Arcan - [Display Server, Multimedia Framework, Game Engine] -> "Desktop Engine"
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README.md

Arcan

Arcan is a powerful development framework for creating virtually anything from user interfaces for specialized embedded applications all the way to full-blown standalone desktop environments.

At its heart lies a robust and portable multimedia engine, with a well-tested and well-documented Lua scripting interface. The development emphasizes security, debuggability and performance -- guided by a principle of least surprise in terms of API design.

For more details about capabilities, design, goals, current development, roadmap, changelogs, notes on contributing and so on, please refer to the arcan-wiki.

There is also a website that collects other links, announcements, releases, videos / presentations and so on.

For developer contact, check out the IRC channel #arcan on irc.freenode.net.

The current release version is roughly that the master branch will always have bugfixes pushed as soon as possible, and after a new (0.n.m.p) update to the m version number - a blog post is pushed and then a set of the next big features gets selected, and each pushed as a new branch. When that feature is completed we interactive-rebase-squish-merge unto the main branch and tag a '.p' version update. When there are no branches left, the 'm' version gets a new update and the cycle repeats anew.

Table of Contents

  1. Getting Started
    1. Compiling
  2. Database / Configuration
  3. Compatibility
  4. Other Tools
  5. Appls to Try
  6. Git layout

Getting Started

The rest of this readme is directed towards developers or very advanced end- users as there is no real work or priority being placed on wrapping/packaging the project and all its pieces at this stage.

Compiling

There are a lot of build options for fine-grained control over your Arcan build. In this section we will just provide the bare essentials for a build on Linux, BSD or OSX. and you can check out the relevant sections in the wiki for more detailed documentation on specialized build environments, e.g. an X.org-free KMS/DRM. (https://github.com/letoram/arcan/wiki/egl-dri)

For starters, the easiest approach is to do the following:

 git clone https://github.com/letoram/arcan.git
 cd arcan
 mkdir build
 cd build
 cmake -DCMAKE_BUILD_TYPE="Debug" -DVIDEO_PLATFORM=sdl ../src
 make -j 12

The required dependencies for this build are: cmake for compilation, libsdl1.2, openal-soft, opengl and freetype. There is also support for building some of these dependencies statically:

 git clone https://github.com/letoram/arcan.git
 cd arcan/external/git
 ./clone.sh
 cd ../../
 mkdir build
 cd build
 cmake -DCMAKE_BUILD_TYPE="Debug" -DVIDEO_PLATFORM=sdl
  -DSTATIC_SQLITE3=ON -DSTATIC_OPENAL=ON -DSTATIC_FREETYPE=ON ../src
 make -j 12

You can then test the build with:

 ./arcan -p ../data/resources/ ../data/appl/welcome

Which tells us to use shared resources from the ../data/resources directory, and launch an application that resides as ../data/appl/welcome. If this path isn't specified relative to current path (./ or ../) or absolute (/path/to), the engine will try and search in the default 'applbase'. This path varies with the OS, but is typically something like /usr/local/share/arcan/appl or to the current user: /path/to/home/.arcan/appl

The 'recommended' setup is to have a .arcan folder in your user home directory with a resources and appl subdirectory. Symlink/bind-mount the data you want accessible into the .arcan/resources path, and have the runable appls in .arcan/appl.

A big note about compilation is how central the 'shmif' set of libraries are. These are used for all external tools, but are tied to the inner workings of your arcan build, and particularly the 'video' platform used. The short explanation as to why this is, is that there's no universal solution for which packing formats and so on it is that is the most efficient for all possible hardware combinations. Your needs on a low level are possible much different from on a higher level where there's another display system involved. While it is possible to account for such variations at runtime with some degree of uncertainty, the solution contributes to a huge state space explosion in a way that we don't have the resources to support, and it makes application development harder on both sides of the shmif- barrier. Therefore, the decision was made to have a 'native color format' and a 'native audio format' that is as hard-wired as 'endianness' is for CPUs. While builds are made out of source, one file, shmif/arcan_shmif_cfg.h, is generated during build time in the normal source tree. Therefore, trying out multiple builds from the same source checkout will likely result in wrong colors unless you rebuild everything that uses SHMIF. This is one of the reasons why SHMIF- is treated as an 'internal API' rather than some protocol or external API for others to use. This situation is subject to change, but there are no such plans in that direction at the moment.

Database / Configuration

Among the output binaries is one called arcan_db. It is a tool that can be used to manipulate the sqlite- database that the engine requires for some features, e.g. application specific key/value store for settings, but also for whitelisting execution and as a generic configuration mechanism.

