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Ruby-FFI is a gem for programmatically loading dynamically-linked native libraries, binding functions within them, and calling those functions from Ruby code. Moreover, a Ruby-FFI extension works without changes on CRuby (MRI), JRuby, Rubinius and TruffleRuby. Discover why you should write your next extension using Ruby-FFI.


  • Intuitive DSL
  • Supports all C native types
  • C structs (also nested), enums and global variables
  • Callbacks from C to Ruby
  • Automatic garbage collection of native memory
  • Usable in Ractor: How-to-use-FFI-in-Ruby-Ractors


require 'ffi'

module MyLib
  extend FFI::Library
  ffi_lib 'c'
  attach_function :puts, [ :string ], :int

MyLib.puts 'Hello, World using libc!'

For less minimalistic and more examples you may look at:


When installing the gem on CRuby (MRI), you will need:

  • A C compiler (e.g., Xcode on macOS, gcc or clang on everything else) Optionally (speeds up installation):
  • The libffi library and development headers - this is commonly in the libffi-dev or libffi-devel packages

The ffi gem comes with a builtin libffi version, which is used, when the system libffi library is not available or too old. Use of the system libffi can be enforced by:

gem install ffi -- --enable-system-libffi        # to install the gem manually
bundle config build.ffi --enable-system-libffi   # for bundle install

or prevented by --disable-system-libffi.

On Linux systems running with PaX (Gentoo, Alpine, etc.), FFI may trigger mprotect errors. You may need to disable mprotect for ruby (paxctl -m [/path/to/ruby]) for the time being until a solution is found.

On FreeBSD systems pkgconf must be installed for the gem to be able to compile using clang. Install either via packages pkg install pkgconf or from ports via devel/pkgconf.

On JRuby and TruffleRuby, there are no requirements to install the FFI gem, and require 'ffi' works even without installing the gem (i.e., the gem is preinstalled on these implementations).


From rubygems:

[sudo] gem install ffi

From a Gemfile using git or GitHub

gem 'ffi', github: 'ffi/ffi', submodules: true

or from the git repository on github:

git clone git://
cd ffi
git submodule update --init --recursive
bundle install
rake install

Install options:

  • --enable-system-libffi : Force usage of system libffi
  • --disable-system-libffi : Force usage of builtin libffi
  • --enable-libffi-alloc : Force closure allocation by libffi
  • --disable-libffi-alloc : Force closure allocation by builtin method


The ffi library is covered by the BSD license, also see the LICENSE file. The specs are covered by the same license as ruby/spec, the MIT license.


The following people have submitted code, bug reports, or otherwise contributed to the success of this project: