xrxs is an experimental game server using the Plan 9 protocol 9p.

The client is intended to be a specialized uxn ROM that can load other ROMs (possibly preserving its own code to maintain a common menu system or interface to xrxs).


This is the working structure of the 9p filesystem:

  • /ctl: Read/write control file for inputing system commands. Reading the file shows the status of the last input command: 1 for success, 0 for failure; logout is a special case, and the status code will be -1 if it was succesful. the following are valid command syntax:

    • login PW: Authenticate with xrxs – password is hashed against realm password hash.
    • logout: Gracefully remove yourself from the users table.
    • load CART: Load a cartridge.
    • chunk TYPE N: Load data of type TYPE and chunk number N.
    • create REALM: Create a new realm (start a new game) – must have a cartridge loaded
    • protect PW: Protect the curent realm with a password if you are the master.
    • transfer USER: Transfer ownership of the realm to another user.
    • delete REALM: Delete the realm off the server if you are the master and it is empty.
    • enter REALM: Join an existing realm.
    • leave: Leave the current realm.
    • unload: Unload the cartridge.
  • /users: Read-only; Self and others in the realm are readable from here, one per line. It contains only yourself before joining a realm. Your username on your machine is used as your username in xrxs – if your name is taken, you will get an error on attaching.

  • /carts: Available game/app cartridges for this server, read only; Carts are listed per line upon reading the file. It is backed by files on the server in a directory structure like carts/CART_NAME/{CART_NAME.rom, data/, realms/}.

  • /slot: After loading the cartridge, its ROM is read from here; Read-only.

  • /data/: Any supporting data that comes with the cartridge will be found here; They are in three parts: sprite, audio, and text. Because uxn is limited to 64-128kb input files, the filesizes of these data blobs should be 64kb max, and the chunk command should be used to page different files into the service when needed. The files on the server should be like TYPEN where TYPE is one of sprite, audio, and text, and N is any sequence of characters (canonically a nonnegative integer). When first loading the cartridge, N == 0. Issuing the command chunk TYPE XXX will attempt to load data from file carts/CART_NAME/data/TYPEXXX into the correct data file. If TYPE is not one of sprite, audio, or text, or the ifle TYPEXXX doesn’t exist in the data directory, the chunk command does nothing.

  • /realms: Open/saved realms, read-only. Realms and their associated universe are backed by real files on the server so that they can be preserved across service instantiations, in a directory structure like: carts/CART_NAME/realms/REALM_NAME/{realm, universe}. Realms can either be solo, open, or protected; Open or protected realms can have limited member numbers. Depending on the cartridge, these settings can be user-managed or managed by the cartridge itself. Realms are listed per line upon reading the file like: REALM_NAME 1 4 1. First would obviously be the name of the realm. The first number is number of members, second is member limit, third is 1 if protected, 0 if not. 0 1 1 represents a protected solo realm that is empty (saved game with password). 0 1 0 represents an unprotected solo realm that is empty (saved game with no password).

  • /universe: Write here to update serverside state for this cart/realm; Read from here to get the complete current state. This is backed by a key-value-pair list on the server.

  • /scope: Write here to tell the server the names of the Atoms (key/value pair of a Universe) you’re interested in (one per line), and read from here to retrieve their values (one per line). In many cases this will be preferrable to fetching the entire Universe.

  • /random: Read-only, get a random number from 0 to 99.

  • /grandom: Read-only, get a random number from 0 to 99 – These are doled out on a per-realm basis, and the number stays the same until everyone in the realm has had a chance to read it. If you’ve already read it this round or aren’t in a realm, it will be empty.

realm format

Each realm directory on the server should have the following files:

  • realm: Basic data for the realm, file should contain only the maximum number of members, the master’s name, and the password hash, if any (otherwise 0), separated by spaces.
  • universe: The actual game state for the realm as key value pairs, one per line, like KEY = VALUE; limit 15 characters for keys, 63 for values.

The realm should be synchronized to disc when realm membership, limit, or password change. Fenagling some periodic autosave should be possible…


config.h in the source contains the following configuration macros:

  • MAX_USERS: the maximum number of simultaneous users able to attach to the xrxs service
  • DATA_DIR: the path to the root of the cartridge and realm storage; can be absolute or relative to the xrxs executable, but must have the trailing /


xrxs is built/tested in a Linux environment with plan9port and the C standard library as the only dependencies. With minimal modifications it will probably run just as well on Plan9, *BSD, WSL, and MacOS.

Running ./build.sh from the repository root should build the xrxs executable.

You can run a local server (for testing, split-screen games, or single-player games) with:

./xrxs -m /path/to/mountpoint

or expose a service on the network (uses 9pserve to support multiple users and gracefully handle disconnects) with:

./xrxs-srv.sh start

Similarly, you can stop the service with:

./xrxs-srv.sh stop