The database itself is split into a number of tables with key=value stores, and a special set of tables (targets and config) for referencing programs that can be launched (also refered to as 'launch targets'). Internally, the tables are prefixed appl_applname, with 'arcan' and 'arcan_lwa' being reservered for engine configuration.

Modifying an appl- kv store is as simple as;

    arcan_db add_appl_kv myappl key value

While the current set of kv pairs can be enumerated via:

    arcan_db show_appl myappl

Refer to the arcan_db tool and manpage for more detailed explanation.

The various input, graphics and event platforms can be all configured either via environment variables or via the database. In case of a conflict, the environment variables will take presedence.

The environment variables follow the pattern: ARCAN_SUBSYS_XXX where SUBSYS can be VIDEO, EVENT, AUDIO, GRAPHICS - or via the database.

Running arcan without any arguments should give you an enumeration of the various values for XXX that the current platform setup accepts.

The database- configuration takes a similar pattern, e.g. subsys_xxx (note that the ARCAN prefix is dropped as it is implied in the database context and the key is in lower case). Example:

    arcan_db add_appl_kv arcan event_verbose 1

Would set the 'verbose' option to enabled for the event subsystem.

For execution targets, it's slightly more complicated. An early design decision was that the Lua VM configuration should be very restrictive -- no arbitrary creation / deletion of files, no arbitrary execution etc. The database tool is used to specify explicitly permitted execution that should not be modifable from the context of the running arcan application.

One target comprise one binary, a binary format and base environment/command-line arguments, including a list of libraries to preload. It also has one or many different configurations that append additional arguments - but both targets and configurations also act as key-value stores in order to track per-application configurations in the same place and with the same toolset/interface as the rest of the system. Furthermore, arguments are subject to namespace expansion.

The following example attempts to illustrate how this works:

    arcan_db db.sqlite add_target example_app /some/binary -some -args
    arcan_db db.sqlite add_config example_app more_args -yes -why -not
    arcan_db add_target mycore RETRO [ARCAN_RESOURCEPATH]/.cores/core.so
    arcan_db add_config mycore myconfig RETRO /path/to/somefile

An arcan application should now be able to:

    launch_target("example_app", "more_args", LAUNCH_EXTERNAL);

or

    vid = launch_target("example_app",
        "more_args", LAUNCH_INTERNAL, callback_function);

The first example would have the engine minimize and release as many resources as possible (while still being able to resume at a later point), execute the specified program and wake up again when the program finishes. This can be used to share the GPU with dedicated fullscreen applications that benefit from minimal overhead, or for use as features like explicit suspend (where the target- you execute is your 'save to mem and shutdown' utility.

The second example would execute the program in the background, expect it to be able to handle the engine shmif- API for audio/video/input cooperatively or through an interposition library.

It can be cumbersome to set up database entries to just test something. Frameservers is a way of separating sensitive or crash-prone functions from the main engine for purposes such as running games or playing back video.

In a default installation, they are prefixed with afsrv_ [game, encode, decode, ...] and while they are best managed from the appl itself, you can run them from the terminal as well. Which ones that are available depend on the dependencies that were available at build time, but for starting a libretro core for instance:

ARCAN_ARG=core=/path/to/core:resource=/path/to/resourcefile afsrv\_game

or video playback:

ARCAN_ARG=file=/path/to/moviefile.mkv afsrv_decode

but they are all best managed from the engine and its respective scripts.

Compatibility

The follow options exist for running 3rd party software that use arcan as a display server:

  1. Arcan-LWA - this allows for nested execution of different scripts, if the dependencies were fulfilled during compile time, you should already have this binary.

  2. Wayland - use the separate tool found in src/tools/waybridge to enable on a per- client basis or as a translation service.

  3. XArcan - there is a patched Xorg server at https://github.com/letoram/xarcan

  4. QEmu - there is a patched QEmu backend at https://github.com/letoram/qemu

  5. SDL2 - there is a patched SDL2 backend at https://github.com/letoram/SDL2 but it is better to run SDL2 applications through wayland.

There is also the option of using hijack (LD_PRELOAD and similar) for hacky ways to access legacy software, should XArcan/Wayland not work. You can enable this with the build-time -DDISABLE_HIJACK=OFF and get access to a SDL1.2 lib (libahijack_sdl12.so).

