FSKit Sample

FSKit is the new framework introduced in macOS Sequoia 15.4 that enables the developer to provide custom filesystem support from user space. The document is almost non-existent at the moment so here is a sample project to show how to use the framework.

UnaryFilesystemExtension is the entry point of your custom filesystem. The implementation should return an instance of your custom filesystem implementation.

The filesystem should be a subclass of FSUnaryFileSystem and conforms to necessary protocols, specifically FSUnaryFileSystemOperations as the system invokes that when deciding if the custom FS should be used, and get the volume from the extension.

From there, you can create your own subclass of FSVolume to help manage your volume and conform to FSVolume.Operations to support the basic volume specific operations. For a fully functional volume implementation, you’ll probably need to conform to most of the FSVolume.*Operations.

Once you build and run the app, enable the File System extension under System Settings -> General -> Login Items & Extensions -> File System Extensions. After that you should be able to mount the FS with something like these (disk18 is a block device)

mkdir /tmp/TestVol
mount -F -t MyFS disk18 /tmp/TestVol

And unmount them with

umount /tmp/TestVol

To create a dummy block device to test this, you can do the following

mkfile -n 100m dummy // create a dummy file
hdiutil attach -imagekey diskimage-class=CRawDiskImage -nomount dummy // mount the newly created file as a raw block device

and then use it with your FS.