Lately I have been going on about how to write Ruby C Extensions and I thought at this point I should share with you how to wire my favourite Gem manager Jeweler and the famous rake-compiler together to make Ruby C extension development easier.

Jeweler Setup

You will need to configure Jeweler for four things:

1. Setting up the extensions flag of the gemspec
2. Including the generated native files
3. Storage of the generated gemspec
4. Fixing the gemspec version

Let's look at each in turn:

The Extensions Flag

Jeweler::Tasks.new do |gem|
    ...
    gem.extensions = FileList['ext/**/extconf.rb']
end

This will enable you to package a gem for all the platforms that you don't provide a native binary for. This gem will perform the well known

Building native extensions.  This could take a while...

task on installation.

Including the Binaries

Jeweler::Tasks.new do |gem|
    ...
    gem.files.include('lib/cranberry.*')
end

This will include all generated canberry.so or cranberry.dll and so on in your generated gem.

Storing the GemSpec

Since rake-compiler needs the gemspec on initialization, we need to store it:

jeweler_tasks = Jeweler::Tasks.new do |gem|
    ...
end
$gemspec = jeweler_tasks.gemspec

This also means that rake-compiler tasks must be set up after the Jeweler tasks.

Fixing the GemSpec Version

(UPDATE: This is a simpler solution. The one posted previously works as well, but this is less invasive)

$gemspec.version = jeweler_tasks.jeweler.version

This fixes an issue in the interaction of rake-compiler and Jeweler. Rake-compiler needs the gemspec to have a version, but Jeweler will only store the current version in the gemspec if certain tasks are called, e.g. gemspec.

If not applied, native building will fail with

Version required (or :noversion)

Rake-Compiler Setup

The rake-compiler itself doesn't need any special configuration, except for passing in the gemspec:

Rake::ExtensionTask.new('craberry', $gemspec) do
    ...
end

But there are some bugs when wiring it to Jeweler.

Gemspec Marshalling Fix

Rake::Task.tasks.each do |task_name|
    case task_name.to_s
    when /^native/
        task_name.prerequisites.unshift("fix_rake_compiler_gemspec_dump")
    end
end

task :fix_rake_compiler_gemspec_dump do
    %w{files extra_rdoc_files test_files}.each do |accessor|
        $gemspec.send(accessor).instance_eval {
            @exclude_procs = Array.new
        }
    end
end

This forces Rake to always execute fix_rake_compiler_gemspec_dump before invoking any native tasks.

Rake-compiler at one point needs to draw a copy of the gemspec. But since gemspecs don't have a dup method, it resorts to YAML-dump-loading the whole thing.

But when using Jeweler, the gemspec will have some Procs attached (to the FileLists), which can't be YAML-loaded (there's no dumping of Ruby code), so the gemspec cloning will fail with

allocator undefined for Proc

The above code snippet prevents that by removing those Procs before the YAML dumping.

The Rest

For the rest, you can follow the rake-compiler tutorial except that with Jeweler the gem task is called build.

Hope this helps some folks out there!