The FSEvents API in macOS allows applications to register for notifications of changes to a given directory tree. Whenever the filesystem is changed, the kernel passes notifications via the special device file /dev/fsevents to a userspace process called fseventsd. This process combines multiple changes to a single directory tree that occur within a short period of time, then notifies applications that have registered for changes to the affected directory. Mac OS X 10.7 (Lion) added the ability to register for file modification notifications.