Learning Lab: Liberation Circuit

This is my first learning lab that I made, and this is the solution/write up for the lab.

The first of the tasks to complete is to compile Liberation Circuit. After running ./init build && ./init container to get into the docker container, all I needed to do was run make. Easy enough.

The next task is to configure Liberation Circuit. If this step is skipped and I just try to run the compiled binary, the error I get when trying to play the first level is the following.

Player 0 spawn failure
Template 0 could not be locked.
Check the template and make sure it's loaded properly.

This can be resolved by either running the compiled binary again, or before running the binary running the following two commands

mkdir -p ~/.config/linleyh/libcirc

cp bin/init.txt ~/.config/linleyh/libcirc/init.txt

With that, Liberation Circuit is now running!

The last task is to complete the first level, which I just made a bunch of attacker units and rushed the opponent.

There are two extra tasks that are optional, but why not.

The updated Dockerfile to compile and run the game as part of the build looks like the following.

FROM alpine:latest
RUN apk update && apk add allegro-dev \
    build-base \
    cmake \
    git \
    make \
    mesa-dri-intel \
    sdl2-dev
RUN git clone https://github.com/linleyh/liberation-circuit.git
WORKDIR liberation-circuit
RUN make -j$(nproc)
RUN mkdir -p ~/.config/linleyh/libcirc
RUN cp ./bin/init.txt ~/.config/linleyh/libcirc/init.txt
WORKDIR bin
CMD ["./libcirc"]

The init container command does a couple of things. First it checks to see if the docker container ($NAME) is running, and if it is, then stop/remove it.

Second, it executes xhost local:root. Doing man xhost shows that xhost is the server access control program for X. Doing xhost local:root is giving the local family access to the X server. It is important to note that the documentation states the “local family specifies all the local connections at one. Hoever, the server interpreted address ‘si:localuser:username’ can be used to specify a single local user. (See the Xsecurity(7) manual page for more details.)”.

The third thing that is happening is a docker run command passing a few arguments.

-v /tmp/.X11-unix:/tmp/.X11-unix is a volume, but what is /tmp.X11-unix? When I do file /tmp/.X11-unix, it outputs /tmp/.X11-unix/: sticky, directory. Doing file /tmp/.X11-unix/* results in the following.

/tmp/.X11-unix/X0:    socket
/tmp/.X11-unix/X1:    socket
/tmp/.X11-unix/X1024: socket
/tmp/.X11-unix/X1025: socket

After some searching, it looks like its a unix domain socket for the X11 server. X11 is the common window system used on Linux systems. I will have to ask someone with more Docker experience, why you pass a unix domain socket as a volume (TODO).

-e "DISPLAY=unix${DISPLAY}" is setting an environment variable for the container called DISPLAY. After some searching, I found out that $DISPLAY is an environment variable used by the X window system. In man x, under DISPLAY NAMES, it states that the “information is used by the application to determine how it should connect to the server and which screen it should use by default (on displays with multiple monitors).” The display name format is hostname:displaynumber.screennumber. It also states that on “POSIX systems, the default display name is stored in your DISPLAY environment variable.” Bingo. When I do echo $DISPLAY, it outputs :0. This means that the argument is going to result in DISPLAY=unix:0, which is setting the display name for the X window system.

--device /dev/dri is adding a host device, but what is /dev/dri? From searching, I have not really found a direct answer, but what it looks like is direct rendering infrastructure (DRI). DRI is a framework that the X window system uses to access graphics hardware. This will have to be another thing I confirm with someone who has more knowledge of Linux/X (TODO).

--name $NAME and $IMAGE are self-explanatory.

The fourth, and last thing happening is removing local permissions via xhost.

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Tags:

#c #ocms #solution #docker #x #gaming