This talk shows how, using native extensions and a C library called libclipboard, we can access a user's clipboard from Ruby code.

1. Writing a Gem
with native extensions
Tristan Penman
Melbourne Ruby Meetup, August 2018
A whirlwind tour

2.  What are they?
 Native extensions allow you to extend the functionality
available in Ruby by writing code in natively compiled
languages such as C, C++, or even Rust.
 This talk shows how native extensions can be used to
interact with a third-party library written in C.
 The final code from this talk can be found on Github:
 https://github.com/tristanpenman/simple-clipboard
Native extensions
A brief introduction

3. Native extensions
Examples
• Byebug - a debugger for Ruby, that uses Ruby's TracePoint API
for execution control and the Debug Inspector API for call
stack navigation. Written as a C extension for speed.
• nokogiri - an HTML and XML parser. Uses native libraries for
speed and ensure standards compliance.
• RMagick - bindings for the ImageMagick image manipulation
library.
• sqlite3 - bindings for the SQLite3 database engine.

4. libclipboard
 libclipboard is a cross-platform clipboard library
 Simple C-based API:
 clipboard_new - create a context through which to access
the user's clipboard
 clipboard_free - free any memory allocated by
clipboard_new
 clipboard_text - read the contents of the clipboard as text
 clipboard_set_text - replace the contents of the clipboard
with new text

5. libclipboard
git clone https://github.com/jtanx/libclipboard
cd libclipboard
mkdir build
cd build
cmake ..
make -j4
sudo make install
 Must be built from source
 Requires git, cmake, and a C++ compiler tool-chain:

6. simple_clipboard
 We will use a native extension to wrap libclipboard with a
Module that we can use in Ruby code:
module SimpleClipboard
def get_text
#TODO: Return current contents ofclipboard,ornil
raise NotImplementedError
end
def set_text(new_text)
#TODO: Update clipboard; return previous contents, ornil
raise NotImplementedError
end
end

7. Extending Ruby using C
 To create our native extension we need two files:
 extconf.rb
 simple_clipboard.c

8. Extending Ruby using C
require 'mkmf'
$LOCAL_LIBS << '-lclipboard'
if RUBY_PLATFORM =~ /darwin/
$LDFLAGS <<' -framework AppKit'
end
create_header
create_makefile 'simple_clipboard/simple_clipboard'
extconf.rb

9. Extending Ruby using C
simple_clipboard.c (1/2)
#include <ruby.h>
#include <libclipboard.h>
VALUEget_text(VALUE _self) {
VALUE result = Qnil;
char *text = clipboard_text(cb);
if (text) {
result =rb_str_new(text, strlen(text));
free(text);
}
return result;
}
// continued on next slide...

10. Extending Ruby using C
simple_clipboard.c (2/2)
// continued from previous slide
VALUEset_text(VALUE _self, VALUE str) {
// omitted, since it is similar to get_text
}
voidInit_simple_clipboard() {
VALUE m =rb_define_module("SimpleClipboard");
rb_define_module_function(m, "get_text", get_text, 0);
rb_define_module_function(m, "set_text", set_text, 1);
}

11.  Run 'ruby extconf.rb' to generate:
 Header file (extconf.h, which is redundant in this case)
 Makefile
 (also mkmf.log)
 Run ’make’ to compile the extension
 On Mac OS X, creates a .bundle file
 On Linux, creates a .so
 On Windows, creates a .dll
Extending Ruby using C

12. 2.3.3:001> require './simple_clipboard'
=>true
2.3.3:002> SimpleClipboard.get_text
=>"Extending Ruby using C"
2.3.3:003> SimpleClipboard.set_text "Hello world"
=>"Extending Ruby using C"
2.3.3:004> SimpleClipboard.get_text
=>"Hello world"
Extending Ruby using C
IRB session

13. Extending Ruby using C
In a nutshell
 For an extension named 'xyz' we need:
 An extconf.rb file and source file called 'xyz.c'
 In the 'xyz.c' file, a function called Init_xyz
 A native extension is free to:
 define modules, classes and methods that operate on
Ruby values, via an opaque C datatype 'VALUE'
 Call existing Ruby code
 Crash the current process
 And more generally, call 'undefined behavior'

14. How to include C code in a gem
 Okay, safety be damned, we want performance…
or legacy functionality…
or something.

15.  lib/
 simple_clipboard/
 version.rb
 simple_clipboard.rb
 simple_clipboard.gemspec
How to include C code in a gem
Layout without a native extension

