It can sometimes be necessary or desirable to use platform-specific calls for some file system operations, despite the fact that this will result in nonportable code. Services are provided to permit cooperation between Common File System and platform-specific calls, thereby minimizing the portability costs associated with platform-specific calls.

These operations should only be used by experts, because there are hidden pitfalls. Some platforms can have multiple valid file descriptor representations, thus some calls might not be compatible with the particular representation chosen by the CfsFileDescriptor. Furthermore, on such platforms, the possibility exists that the underlying representation might change.