Mac OS X for Unix Geeks

Two-level Namespaces

In Mac OS X 10.0, the dynamic linker merged symbols into a single (flat) namespace. So, if you link against two different libraries that both define the same function, the dynamic linker complains, because the same symbol was defined in both places. This approach prevented collisions that were known at compile time. However, if there wasn’t a conflict at compile time, there is no guarantee that a future version of the library won’t introduce a conflict.

Suppose you linked your application against version 1 of libfoo and version 1 of libbar. At the time you compiled your application, libfoo defined a function called logerror( ), and libbar did not. But when version 2 of libbar came out, it included a function called logerror( ). Since the conflict was not known at compile time, your application doesn’t expect libbar to contain this function. If your application happens to load libbar before libfoo, it will call libbar’s logerror( ) method, which is not what you want.

So, Mac OS X 10.1 introduced two-level namespaces, which the compiler uses by default. (Mac OS X 10.2 does not introduce any changes to two-level namespaces.) With this feature, you can link against version 1 of libfoo and libbar. The linker creates an application that knows logerror( ) lives in libfoo. So, even if a future version of libbar includes a logerror( ) function, your application will know which logerror( ) it should use.

If you want to build an application using a flat namespace, use ...