|Initial release||Fourth Edition / April 28, 2002|
|Operating system||Plan 9 from Bell Labs|
rio is Plan 9 from Bell Labs' windowing system. It is well known for making its window management transparent to the application. This allows running rio inside of another window manager.
rio is the latest in a long series of graphical user interfaces developed at Bell Labs, mostly developed by Rob Pike, the concurrent window system, and the Blit (which predated X).
rio was a complete rewrite of 8½ in Alef. Its main change was that it stopped parsing and rewriting graphical commands and let the client write pixels directly. This was done mainly for efficiency. As Alef disappeared due to being too difficult to maintain given the number of people working on Plan 9 at the time, rio was rewritten in C. This was done using the Plan 9 thread library which was inspired by Alef and had most of its features, such as blocking channels for interthread and interprocess communication. Another important change, due more to the environment than to rio per se, is that rio supports full colour, using alpha compositing, whereas 8½ uses bitblt operations.
Many of its features embody key Plan 9 design concepts: