A long-lived project that still receives updates
Easy to use and reliable cross compiler environment for building Windows and Linux binary gems. Use rake-compiler-dock to enter an interactive shell session or add a task to your Rakefile to automate your cross build.


~> 3.0
>= 1.7, < 3.0
>= 12
 Project Readme


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

It provides cross compilers and Ruby environments for 2.4 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.

Supported platforms

The following platforms are supported for cross-compilation by rake-compiler-dock:

  • aarch64-linux and aarch64-linux-gnu
  • aarch64-linux-musl
  • arm-linux and arm-linux-gnu
  • arm-linux-musl
  • arm64-darwin
  • jruby
  • x64-mingw-ucrt
  • x64-mingw32
  • x86-linux and x86-linux-gnu
  • x86-linux-musl
  • x86-mingw32
  • x86_64-darwin
  • x86_64-linux and x86_64-linux-gnu
  • x86_64-linux-musl


x64-mingw-ucrt should be used for Ruby 3.1 and later on windows. x64-mingw32 should be used for Ruby 3.0 and earlier. This is to match the changed platform of RubyInstaller-3.1.

Linux GNU and Musl

Platform names with a *-linux suffix are aliases for *-linux-gnu, since the Rubygems default is to assume gnu if no libc is specified.

Some C extensions may not require separate GNU and Musl builds, in which case it's acceptable to ship a single *-linux gem to cover both platforms. We recommend you set up automated testing for both GNU and Musl to determine whether you need to ship distinct native gems for these platforms (see flavorjones/ruby-c-extensions-explained for an example of how to set up testing pipelines on Github Actions).

Linux GNU and Musl: important details


If you ship -linux-gnu and -linux-musl gems:

  • Use rake-compiler >= 1.2.7 to build your gems
  • Recommend your musl users have bundler >= 2.5.6
  • Require your linux users to have rubygems >= 3.3.22 and bundler >= 2.3.21

⚠ DO NOT ship a -linux-musl gem with a -linux gem. See #117 for more context.

Warning about combining -linux-musl with other linux native platforms

After evaluating many ruby, rubygems, and bundler versions, we strongly recommend that gem maintainers choose one of the following two options:

  • ship a -linux platform gem that works for both gnu and musl systems if you can,
  • or ship both -linux-gnu and -linux-musl platform gems

Do NOT ship -linux and -linux-musl gems together, some versions of bundler or rubygems will not work properly for your users.

Warning about required versions of rubygems

The *-linux-gnu and *-linux-musl platform gems require Rubygems 3.3.22 or later (or Bundler 2.3.21 or later) at installation time.

Ruby version 3.1 and later ship with a sufficient Rubygems version. For earlier versions of Ruby, here are the versions of Rubygems you should recommend to your users:

  • ruby: "3.0", rubygems: "3.5.5" # or possibly higher
  • ruby: "2.7", rubygems: "3.4.22"
  • ruby: "2.6", rubygems: "3.4.22"
  • ruby: "2.5", rubygems: "3.3.26"
  • ruby: "2.4", rubygems: "3.3.26"

It's strongly suggested that you use rake-compiler v1.2.7 or later to build linux-musl and/or linux-gnu native gems. That version of rake-compiler sets required_rubygems_version automatically in the native platform gems' gemspecs.

Warning about required versions of bundler

Finally, there is a known bug in bundler < 2.5.6 that may make it difficult for users on musl systems to resolve their linux-musl dependencies correctly. You can read a description of this problem at rubygems/rubygems#7432, but in summary you should recommend your musl users have bundler v2.5.6 or later.


Install docker following the instructions on the docker website ... 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:

exttask = Rake::ExtensionTask.new('my_extension', my_gem_spec) do |ext|
  ext.cross_compile = true
  ext.cross_platform = %w[x86-mingw32 x64-mingw-ucrt x64-mingw32 x86-linux x86_64-linux x86_64-darwin arm64-darwin]

where you should choose your platforms from the list in the "Supported platforms" section.

See below, how to invoke cross builds in your Rakefile.

Additionally 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 Windows and x86_64 Linux binary gems interactively, it can be called like this:

user@host:$ cd your-gem-dir/
user@host:$ rake-compiler-dock   # this enters a container with an interactive shell for x86 Windows (default)
user@5b53794ada92:$ bundle
user@5b53794ada92:$ rake cross native gem
user@5b53794ada92:$ exit
user@host:$ ls pkg/*.gem
your-gem-1.0.0.gem  your-gem-1.0.0-x86-mingw32.gem

user@host:$ RCD_PLATFORM=x86_64-linux-gnu rake-compiler-dock   # this enters a container for amd64 Linux GNU target
user@adc55b2b92a9:$ bundle
user@adc55b2b92a9:$ rake cross native gem
user@adc55b2b92a9:$ exit
user@host:$ ls pkg/*.gem
your-gem-1.0.0.gem  your-gem-1.0.0-x86_64-linux-gnu.gem

Or non-interactive:

user@host:$ 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 can be installed, prior to the build command. This is local to the running session, only.