Other Tools

Depending on build-time configuration and dependencies, a number of other binaries may also have been produced. The particularly relevant ones are:

arcan_lwa: specialized build that connects/renders to an existing Arcan instance, similar in some ways to Xnest or ephyr.

arcan_frameserver: a chainloader used to setup/configure the environment for the individual frameservers.

Frameservers are an important part of the engine. They can be considered specialized or privileged separate processes for offloading or isolating sensitive and bug-prone tasks like parsing and decoding media files. One frameserver implements a single 'archetype' out of the set (decode, net, encode, remoting, terminal, game, avfeed). The running appl- scripts can then use these to implement features like desktop sharing, accessibility tools, screen recorders, etc. with a uniform interface for system-access policies and granular sandboxing controls.

afsrv_terminal: the default terminal emulator implementation.

afsrv_decode: media decoding and rendering implementation, default version uses libvlc.

afsrv_encode: used for transforming/recording/streaming media.

afsrv_net: (experimental/broken) used for negotiating/discovering local networking services.

afsrv_remoting: client-side for bridging with remote desktop style protocols, with the default using VNC.

afsrv_game: implementation of the libretro API that allows you to run a large number of game engines and emulators.

afsrv_avfeed: custom skeleton for testing/ quick- wrapping some A/V device.

Inside the tools directory, there are a number of additional tools that are built separately. These are:

aclip: clipboard manager / CLI integration

aloadimage: sandboxed image loader

shmmon: shmif- dump/inspection tool

waybridge: wayland client support

leddec: example of an external LED controller

vrbridge: add support for virtual-reality related hardware

Appls to try

With the engine built, and the welcome- test appl running, what to try next? That depends on your fancy. For appl- development you have some basic scripting tutorials and introduction documentation on the wiki.

For desktop environment use, there are two usable ones available right now, 'durden' and 'prio'. 'Durden' is an attempt at evolving a complete, customizable, heavily integrated approach to the keyboard dominant management/use style promoted by tiling window managers like Xmonad or i3.

'Prio' is instead a much simpler skeleton for a composable desktop where third party providers can be set to be responsible for different parts of the UI. Its window management model is an homage to the 'Rio' system used as part of the Plan9 operating system.

Demonstrating how more advanced applications can be built, there is also senseye, which is an research- level data visualization, debugging and reverse engineering tool - but senseye is likely to only be of use to those few that have an unhealthy interest in such areas.

To try out durden or prio:

git clone https://github.com/letoram/durden.git
arcan -p /my/home /path/to/checkout/durden/durden

git clone https://github.com/letoram/prio.git
arcan -p /my/home /path/to/checkout/prio

The basic format for starting is arcan: [engine arguments] applname [appl arguments]

Note that it's the durden subdirectory in the git, not the root. The reason for the different start path (-p /my/home) is to give read-only access to the appl for the built-in resource browser. It is possible that (depending on platform, time of day, the use of bastard devices like KVMs etc.) the detected resolution is wrong. You can explicitly override that for now by using -w desiredwidth -h desiredheight as arguments to the engine.

For details on configuring and using durden or prio, please refer to the respective README.md provided in each git. There are also demonstration videos on the youtube-channel.

Filesystem Layout

The git-tree has the following structure:

data/ -- files used for default packaging and installation (icons, etc.)

doc/*.lua -- specially formatted script files (1:1 between Lua API
             functions and .lua file in this folder) that is used
             by doc/mangen.rb to generate manpages, test cases,
             editor syntax highlighting etc.

doc/*.1   -- manpages for the binaries
doc/*.pdf -- presentation slides (updated yearly)
external/ -- external dependencies that may be built in-source

src/
    engine/ -- main engine code-base
    frameserver/ -- individual frameservers and support functions
    hijack/ -- interpositioning libraries for different data sources
    platform/ -- os/audio/video/etc. interfacing
    tools/ -- database tools, keymap conversion, protocol/device bridges
    shmif/ -- engine<->frameserver IPC

tests/
      api_coverage -- dynamically populated with contents from doc/
      benchmark -- scripts to pinpoint bottlenecks, driver problems etc.
      core -- for core libraries (shmif-server etc.)
      interactive -- quick/messy tests that are thrown together during
                     development to test/experiment with some features.
      frameservers -- specialized testing clients
      security -- fuzzing tools, regression tests for possible CVEs etc.
      regression -- populated with test-cases that highlight reported bugs.
      exercises -- solutions to the exercises in the wiki.
      examples -- quick examples / snippets
      modules -- system_load()able lua extensions