Calling Native Library Code from Ruby


If you have encountered a situation where you want to wrap C, C++ library in Ruby and create a gem that you can use anywhere, then ffi-gem is for you. Without this gem you have to write your own C extension for Ruby to wrap an existing C library (if it's not already available). This is not easy, especially if you are not familiar with C.

What is FFI?

FFI stands for Foreign Function Interface. A foreign function interface is the popular name for the interface that allows code written in one language to call code written in another language.

Advantages of FFI

  • You often don't have to write C/C++ code.
  • FFI will run on MRI, Rubinius and JRuby.
  • The end users don't need development headers or a C/C++ compiler.

For a discussion on when it is not the right tool, read the Detecting Faces with Ruby: FFI in a Nutshell article.

What is libffi?

The libffi library provides a portable, high level programming interface to various calling conventions. This allows a programmer to call any function specified by a call interface description at run-time. The libffi library is useful to anyone trying to build a bridge between interpreted and natively compiled code. Some notable users include:

Java Native Access (JNA) - the JNI-free way to call native code from Java.
Ruby-FFI - a Foreign Function Interface extension for Ruby.

What is Ruby-FFI?

Ruby-FFI is a ruby extension for programmatically loading dynamic libraries, binding functions within them, and calling those functions from Ruby code. Moreover, a Ruby-FFI extension works without changes on Ruby and JRuby.

Simple Example

Install the FFI gem.

$ gem install ffi

Let's now write a program to use a function from LIBC. By the way, LIBC is the GNU C Library. Our program will print the process ID of the current process. LIBC provides a getpid function that we can use in our program through FFI. According to the GNU docs, this function returns an integer and does not take any parameter.

Function: pid_t getpid (void)

We use that fact in the attach_function, the first parameter is the symbol of the function inside the native library, the second parameter is empty since it does not take any input parameter. The third parameter is int, the return value from the function.

require 'ffi'

module Foo
  extend FFI::Library

  ffi_lib FFI::Library::LIBC
  attach_function :getpid, [ ], :int
puts "My pid=#{Foo.getpid}"
puts $$

My pid=18185

We first require the FFI gem. This will load and initialize the FFI library. We then define a module that extends the FFI::Library module. This attaches the native functions to the Foo module. The ffilib call can take multiple arguments to load native libraries. In this case we are loading just one native library, namely FFI::Library::LIBC. Now we can use the methods from this module to bind to the LIBC library. The attachfunction will link the native C functions into the Foo module. In our example, it attaches the getpid function to our module. Now the getpid function can be accessed as a static function of our module. The value returned by the C function is the same as the Ruby global variable $$ that returns the process ID.

Example 2

The getuid function returns the real user ID of the process. Let's write a program to print this value.

require 'ffi'

module RubyFFI
  extend FFI::Library
  attach_function :getuid, [], :uint

You will get the error:

LoadError: no library specified
    from /home/zepho/.rvm/gems/ruby-2.1.4@ffi/gems/ffi-1.9.6/lib/ffi/library.rb:162:in `ffi_libraries'
    from /home/zepho/.rvm/gems/ruby-2.1.4@ffi/gems/ffi-1.9.6/lib/ffi/library.rb:240:in `attach_function'
    from (irb):4:in `<module:RubyFFI>'
    from (irb):2
    from /home/zepho/.rvm/rubies/ruby-2.1.4/bin/irb:11:in `<main>'

We know that getuid function is provided by the GNU C library. We need to specify this before the attach_function.

require 'ffi'

module RubyFFI
  extend FFI::Library
  ffi_lib FFI::Library::LIBC

  attach_function :getuid, [], :uint

On my machine the output is :

Uid is 1000

Example 3

Let's write a program to call the standard C library function puts().

module MyLib
  extend FFI::Library
  ffi_lib 'c'

  attach_function :puts, [ :string ], :int

MyLib.puts 'Hello, World using libc!'
Hello, World using libc!
 => 25

Example 4

The getlogin identifies who logged in. Let's write a program to print who is logged in.

require 'ffi'
module Foo
  extend FFI::Library
  ffi_lib FFI::Library::LIBC

  attach_function :getlogin, [ ], :string
puts "getlogin=#{Foo.getlogin}"


Example 5

Let's write a program to wrap the power math function in Ruby.

require 'ffi'

module FfiMathTest
  extend FFI::Library
  ffi_lib 'c'
  ffi_lib 'm'
  attach_function :pow, [ :double, :double ], :double
puts FfiMathTest.pow(2.2, 10)


In this example we are using the ffi_lib 'm' that loads the pow(double x, double, y) C library function in math.h that returns x raised to the power of y.

