Category Archives: Jruby

Create ruby/jruby bindings of C++/C with ffi

Let’s start with the basic. What are bindings? It’s a way to call low level libraries (normally in C or C++) from another language (high level one, like ruby, java or whatever). This simple step requires two important things: 1) first, to know how we should call the method itself and 2) second, how to map the type from the high level language to C primitives types. To accomplish this, we have what is called foreign function interface which trivialize this task.

In ruby/jruby world we are gonna need ffi gem (jruby also has a compatible ffi gem), besides that the most important part is to have a clear interface in C. You can do bindings for a C++ library if you create a C interface first (because the bindings between C++ and C are free, the compiler knows how to do it by itself). So let’s cut the crap and write some code

First our C++ library which can be this few lines or thousands:

class awesome_object {
    awesome_object(int a, int b) : a_(a), b_(b) {}
    int awesome_method() {
      return a_ + b_;
    int a_, b_;

Now the C bindings which will be our façade for our c++ library:

extern "C" {
  typedef awesome_object_p void *;
  awesome_object_p create_awesome_object(int a, int b) {
    return static_cast<void *>(new awesome_object(a, b));
  void delete_pointer_to_awesome_object(awesome_object_p p) {
    delete static_cast<awesome_object *>(p);
  int awesome_method(awesome_object_p p) {
    awesome_object o = *static_cast<awesome_object *>(p);
    return o.awesome_method();

Now ruby code that use ffi gem to be able to call the methods defined in C:

require 'ffi'
module AwesomeObject
  include FFI
  extend FFI::Library
  ffi_lib "" # .dll if you are in windows, it doesn't matter the OS
  attach_function "delete_pointer_to_awesome_object", "delete_pointer_to_awesome_object", [:pointer], :void
  attach_function "awesome_method", "awesome_method", [:pointer], :int
  attach_function "create_awesome_object", "create_awesome_object", [:int, :int], :pointer

With this you can use your C++ library from ruby code (I’d recommend package everything as a gem) just like this:

class MyTest
  include AwesomeObject
  def test!
    object = create_awesome_object(11, 31)
    awesome_method object
    delete_pointer_to_awesome_object object

So, if you have read until now, probably you just want something working, say no more. Download this tar.gz, and see all this by yourself. In the tar you have the code split into c/h cpp/hpp files as it should be, in the post I’ve put all together to simplify things. Just execute make test and if you have ruby, ffi gem and g++ installed on your system, you’ll see something like:

$ make test
g++ -g -Wall -fPIC -c awesome_object.cpp
g++ -g -Wall -fPIC -c awesome.c
g++ -g -Wall -fPIC -shared -o awesome.o awesome_object.o
ruby test.rb

A new gem: rubygems is it jruby?

A few days ago after seeing this tweet, I’ve decided to migrate the gem rubygems-isit19 to jruby. Thanks to this, when you install another gem you’ll be able to read about compability of this gem with jruby, the information is fetched from website.

Once you have installed the gem you will have two main features, it’s easier if I just show you. First the messages when you install gems:

$ jgem install daemons
daemons 1.0.10 probably might not work, blaxter says 1.0 fails
Update with your experiences!
Successfully installed daemons-1.0.10
1 gem installed
$ jgem install cucumber
cucumber 0.3.97 might work, 100% say works on jruby
Update with your experiences!
Successfully installed cucumber-0.3.97
1 gem installed
$ jgem install faker
faker 0.3.1 is 100% verified jruby
Update with your experiences!
Successfully installed faker-0.3.1
1 gem installed

And also we have an explicit gem command:

$ jgem isitjruby faker
faker 0.3.1:
    Blaxter says 0.3.1 works on GNU/Linux
        it works perfectly, and it passes all test.
$ jgem isitjruby daemons
daemons 1.0.10:
    blaxter says 1.0 fails on GNU/Linux
        Daemons use fork, jruby doesn't support fork, so it doesn't work and
        never will

And that’s it. The code is on github be free of fork me or open any issues. The important thing is populate with relevant information for benefit of all jruby community. Oh I forgot to tell you how to install it, very easy, I’ve uploaded the gem to gemcutter so you can install it with something like

$ jgem install rubygems-isitjruby -s

It’s also in github, but I think (and I hope) gemcutter will be the next rubyforge, so better gemcutter :).