The Screen Becomes a File
The Screen Becomes a File
Seed: Plan 9 graphical subsystem (/dev/draw, /dev/mouse, Rio).
Central fact: Plan 9 replaces a monolithic display server API with /dev/draw, a synthetic file exposing image buffers and compositing operations as a 9P stream, so that Rio manages windows by multiplexing ordinary device files into isolated per-process namespaces rather than running a privileged graphics protocol above the filesystem.
A window system usually lives outside the filesystem's law — a special server, a special protocol, a special API a program must be compiled against to draw so much as a line. Plan 9 will not grant graphics that exemption. /dev/draw exposes the frame buffer itself as a structured stream any process can open, describing rectangles, clips, and Porter-Duff compositing the same way any other file describes its contents: by being read and written.
Rio, the window manager, is not a separate kind of thing sitting above this. It is a process like any other, multiplexing /dev/draw and /dev/mouse and /dev/cons into private namespaces for its children, so that a window nested inside a window is not a special case — it is just another namespace, walking the same small set of files. The picture on the glass is never anything but a file answering to read and write.