File System API - Kernel-level API

Kernel-level API

The API is "kernel-level" when the kernel not only provides the interfaces for the filesystems developers but is also the space in which the filesystem code resides.

It differs with the old schema in that the kernel itself uses its own facilities to talk with the filesystem driver and vice-versa, as contrary to the kernel being the one that handles the filesystem layout and the filesystem the one that directly access the hardware.

It is not the cleanest scheme but resolves the difficulties of major rewrite that has the old scheme.

With modular kernels it allows adding filesystems as any kernel module, even third party ones. With non-modular kernels however it requires the kernel to be recompiled with the new filesystem code (and in closed-source kernels, this makes third party filesystem impossible).

Unixes and Unix-like systems such as Linux have used this modular scheme.

There is a variation of this scheme used in MS-DOS (DOS 4.0 onward) and compatibles to support CD-ROM and network file systems. Instead of adding code to the kernel, as in the old scheme, or using kernel facilities as in the kernel-based scheme, it traps all calls to a file and identifies if it should be redirected to the kernel's equivalent function or if it has to be handled by the specific filesystem driver, and the filesystem driver "directly" access the disk contents using low-level BIOS functions.