There are at least 3 ways to tag a structure such that the
valid fields allocated for a pointer to the structure at
an arbitrary "release level" can be identified at run-time.
They all involve adding a field in the structure that contains
one of the following forms of identification.

1. a release/version number
2. the allocation size of the structure in bytes
3. a bit-mask specifying attributes present in the structure

Pros and cons of each:

1. a release/version number
   Pro: A release or version number is simple, easy to document,
   and easy for both users and implementers to understand and
   reference.
   Con: A release or version number must be maintained "by hand"
   and requires other documentation to indicate what should or
   should not be present in the structure.  In particular, the
   allocation size of the structure must be linked to the release
   number "by hand".  There is no easy way to verify that the
   code implementing the structure conforms to the release or
   version number.

2. the allocation size of the structure in bytes
   Pro: Easy to guarantee correct initialization at
   compile-time using sizeof().
   Con: Requires other documentation to indicate what should or
   should not be present in the structure.  There is no easy way
   to verify that the code implementing the structure conforms to
   the size.  No obvious correlation with a release or version
   number.

3. a bit-mask specifying attributes present in the structure
   Pro: Makes it easy for implementer to code new features, and to
   know and test dynamically at run-time what features are or not
   present in a structure that may have been compiled by a different
   release from the code accessing it.
   Con: A bit-mask must be defined and maintained "by hand",
   including the definition and exposure of all the bit symbols.
   Requires other documentation to correlate with a release or
   version number.  The allocation size of the structure must be
   linked to the attributes "by implication" (i.e., given a pointer
   to a structure at run-time, the presence of an attribute that
   implies the presence of field xxx indicates that the allocation
   size of the structure is at least the sum of the offset to xxx
   within the structure plus the size of field xxx).