 3, bar__ns, &spoint, "/foo")
is equivalent to
cdb_subscribe(s, 3, <any valid namespace hash>, &spoint, "/bar:foo")
But it doesn't do anything for "/", since there can only be one root.