Easy to use and reliable cross compiler environment for building Windows, Linux and JRuby binary gems.

It provides cross compilers and Ruby environments for 2.2 and newer versions of the RubyInstaller and Linux runtime environments. They are prepared for use with rake-compiler. It is used by many gems with C or JRuby extentions.

This is kind of successor of rake-compiler-dev-box. It is wrapped as a gem for easier setup, usage and integration and is based on lightweight Docker containers. It is also more reliable, since the underlying docker images are versioned and immutable.


Install docker natively on Linux:

$ sudo apt-get install docker.io

... or install docker-toolbox for Windows and OSX or boot2docker on Windows or OS X .

Install rake-compiler-dock as a gem. The docker image is downloaded later on demand:

$ gem install rake-compiler-dock

... or build your own gem and docker image:

$ git clone https://github.com/rake-compiler/rake-compiler-dock
$ rake install


Rake-compiler-dock provides the necessary tools to build Ruby extensions for Windows and Linux written in C and C++ and JRuby written in Java. It is intended to be used in conjunction with rake-compiler's cross build capability. Your Rakefile should enable cross compilation like so:

Rake::ExtensionTask.new('my_extension', my_gem_spec) do |ext|
  ext.cross_compile = true
  ext.cross_platform = %w[x86-mingw32 x64-mingw32 x86-linux x86_64-linux]

It may also be used to build ffi based binary gems like libusb, but currently doesn't provide any additional build helpers for this use case, beyond docker invocation and cross compilers.

Interactive Usage

Rake-compiler-dock offers the shell command rake-compiler-dock and a ruby API for issuing commands within the docker image, described below.

rake-compiler-dock without arguments starts an interactive shell session. This is best suited to try out and debug a build. It mounts the current working directory into the docker environment. All changes below the current working directory are shared with the host. But note, that all other changes to the file system of the container are dropped at the end of the session - the docker image is static for a given version. rake-compiler-dock can also take the build command(s) from STDIN or as command arguments.

All commands are executed with the same user and group of the host. This is done by copying user account data into the container and sudo to it.

To build x86 and x64 Windows and Linux binary gems interactively, it can be called like this:

[email protected]:$ cd your-gem-dir/
[email protected]:$ rake-compiler-dock   # this enters a container with an interactive shell
[email protected]:$ bundle
[email protected]:$ rake cross native gem
[email protected]:$ exit
[email protected]:$ ls pkg/*.gem
your-gem-1.0.0.gem  your-gem-1.0.0-x64-mingw32.gem  your-gem-1.0.0-x86-mingw32.gem
your-gem-1.0.0-x86_64-linux.gem  your-gem-1.0.0-x86-linux.gem

Or non-interactive:

[email protected]:$ rake-compiler-dock bash -c "bundle && rake cross native gem"

The environment variable RUBY_CC_VERSION is predefined as described below.

If necessary, additional software from the Ubuntu repositories can be installed, prior to the build command. This is local to the running session, only:

sudo apt-get update && sudo apt-get install your-package

You can also choose between different executable ruby versions by rvm use <version> . The current default is 2.5.

JRuby support

Rake-compiler-dock offers a dedicated docker image for JRuby. JRuby doesn't need a complicated cross build environment like C-ruby, but using Rake-compiler-dock for JRuby makes building binary gems more consistent.

To build java binary gems interactively, it can be called like this:

[email protected]:$ cd your-gem-dir/
[email protected]:$ RCD_RUBYVM=jruby rake-compiler-dock   # this enters a container with an interactive shell
[email protected]:$ ruby -v
jruby (2.5.0) 2018-12-06 6d5a228 OpenJDK 64-Bit Server VM 10.0.2+13-Ubuntu-1ubuntu0.18.04.4 on 10.0.2+13-Ubuntu-1ubuntu0.18.04.4 +jit [linux-x86_64]
[email protected]:$ bundle
[email protected]:$ rake java gem
[email protected]:$ exit
[email protected]:$ ls pkg/*.gem
your-gem-1.0.0.gem  your-gem-1.0.0-java.gem

Add to your Rakefile

To make the build process reproducible for other parties, it is recommended to add rake-compiler-dock to your Rakefile. This can be done like this:

task 'gem:native' do
  require 'rake_compiler_dock'
  sh "bundle package"   # Avoid repeated downloads of gems by using gem files from the host.
  RakeCompilerDock.sh "bundle --local && rake cross native gem"
  RakeCompilerDock.sh "bundle --local && rake java gem", rubyvm: :jruby

Rake-compiler-dock uses semantic versioning, so you should add it into your Gemfile, to make sure, that future changes will not break your build.

gem 'rake-compiler-dock', '~> 0.7.0'

See the wiki for projects which make use of rake-compiler-dock.

Environment Variables

Rake-compiler-dock makes use of several environment variables.

The following variables are recognized by rake-compiler-dock:

  • RCD_RUBYVM - The flavour of docker image to be used. Must be one of mri, jruby.
  • RCD_IMAGE - The docker image that is downloaded and started. Defaults to "larskanis/rake-compiler-dock:IMAGE_VERSION" with an image version that is determined by the gem version.

The following variables are passed through to the docker container without modification:

The following variables are provided to the running docker container:

  • RCD_IMAGE - The full docker image name the container is running on.
  • RCD_HOST_RUBY_PLATFORM - The RUBY_PLATFORM of the host ruby.
  • RCD_HOST_RUBY_VERSION - The RUBY_VERSION of the host ruby.
  • RUBY_CC_VERSION - The target ruby versions for rake-compiler. The default is defined in the Dockerfile, but can be changed as a parameter to rake.
  • RCD_MOUNTDIR - The directory which is mounted into the docker container. Defaults to pwd.
  • RCD_WORKDIR - The working directory within the docker container. Defaults to pwd.

Other environment variables can be set or passed through to the container like this:

RakeCompilerDock.sh "rake cross native gem OPENSSL_VERSION=#{ENV['OPENSSL_VERSION']}"

More information

See Frequently asked questions and Join the chat at https://gitter.im/larskanis/rake-compiler-dock


  1. Fork it ( https://github.com/rake-compiler/rake-compiler-dock/fork )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request