One of the largest problems plaguing Ruby implementations (and plaguing some other language implementations, so I hear from my Pythonista friends) is the ever-painful story of "extensions". In general, these take the form of a dynamic library, usually written in C, that plugs into and calls Ruby's native API as exposed through ruby.h and libruby. Ignoring for the moment the fact that this API exposes way more of Ruby's internals than it should, extensions present a very difficult problem for other implementations:

Do we support them or not?

In many cases, this question is answered for us; most extensions require access to object internals we can't expose, or can't expose without extremely expensive copying back and forth. But there's also a silver lining: the vast majority of C-based extensions exist solely to wrap another library.

Isn't it obvious what's needed here?

This problem has been tackled by a number of libraries on a number of platforms. On the JVM, there's Java Native Access (JNA). On Python, there's ctypes. And even on Ruby, there's the "dl" stdlib, wrapping libdl for programmatic access to dynamic libraries. But dl is not widely used, because of real or perceived bugs and a rather arcane API. Something better is needed.

Enter FFI.

FFI stands for Foreign Function Interface. FFI has been implemented in various libraries; one of them, libffi, actually serves as the core of JNA, allowing Java code to load and call arbitrary C libraries. libffi allows code to load a library by name, retrieve a pointer to a function within that library, and invoke it, all without static bindings, header files, or any compile phase.

In order to address a need early in Rubinius's dev cycle, Evan Phoenix came up with an FFI library for Rubinius, wrapping the functionality of libffi in a friendly Ruby DSL-like API.

A simple FFI script calling the C "getpid" function:

require 'ffi'

module GetPid
  extend FFI::Library

  attach_function :getpid, [], :uint
end

puts GetPid.getpid

Because JRuby already ships with JNA, and because FFI could fulfill the C-extension needs of almost all Ruby users, we endeavored to create a compatible implementation. And by we I mean Wayne Meissner.

Wayne is one of the primary maintainers of JNA, and has recently spent time on a new higher-performance version of it called JFFI. Wayne also became a JRuby committer this spring, and perhaps his most impressive contribution to date is a full FFI library for JRuby, based on JNA (eventually JFFI, once we migrate fully) and implementing the full set of what we and Evan agreed would be "FFI API 1.0". We shipped the completed FFI support in JRuby 1.1.4.

The "Passwd" and "Group" structures for functions like 'getpwuid':

module Etc
  class Passwd < FFI::Struct
    layout :pw_name, :string, 0,
           :pw_passwd, :string, 4,
           :pw_uid, :uint, 8,
           :pw_gid, :uint, 12,
           :pw_dir, :string, 20,
           :pw_shell, :string, 24
  end
  class Group < FFI::Struct
    layout :gr_name, :string, 0,
           :gr_gid, :uint, 8
  end
end

In JRuby 1.1.5, we've taken another step forward with the API, adding support for callbacks. How would you represent a callback you pass into a C function from Ruby? How else! As a block!

Binding and calling "qsort" with an array of integers:

require 'ffi'

module LibC
  extend FFI::Library
  callback :qsort_cmp, [ :pointer, :pointer ], :int
  attach_function :qsort, [ :pointer, :int, :int, :qsort_cmp ], :int
end

p = MemoryPointer.new(:int, 2)
p.put_array_of_int32(0, [ 2, 1 ])
puts "Before qsort #{p.get_array_of_int32(0, 2).join(', ')}"
LibC.qsort(p, 2, 4) do |p1, p2|
  i1 = p1.get_int32(0)
  i2 = p2.get_int32(0)
  i1 < i2 ? -1 : i1 > i2 ? 1 : 0
end
puts "After qsort #{p.get_array_of_int32(0, 2).join(', ')}"

But what good is having such a library if it doesn't run everywhere? Up until recently, only Rubinius and JRuby supported FFI, which made our case for cross-implementation use pretty weak. Even though we were getting good use out of FFI, there was no motivation for anyone to use it in general, since the standard Ruby implementation had no support.

That is, until Wayne pulled another rabbit out of his hat and implemented FFI for C Ruby as well. The JRuby team is proud to announce a wholly non-JRuby library: FFI is now available on Ruby 1.9 and Ruby 1.8.6/7, in addition to JRuby 1.1.4+ and Rubinius (though Rubinius does not yet support callbacks).

Session showing installation and use of FFI in C Ruby:

$ sudo gem install ffi
Password:
Building native extensions.  This could take a while...
Successfully installed ffi-0.1.1
1 gem installed
Installing ri documentation for ffi-0.1.1...
Installing RDoc documentation for ffi-0.1.1...
[headius @ cnutter:~]
$ irb
>> require 'ffi'
=> true
>> module RubyFFI
>> extend FFI::Library
>> attach_function :getuid, [], :uint
>> end
=> #<FFI::Invoker:0x1fe8c>
>> puts RubyFFI.getuid
501
=> nil
>> 

Our hope with JRuby's support of FFI and our release of FFI for C Ruby is that we may finally escape the hell of C extensions. Next time you need to call out to a C library, don't write a wrapper shim in C! Write it using FFI, and it will work across implementations without recompile.

Here's some links to docs on FFI. As with most open-source projects, documentation is a little light right now, but hopefully that will change.

Calling C from JRuby
Rubinius's Foreign Function Interface
On the Rubinius FFI

A key feature that's not well documented is the use of FFI's templating system to generate bindings based on the current platform's header files. Here's a sample from the "Etc" module above.

Etc module template, showing how to pull in header files and inspect a struct definition:

module Etc
  class Passwd < FFI::Struct
    @@@
    struct do |s|
      s.include "sys/types.h"
      s.include "pwd.h"

      s.name "struct passwd"
      s.field :pw_name, :string
      s.field :pw_passwd, :string
      s.field :pw_uid, :uint
      s.field :pw_gid, :uint
      s.field :pw_dir, :string
      s.field :pw_shell, :string
    end
    @@@
  end
  class Group < FFI::Struct
    @@@
    struct do |s|
      s.include "sys/types.h"
      s.include "grp.h"

      s.name "struct group"
      s.field :gr_name, :string
      s.field :gr_gid, :uint
    end
    @@@
  end
end

As more docs come to my attention, I'll update this post and add links to the JRuby wiki page. For those of you interested in the Ruby FFI project itself, check out the Ruby FFI project on Kenai. And feel free to hunt down any of the JRuby team, including Wayne Meissner, on the JRuby mailing lists or in #jruby on FreeNode.

Update: Wayne has posted a follow-up with more details here: More on Ruby FFI