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by ryan davis



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Writing Extensions using Hoe

Published 2008-08-07 @ 12:59

Tagged hoe, ruby, rubyinline

Despite popular belief, it is possible and easy to write extensions with projects based on Hoe. It is possible to do both classic and inline extensions. Let’s take a look at what both of them look like.

Classic Extensions

This is a standard extconf.rb based extension, with the build mechanism folded into the Rakefile instead of also having a Makefile in the ext directory.

Rakefile

  EXT = "ext/blah.#{Hoe::DLEXT}"                        # 1
  
  class Blah
    VERSION = '1.0.0'                                   # 2
  end
  
  Hoe.new('blah', Blah::VERSION) do |p|
    p.developer('FIX', 'FIX@example.com')
  
    p.spec_extras[:extensions] = "ext/extconf.rb"       # 3
    p.clean_globs << EXT << "ext/*.o" << "ext/Makefile"
  end
  
  task :test => EXT
  
  file EXT => ["ext/extconf.rb", "ext/blah.c"] do       # 4
    Dir.chdir "ext" do
      ruby "extconf.rb"
      sh "make"
    end
  end

1) EXT defines the path to the built extension, used below for dependencies and the clean_globs array.

2) Normal Hoe-based projects require the library in the Rakefile in order to grab the VERSION constant. This means you only have to update the version in a single location, but can use that value for packaging and deploying. We can’t do that here (since the extension isn’t necessarily built yet), so we cheat by duplicating the version constant. Violates DRY, but it is not that big of a deal.

3) The hoe spec also uses spec_extras to define :extensions to specify that we’ve got an extconf.rb file to use.

4) Finally, and most importantly, we have a file rule for EXT that executes extconf.rb to generate a Makefile and then executes make to build the extension binary.

ext/ext/extconf.rb

  require 'mkmf'
  create_makefile("blah")

Nothing special here to explain.

ext/ext/blah.c

  #include "ruby.h"
  
  static VALUE blah(VALUE self) {
    puts("hello world");
    return Qnil;
  }
  
  #ifdef __cplusplus
  extern "C" {
  #endif
    void Init_blah() {
      VALUE c = rb_define_class("Blah", rb_cObject);
      rb_define_method(c, "blah", (VALUE(*)(ANYARGS))blah, 0);
    }
  #ifdef __cplusplus
  }
  #endif

Here is your basic C extension for ruby. We’ve got a function blah and an Init_blah function that declares the Blah class with a blah method and binds it to the blah C function.

Remember, this is as easy as it gets. blah doesn’t take any arguments and it doesn’t actually do anything.

Inline Extensions

Inline makes your life a lot easier than the above.

Rakefile

  require './lib/blah.rb'
  
  Hoe.new('blah', Blah::VERSION) do |p|
    p.developer('FIX', 'FIX@example.com')
  end

Every mechanism above is replaced with a simple Hoe spec. It gets its VERSION from lib/blah.rb as usual because everything is just happy ruby and there are no bootstrap steps. There are no extra dependencies or anything to take care of because inline does everything for you.

lib/blah.rb

  require 'rubygems'
  require 'inline'
  
  class Blah
    VERSION = '1.0.0'
  
    inline(:C) do |builder|
      builder.c %{
        void blah() {
          puts("hello world");
        }
      }
    end
  end

Here we have a regular ruby class Blah with a VERSION constant defined. The only thing special is the call to inline with :C as the specifier of what type of builder we want. That builder is then told to define a C function with a C function signature of void blah(). Our simple hello world is inside and pretty much the same as above. The main difference is that there is no return value, that’s because Inline knows that a C function returning void should return nil automatically. Inline deals with all arguments and return values automatically.

Another difference with the classic extension example, is I’m not forced to always run rake before I can play. I can run a test file directly, and if the C source has changed, inline deals with it automatically behind the scenes.

Conclusion

For everything I do, there is little reason to use anything but inline. There are times where you need to write a classic extension. Luckily, it isn’t much work to make it work in a classic Rake/Hoe setup.