Who are both of us? The important point is that "everything is a file" is just a short way to say "everything is a file system". Your interface is not a file d

Who are both of us? The important point is that "everything is a file" is just a short way to say "everything is a file system". Your interface is not a file descriptor to which you read and write, but a whole tree where there are different files on which you can perform the operations defined in the 9P protocol (create/read/write/...). For example, in the windowing system, you open a file to create a new window, and the new window will have associated a directory with files that represent the screen, mouse and keyboard (and the process running in that window will work with those files exactly as it does with native devices). You can have a look at the man pages (sections 3 and 4) to see how these filesystems work.