Ruby LanguageC Extensions

Your first extension

C extensions are comprised of two general pieces:

  1. The C Code itself.
  2. The extension configuration file.

To get started with your first extension put the following in a file named extconf.rb:

require 'mkmf'


A couple of things to point out:

First, the name hello_c is what the output of your compiled extension is going to be named. It will be what you use in conjunction with require.

Second, the extconf.rb file can actually be named anything, it's just traditionally what is used to build gems that have native code, the file that is actually going to compile the extension is the Makefile generated when running ruby extconf.rb. The default Makefile that is generated compiles all .c files in the current directory.

Put the following in a file named hello.c and run ruby extconf.rb && make

#include <stdio.h>
#include "ruby.h"

VALUE world(VALUE self) {
  printf("Hello World!\n");
  return Qnil;

// The initialization method for this module
void Init_hello_c() {
  VALUE HelloC = rb_define_module("HelloC");
  rb_define_singleton_method(HelloC, "world", world, 0);

A breakdown of the code:

The name Init_hello_c must match the name defined in your extconf.rb file, otherwise when dynamically loading the extension, Ruby won't be able to find the symbol to bootstrap your extension.

The call to rb_define_module is creating a Ruby module named HelloC which we're going to namespace our C functions under.

Finally, the call to rb_define_singleton_method makes a module level method tied directly to the HelloC module which we can invoke from ruby with

After having compiled the extension with the call to make we can run the code in our C extension.

Fire up a console!

irb(main):001:0> require './hello_c'
=> true
Hello World!
=> nil

Working with C Structs

In order to be able to work with C structs as Ruby objects, you need to wrap them with calls to Data_Wrap_Struct and Data_Get_Struct.

Data_Wrap_Struct wraps a C data structure in a Ruby object. It takes a pointer to your data structure, along with a few pointers to callback functions, and returns a VALUE. The Data_Get_Struct macro takes that VALUE and gives you back a pointer to your C data structure.

Here's a simple example:

#include <stdio.h>
#include <ruby.h>

typedef struct example_struct {
  char *name;
} example_struct;

void example_struct_free(example_struct * self) {
  if (self->name != NULL) {

static VALUE rb_example_struct_alloc(VALUE klass) {
  return Data_Wrap_Struct(klass, NULL, example_struct_free, ruby_xmalloc(sizeof(example_struct)));

static VALUE rb_example_struct_init(VALUE self, VALUE name) {
  example_struct* p;

  Check_Type(name, T_STRING);

  Data_Get_Struct(self, example_struct, p);
  p->name = (char *)malloc(RSTRING_LEN(name) + 1);
  memcpy(p->name, StringValuePtr(name), RSTRING_LEN(name) + 1);

  return self;

static VALUE rb_example_struct_name(VALUE self) {
  example_struct* p;
  Data_Get_Struct(self, example_struct, p);

  printf("%s\n", p->name);

  return Qnil;

void Init_example()
  VALUE mExample = rb_define_module("Example");
  VALUE cStruct = rb_define_class_under(mExample, "Struct", rb_cObject);

  rb_define_alloc_func(cStruct, rb_example_struct_alloc);
  rb_define_method(cStruct, "initialize", rb_example_struct_init, 1);
  rb_define_method(cStruct, "name", rb_example_struct_name, 0);

And the extconf.rb:

require 'mkmf'


After compiling the extension:

irb(main):001:0> require './example'
=> true
irb(main):002:0> test_struct ="Test Struct")
=> #<Example::Struct:0x007fc741965068>
Test Struct
=> nil

Writing Inline C - RubyInLine

RubyInline is a framework that lets you embed other languages inside your Ruby code. It defines the Module# inline method, which returns a builder object. You pass the builder a string containing code written in a language other than Ruby, and the builder transforms it into something that you can call from Ruby.

When given C or C++ code (the two languages supported in the default RubyInline install), the builder objects writes a small extension to disk, compiles it, and loads it. You don't have to deal with the compilation yourself, but you can see the generated code and compiled extensions in the .ruby_inline subdirectory of your home directory.

Embed C code right in your Ruby program:

  • RubyInline (available as the rubyinline gem) create an extension automatically

RubyInline won't work from within irb

#!/usr/bin/ruby -w
    # copy.rb
    require 'rubygems'
    require 'inline'

    class Copier
    inline do |builder|
      builder.c <<END
    void copy_file(const char *source, const char *dest)
      FILE *source_f = fopen(source, "r");
      if (!source_f)
        rb_raise(rb_eIOError, "Could not open source : '%s'", source);

      FILE *dest_f = fopen(dest, "w+");
      if (!dest_f)
        rb_raise(rb_eIOError, "Could not open destination : '%s'", dest);

      char buffer[1024];

      int nread = fread(buffer, 1, 1024, source_f);
      while (nread > 0)
        fwrite(buffer, 1, nread, dest_f);
        nread = fread(buffer, 1, 1024, source_f);

C function copy_file now exists as an instance method of Copier:

open('source.txt', 'w') { |f| f << 'Some text.' }'source.txt', 'dest.txt')
puts open('dest.txt') { |f| }