16. How to include C code in a gem
Layout with a native extension
 ext/
 simple_clipboard/
 extconf.rb
 simple_clipboard.c
 lib/
 simple_clipboard/
 version.rb
 simple_clipboard.rb
 simple_clipboard.gemspec

17. #Boilerplate omitted
Gem::Specification.new do |s|
s.name ='simple_clipboard'
s.version =SimpleClipboard::VERSION
s.date ='2018-07-24'
s.summary ='Simple clipboardexample gem'
s.authors =['TristanPenman']
s.email ='tristan@tristanpenman.com'
s.licenses =['MIT']
#continued on next slide...
How to include C code in a gem
simple_clipboard.gemspec (1/2)

18. #continued from previous slide...
s.extensions =['ext/simple_clipboard/extconf.rb']
#Tell bundler where to findthe code forour gem
s.require_paths =['lib']
#Files toinclude in bundle
s.files =['ext/simple_clipboard/simple_clipboard.c',
'lib/simple_clipboard.rb',
'lib/simple_clipboard/version.rb']
end
How to include C code in a gem
simple_clipboard.gemspec (2/2)

19. How to include C code in a gem
 Run 'gem build simple_clipboard.gemspec':
 Does not actually compile native extension
 Creates 'simple_clipboard-0.0.1.gem'
 Run 'gem install simple_clipboard-0.0.1.gem'
 This is when bundler will build the native extension
 And this why, when things go wrong while building gems
such as nokigiri, that you can get very complex error
messages

20. $ gem install simple_clipboard-0.0.1.gem
How to include C code in a gem
Example
Building native extensions. This could take a while...
Successfully installed simple_clipboard-0.0.1
Parsing documentation forsimple_clipboard-0.0.1
Done installing documentation for simple_clipboard after 0 seconds
1 gem installed

21. 2.3.3 :001 >require 'simple_clipboard'
=> true
2.3.3 :002 > SimpleClipboard.get_text
=> "Extending Ruby using C"
2.3.3 :003 > SimpleClipboard.set_text "Hello world"
=> "Extending Ruby using C"
2.3.3 :004 > SimpleClipboard.get_text
=> "Hello world"
Extending Ruby using C
IRB session

22. Testing native extensions
require "bundler/gem_tasks"
require "rspec/core/rake_task"
require 'rake/extensiontask'
desc "simple_clipboard unit tests"
RSpec::Core::RakeTask.new(:spec) do|t|
t.pattern ="spec/*_spec.rb"
t.verbose =true
End
#continued on next slide...
Rakefile (1/2)

23. Testing native extensions
#continued fromprevious slide
Rake::ExtensionTask.new do|ext|
ext.name ='simple_clipboard'
ext.source_pattern ="*.{c,h}"
ext.ext_dir ='ext/simple_clipboard'
ext.lib_dir ='lib/simple_clipboard'
ext.gem_spec =
Gem::Specification.load('simple_clipboard.gemspec')
end
#Default is tocompile native extension then runtests
task :default =>[:compile, :spec]
Rakefile (2/2)

24. Testing native extensions
1. Run 'rake' to compile and run tests
2. Run 'rake compile' to only compile native extension
 Compiles the native extension, then copies
simple_clipboard.[bundle|so|dll] file into 'lib/simple_clipboard'
3. Run 'rake spec' to only run tests
 Assumes that native extension (e.g. simple_clipboard.bundle) has
already been copied to 'lib/simple_clipboard' directory
Rake tasks

25. Resources
• Useful reference implementation of a
Gem with a native extension:
https://github.com/neilslater/ruby_nex_c
• Core documentation:
https://ruby-doc.org/core-
2.3.3/doc/extension_rdoc.html
(Beware the version number in this link)
• Pat Shaughnessy’s book:
Ruby Under a Microscope

26. Resources
• RubyGems documentation
https://guides.rubygems.org/gems-with-extensions/
• Aaron Bedra's Extending Ruby guide
http://aaronbedra.com/extending-ruby
• Chris Lalancette's in-depth series on writing Ruby extensions
in C, which covers numerous topics:
http://clalance.blogspot.com/2011/01/writing-ruby-
extensions-in-c-part-1.html
(12 parts in total)

27. Thanks for listening