Controlling Symbol Visibility

A shared library should only export (i.e., make visible outside the library) those symbols (functions, variables) that form its specified ABI (Application Binary Interface).

Exposing other symbols "accidentally" can lead to various problems:

Symbols (i.e., unspecified interfaces) might be used by application developers using library.
Conflict: shared library maintainer wants to change the unspecified interfaces, but application developer expects to be able to continue using them.
Symbols might (unexpectedly) interpose definitions provided in other shared libraries.
Exporting unnecessary symbols increases size of dynamic symbol table that is loaded at run-time.