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Cross Compiling Ruby Gems for win32

While I was developing nokogiri, I had to learn how to cross compile gems for win32. I don’t have a compiler on windows, so I had to do this on OS X. I just want to dump a few notes here so that other people might benefit, and so that I won’t forget in the future.

As far as I can tell, there are 4 major steps to getting your native gem cross compiled for windows:

After a while, I could run i386-mingw32-gcc to compile stuff. Next up, cross compiling ruby.

First, you have to download ruby, so I wrote a rake task to do just that. This rake task downloads ruby in to a “stash” directory:

namespace :build do
  file "stash/ruby-1.8.6-p287.tar.gz" do |t|
    puts "downloading ruby"
    Dir.chdir('stash') do 
      url = ("ftp://ftp.ruby-lang.org/pub/ruby/1.8/ruby-1.8.6-p287.tar.gz")
      system("wget #{url} || curl -O #{url}")

Next you have to apply a patch to Makefile.in so that it will work with the cross compiler. Once that patch is applied, you can compile ruby with mingw32. Here is my rake task to do that, and unfortunately the strange Makefile.in patch is very necessary:

namespace :build do
  namespace :win32 do
    file 'cross/bin/ruby.exe' => ['cross/ruby-1.8.6-p287'] do
      Dir.chdir('cross/ruby-1.8.6-p287') do
        str = ''
        File.open('Makefile.in', 'rb') do |f|
          f.each_line do |line|
            if line =~ /^\s*ALT_SEPARATOR =/
              str += "\t\t    " + 'ALT_SEPARATOR = "\\\\\"; \\'
              str += "\n"
              str += line
        File.open('Makefile.in', 'wb') { |f| f.write str }
        buildopts = if File.exists?('/usr/bin/i586-mingw32msvc-gcc')
                      "--host=i586-mingw32msvc --target=i386-mingw32 --build=i686-linux"
                      "--host=i386-mingw32 --target=i386-mingw32"
          env ac_cv_func_getpgrp_void=no \
            ac_cv_func_setpgrp_void=yes \
            rb_cv_negative_time_t=no \
            ac_cv_func_memcmp_working=yes \
            rb_cv_binary_elf=no \
            ./configure \
            #{buildopts} \
            --prefix=#{File.expand_path(File.join(Dir.pwd, '..'))}
        sh 'make'
        sh 'make install'

    desc 'build cross compiled ruby'
    task :ruby => 'cross/bin/ruby.exe'

After executing that task (which will take a while), you should have a cross compiled ruby that you can link against.

To modify the gemspec, what I do is assign the new Hoe object to a constant like so:

HOE = Hoe.new('nokogiri', Nokogiri::VERSION) do |p|
  p.developer('Aaron Patterson', 'aaronp@rubyforge.org')
  p.developer('Mike Dalessio', 'mike.dalessio@gmail.com')
  p.clean_globs = [
  p.spec_extras = { :extensions => ["Rakefile"] }

Then when I’m building my win32 gemspec, I modify the gemspec with win32 specific bits and write out the gemspec. This task modifies the gemspec file list to include any binary files such as dll’s and so files that I’ve built, assigns the platform to mswin32, and tells the gemspec that there are no extensions to be built:

namespace :gem do
  namespace :win32 do
    task :spec => ['build:win32'] do
      File.open("#{HOE.name}.gemspec", 'w') do |f|
        HOE.spec.files += Dir['ext/nokogiri/**.{dll,so}']
        HOE.spec.platform = 'x86-mswin32-60'
        HOE.spec.extensions = []

We have to modify the file list and remove any extension building tasks because the gem is going to be shipped with the pre-built windows binaries. Setting the platform to that hardcoded string is a total hack, but I couldn’t figure out a different way. If you were building this spec on windows, you should use “Gem::Platform::CURRENT” instead of that string. After executing this task, you should end up with a file named “packagename.gemspec”. Just run “gem build packagename.gemspec”, and you’ll have your win32 gem, completely windows free!

One final thing…. Nokogiri ships with the libxml and libxslt dll files. In order to get those files to be found with dlopen (or whatever it is that windows uses), they must be in your PATH. Yes. Your PATH. So Nokogiri changes the environment’s PATH to include the directory where the DLL’s are located. You can see the hot PATH manipulation code here.

If you want to see all of the uncensored nitty gritty of the cross compilation action, check out the Nokogiri Rakefile located here.

Good luck, and don’t forget about those windows people.

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