For Windows and Mac:

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

For Linux:

sudo yum install your-package

You can also choose between different executable ruby versions by rbenv shell <version> . The current default is 3.1.

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:

user@host:$ cd your-gem-dir/
user@host:$ RCD_RUBYVM=jruby rake-compiler-dock   # this enters a container with an interactive shell
user@5b53794ada92:$ 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]
user@5b53794ada92:$ bundle
user@5b53794ada92:$ rake java gem
user@5b53794ada92:$ exit
user@host:$ 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 --all"   # Avoid repeated downloads of gems by using gem files from the host.
  PLATFORMS.each do |plat|
    RakeCompilerDock.sh "bundle --local && rake native:#{plat} gem", platform: plat
  RakeCompilerDock.sh "bundle --local && rake java gem", rubyvm: :jruby

This runs the bundle and rake commands once for each platform. That is once for the jruby gems and 6 times for the specified MRI platforms.

Run builds in parallel

rake-compiler-dock uses dedicated docker images per build target (since rake-compiler-dock-1.0). Because each target runs in a separate docker container, it is simple to run all targets in parallel. The following example defines rake gem:native as a multitask and separates the preparation which should run only once. It also shows how gem signing can be done with parallel builds. Please note, that parallel builds only work reliable, if the specific platform gem is requested (instead of just "rake gem").

  namespace "gem" do
    task 'prepare' do
      require 'rake_compiler_dock'
      require 'io/console'
      sh "bundle package --all"
      sh "cp ~/.gem/gem-*.pem build/gem/ || true"
      ENV["GEM_PRIVATE_KEY_PASSPHRASE"] = STDIN.getpass("Enter passphrase of gem signature key: ")

    exttask.cross_platform.each do |plat|
      desc "Build all native binary gems in parallel"
      multitask 'native' => plat

      desc "Build the native gem for #{plat}"
      task plat => 'prepare' do
        RakeCompilerDock.sh <<-EOT, platform: plat
          (cp build/gem/gem-*.pem ~/.gem/ || true) &&
          bundle --local &&
          rake native:#{plat} pkg/#{exttask.gem_spec.full_name}-#{plat}.gem

Add to your Gemfile

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', '~> 1.2'

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

As a CI System Container

The OCI images provided by rake-compiler-dock can be used without the rake-compiler-dock gem or wrapper. This may be useful if your CI pipeline is building native gems.

For example, a Github Actions job might look like this:

    name: "native-gem"
    runs-on: ubuntu-latest
      image: "ghcr.io/rake-compiler/rake-compiler-dock-image:1.2.2-mri-x86_64-linux-gnu"
      - uses: actions/checkout@v2
      - run: bundle install && bundle exec rake gem:x86_64-linux-gnu:rcd
      - uses: actions/upload-artifact@v2
          name: native-gem
          path: gems
          retention-days: 1

Where the referenced rake task might be defined by something like:

cross_platforms = ["x64-mingw32", "x86_64-linux", "x86_64-darwin", "arm64-darwin"]

namespace "gem" do
  cross_platforms.each do |platform|
    namespace platform do
      task "rcd" do

For an example of rake tasks that support this style of invocation, visit https://github.com/flavorjones/ruby-c-extensions-explained/tree/main/precompiled

Living on the edge: using weekly snapshots

OCI images snapshotted from main are published weekly to Github Container Registry with the string "snapshot" in place of the version number in the tag name, e.g.:

  • ghcr.io/rake-compiler/rake-compiler-dock-image:snapshot-mri-x86_64-linux-gnu

These images are intended for integration testing. They may not work properly and should not be considered production ready.

Environment Variables

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

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

  • RCD_RUBYVM - The ruby VM and toolchain to be used. Must be one of mri, jruby.
  • RCD_PLATFORM - The target rubygems platform. Must be a space separated list out of the platforms listed under "Supported platforms" above. It is ignored when rubyvm is set to :jruby.
  • RCD_IMAGE - The docker image that is downloaded and started. Defaults to "ghcr.io/rake-compiler/rake-compiler-dock-image:IMAGE_VERSION-PLATFORM" with an image version that is determined by the gem version.

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

  • http_proxy, https_proxy, ftp_proxy - See Frequently asked questions for more details.
  • GEM_PRIVATE_KEY_PASSPHRASE - To avoid interactive password prompts in the container.

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