Rethinking /dev and devices in the UNIX kernel

Poul-Henning Kamp <>

An outstanding novelty in UNIX at its introduction was the notion of ‘‘a file is a file is a file and even a device is a file.’’ Going from ‘‘hardware only changes when the DEC Field engineer is here’’ to ‘‘my toaster has USB’’ has put serious strain on the rather crude implementation of the ‘‘devices as files’’ concept, an implementation which has survived practically unchanged for 30 years in most UNIX variants. Starting from a high-level view of devices and the semantics that have grown around them over the years, this paper takes the audience on a grand tour of the redesigned FreeBSD device-I/O system, to convey an overview of how it all fits together, and to explain why things ended up as they did, how to use the new features and in particular how not to.