Clone this repo:


  1. a82e9e4 [base, cast] Error handling cleanup by Jordan Bayles · 2 days ago main
  2. d7b6a03 [Cast Remoting] Add Proto Documentation by Ryan Keane · 3 days ago
  3. 8803ea4 C++20 fixes. by Peter Kasting · 3 days ago
  4. d3187ee [Open Screen] Synchronize code coverage scripts with Chromium. by mark a. foltz · 4 days ago
  5. 843d7cf [Cast Remoting] Add new Proto message type by Ryan Keane · 4 days ago

Open Screen Library

The Open Screen Library implements the Open Screen Protocol and the Chromecast protocols (discovery, application control, and media streaming).

Information about the Open Screen Protocol and its specification can be found on GitHub.

Getting the code

Installing depot_tools

Library dependencies are managed using gclient, from the depot_tools repo.

To get gclient, run the following command in your terminal:

    git clone

Then add the depot_tools folder to your PATH environment variable.

Note that openscreen does not use other features of depot_tools like repo or drover. However, some git-cl functions do work, like git cl try, git cl format, git cl lint, and git cl upload.

Checking out code

From the parent directory of where you want the openscreen checkout (e.g., ~/my_project_dir), configure gclient and check out openscreen with the following commands:

    cd ~/my_project_dir
    gclient config
    gclient sync

The first gclient command will create a default .gclient file in ~/my_project_dir that describes how to pull down the openscreen repository. The second command creates an openscreen/ subdirectory, downloads the source code, all third-party dependencies, and the toolchain needed to build things; and at their appropriate revisions.

Syncing your local checkout

To update your local checkout from the openscreen reference repository, just run

   cd ~/my_project_dir/openscreen
   git pull
   gclient sync

This will rebase any local commits on the remote top-of-tree, and update any dependencies that have changed.

Build setup

The following are the main tools are required for development/builds:

  • Build file generator: gn
  • Code formatter: clang-format
  • Builder: ninja (GitHub releases)
  • Compiler/Linker: clang (installed by default) or gcc (installed by you)

All of these--except gcc as noted above--are automatically downloaded/updated for the Linux and Mac environments via gclient sync as described above. The first two are installed into buildtools/<platform>/.

Mac only: XCode must be installed on the system, to link against its frameworks.

clang-format is used for maintaining consistent coding style, but it is not a complete replacement for adhering to Chromium/Google C++ style (that's on you!). The presubmit script will sanity-check that it has been run on all new/changed code.

Linux clang

On Linux, the build will automatically download the Clang compiler from the Google storage cache, the same way that Chromium does it.

Ensure that libstdc++ 8 is installed, as clang depends on the system instance of it. On Debian flavors, you can run:

   sudo apt-get install libstdc++-8-dev

Linux gcc

Setting the gn argument “is_gcc=true” on Linux enables building using gcc instead.

  mkdir out/debug-gcc
  gn gen out/debug-gcc --args="is_gcc=true"

Note that g++ version 7 or newer must be installed. On Debian flavors you can run:

  sudo apt-get install gcc-7

Mac clang

On Mac OS X, the build will use the clang provided by XCode, which must be installed.

Debug build

Setting the gn argument “is_debug=true” enables debug build.

  gn gen out/debug --args="is_debug=true"

To install debug information for libstdc++ 8 on Debian flavors, you can run:

   sudo apt-get install libstdc++6-8-dbg

gn configuration

Running gn args opens an editor that allows to create a list of arguments passed to every invocation of gn gen.

  gn args out/debug

Building targets

OSP demo

The following commands will build the Open Screen Protocol demo and run it.

  mkdir out/debug
  gn gen out/debug             # Creates the build directory and necessary ninja files
  ninja -C out/debug osp_demo  # Builds the executable with ninja
  ./out/debug/osp_demo          # Runs the executable

The -C argument to ninja works just like it does for GNU Make: it specifies the working directory for the build. So the same could be done as follows:

  ./gn gen out/debug
  cd out/debug
  ninja osp_demo

After editing a file, only ninja needs to be rerun, not gn. If you have edited a file, ninja will re-run gn for you.

We recommend using autoninja instead of ninja, which takes the same command-line arguments but automatically parallelizes the build for your system, depending on number of processor cores, amount of RAM, etc.

Also, while specifying build targets is possible while using ninja, typically for development it is sufficient to just build everything, especially since the Open Screen repository is still quite small. That makes the invokation to the build system simplify to:

  autoninja -C out/debug

For details on running osp_demo, see its

Cast Streaming sender and receiver

The process for running the Cast Streaming sender and receiver applications is detailed in the cast/ The build process is the same as the osp_demo, excepting the choice of targets:

gn gen out/debug
autoninja -C out/debug cast_sender cast_receiver

However invokation is more complicated due to certificate requirements, as well as requiring a valid network address and path to a video for the sender to play. An example is provided below but the cast readme should be consulted for more information.

/path/to/out/Default/cast_receiver -g

./out/Default/cast_receiver -d generated_root_cast_receiver.crt -p generated_root_cast_receiver.key lo0

./out/Default/cast_sender -d generated_root_cast_receiver.crt lo0 ~/video-1080-mp4.mp4

Building other targets

Running ninja -C out/debug gn_all will build all non-test targets in the repository.

gn ls --type=executable out/debug will list all of the executable targets that can be built.

If you want to customize the build further, you can run gn args out/debug to pull up an editor for build flags. gn args --list out/debug prints all of the build flags available.

Building and running unit tests

  ninja -C out/debug openscreen_unittests

Contributing changes

Open Screen library code should follow the Open Screen Library Style Guide.

This library uses Chromium Gerrit for patch management and code review (for better or worse). You will need to register for an account at to upload patches for review.

The following sections contain some tips about dealing with Gerrit for code reviews, specifically when pushing patches for review, getting patches reviewed, and committing patches.

Uploading a patch for review

The git cl tool handles details of interacting with Gerrit (the Chromium code review tool) and is recommended for pushing patches for review. Once you have committed changes locally, simply run:

  git cl format
  git cl upload

The first command will will auto-format the code changes. Then, the second command runs the script to check style and, if it passes, a newcode review will be posted on

If you make additional commits to your local branch, then running git cl upload again in the same branch will merge those commits into the ongoing review as a new patchset.

It's simplest to create a local git branch for each patch you want reviewed separately. git cl keeps track of review status separately for each local branch.

Addressing merge conflicts

If conflicting commits have been landed in the repository for a patch in review, Gerrit will flag the patch as having a merge conflict. In that case, use the instructions above to rebase your commits on top-of-tree and upload a new patchset with the merge conflicts resolved.


Clicking the CQ DRY RUN button (also, confusingly, labeled COMMIT QUEUE +1) will run the current patchset through all LUCI builders and report the results. It is always a good idea get a green tryjob on a patch before sending it for review to avoid extra back-and-forth.

You can also run git cl try from the commandline to submit a tryjob.

Code reviews

Send your patch to one or more committers in the COMMITTERS file for code review. All patches must receive at least one LGTM by a committer before it can be submitted.

Submitting patches

After your patch has received one or more LGTM commit it by clicking the SUBMIT button (or, confusingly, COMMIT QUEUE +2) in Gerrit. This will run your patch through the builders again before committing to the main openscreen repository.

Additional resources