I'm looking for a way to make a directory accessible only to its parent directories. That is, suppose you have two directories, A and B, at the same level in the file hierarchy. Now suppose that you have a directory A' which is a subdirectory of A. I'd like to enforce that A is able to access the contents of A' but B is not.

My problem is that I'd like to use a library (directory A) which builds on top of a legacy version of another library (directory A'). At the same time, I want to be able to use the newest version of this legacy library (directory B). I want to make sure that people aren't somehow using library A and linking against new library B by enforcing that library A must use library A'.

I could just link A against library B, but then I'm risking compatibility.