Exposing Native Libraries to Ruby

We now know enough to write programs using FFI to wrap the existing native code. What if we want to write Ruby wrapper for our custom native code? Let's know write a simple c program that calculates the power, squareroot and factorial. Here is the simplemath.c

#include <math.h>

double power(double base, double power) {
  return pow(base, power);

double square_root(double x) {
  return sqrt(x);

long ffi_factorial(int max) {
  int i = max, result = 1;
  while (i >= 2) {
    result *= i--;
  return result;

Let's compile this as follows:

$gcc -shared -Wl,-soname,libsimplemath -o simple_math.c -lm

You now see the .so file:

~/projects/ffi$ ll
total 24
drwxrwxr-x 2 zepho zepho 4096 Nov  5 23:26 ./
drwxr-xr-x 5 zepho zepho 4096 Nov  5 23:15 ../
-rwxrwxr-x 1 zepho zepho 6781 Nov  5 23:19*
-rw-rw-r-- 1 zepho zepho  262 Nov  5 23:18 simple_math.c

Create simple_math.rb that will wrap the C code.

require 'ffi'

class FFIMath
  extend FFI::Library

  ffi_lib "./"

  functions = [
   [:power, [:double, :double], :double],
   [:square_root, [:double], :double],
   [:ffi_factorial, [:int], :long]

  functions.each do |func|
    rescue Object => e
      puts "Could not attach #{func}, #{e.message}"

puts FFIMath.power(3, 5)
puts FFIMath.square_root(81)
puts FFIMath.ffi_factorial(10)

If you don’t specify ‘./’ in the ffi_lib call, you will get the error:

~/projects/ffi$ ruby simple_math.rb
/home/zepho/.rvm/gems/ruby-2.1.4@ffi/gems/ffi-1.9.6/lib/ffi/library.rb:133:in `block in ffi_lib': Could not open library '': cannot open shared object file: No such file or directory (LoadError)
    from /home/zepho/.rvm/gems/ruby-2.1.4@ffi/gems/ffi-1.9.6/lib/ffi/library.rb:100:in `map'
    from /home/zepho/.rvm/gems/ruby-2.1.4@ffi/gems/ffi-1.9.6/lib/ffi/library.rb:100:in `ffi_lib'
    from simple_math.rb:6:in `<class:FFIMath>'
    from simple_math.rb:3:in `<main>'

Let's look at another example. Create ffi_test.c with the following contents:

#include <stdlib.h>
#include <math.h>

int ffi_pow(int a, int n) {
  return pow(a,n);

int ffi_factorial(int max) {
  int i=max, result=1;
  while (i >= 2) { result *= i--; }
  return result;

int ffi_fibonacci(int n) {
  int a = 1, b = 1, c, i;
  if (n == 0) {
    return 0;

  for (i= 3; i <= n; i++) {
    c = a + b;
    c = b;
    b = c;

  return b;

Compile it :

gcc -c -fPIC ffi_test.c
gcc -shared -o ffi_test.o

Create the Ruby wrapper ffi_test.rb.

require 'ffi'

module FfiCustomTest
  extend FFI::Library
  ffi_lib 'c'
  ffi_lib './'

  attach_function :ffi_pow, [ :int, :int ], :int
  attach_function :ffi_factorial, [ :int ], :int
  attach_function :ffi_fibonacci, [ :int ], :int

puts FfiCustomTest.ffi_factorial(5)
puts FfiCustomTest.ffi_fibonacci(9)
puts FfiCustomTest.ffi_pow(2, 10)

~/projects/ffi$ ruby ffi_test.rb


  1. If you are using rvm, install jruby and rubinius.
  2. Run the code examples above on jruby, rubinius and mri.


In this article we wrapped the native C code from standard library using ffi gem. We also wrapped our own C code so that we know how to wrap code that is developed within our company. If you have suffered due to gem installation problems due to native extensions, the ffi gem provides an easy way to share your native libraries by exposing it to your ruby programs via Ruby wrapper. You can also package your native libraries as a gem using this technique.


Bridging MRI JRuby Rubinius with FFI
Calling C, CPP from Ruby
libffi on github
Extending Ruby with C
FFI Ruby Docs
ffi/samples directory in ffi github repo

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