Calling Native Code from Ruby

This talk gives a quick introduction into the not so well known Fiddle, which has been a part of MRI since 1.9. It allows direct interaction with C code via a Foreign Function Interface, which enables wrapping native code without compiling C extensions, making the lives of users of your projects much easier.

Transcript

1. Calling native code from Ruby
   eurucamp 2015
   Boris Bügling - @NeoNacho
   

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2. We want to use some existing
   native library or system
   functionality
   

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3. We don't want a C extension
   

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4. Because nobody got time to deal
   with user's compiler problems
   

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5. Xcodeproj
   • Part of CocoaPods
   • Interacts with Xcode projects
   • Used to include a C extension
   

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6. Xcodeproj issues
   $ bundle exec rake
   ...
   linking shared-object xcodeproj_ext.bundle
   clang: error: unknown argument: '-multiply_definedsuppress'
   [-Wunused-command-line-argument-hard-error-in-future]
   clang: note: this will be a hard error (cannot be
   downgraded to a warning) in the future
   make: *** [xcodeproj_ext.bundle] Error 1
   

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7. Xcodeproj issues
   $ gem install xcodeproj
   Building native extensions. This could take a while...
   ERROR: Error installing xcodeproj:
   ERROR: Failed to build gem native extension.
   /Users/siuying/.rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
   checking for -std=c99 option to compiler... *** extconf.rb failed ***
   Could not create Makefile due to some reason, probably lack of
   necessary libraries and/or headers. Check the mkmf.log file for more
   details. You may need configuration options.
   

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8. Xcodeproj issues
   creating Makefile
   make
   [...]
   make install
   /usr/bin/install -c -m 0755 xcodeproj_ext.bundle ./.gem.20130406-29555-zd0wy8
   ERROR: While executing gem ... (NoMethodError)
   undefined method `join' for nil:NilClass
   

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10. Fiddle!
    

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11. Somewhat exists in Ruby 1.9.x
    

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12. But you really want 2.x
    

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13. FFI for calling C from Ruby
    

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14. Uses dlopen / dlsym
    

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15. Can also be used to call Objective-
    C !
    

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16. require 'fiddle'
    image = Fiddle.dlopen(PATH)
    function = Fiddle::Function.new(image[symbol],
    parameter_types,
    return_type)
    result = function.call(a, b, c)
    

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17. Types
    • Fiddle::TYPE_VOID
    • Fiddle::TYPE_INT
    • Fiddle::TYPE_CHAR
    • Fiddle::TYPE_VOIDP
    • ...
    

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18. Use Fiddle::Handle.new to refer to
    our own binary
    

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19. real-world example
    Xcodeproj
    

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20. let's look at some code
    Using CoreFoundation
    

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21. def self.extern_image(image, symbol, parameter_types, return_type)
    symbol = symbol.to_s
    create_function = symbol.include?('Create')
    function_cache_key = "@__#{symbol}__"
    define_singleton_method(symbol) do |*args|
    unless function = instance_variable_get(function_cache_key)
    function = Fiddle::Function.new(image[symbol],
    parameter_types,
    return_type)
    instance_variable_set(function_cache_key, function)
    end
    result = function.call(*args)
    create_function ? CFAutoRelease(result) : result
    end
    end
    

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22. PATH = '/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation'
    def self.image
    @image ||= Fiddle.dlopen(PATH)
    end
    def self.extern(symbol, parameter_types, return_type)
    extern_image(image, symbol, parameter_types, return_type)
    end
    extern :CFDataGetLength, [CFTypeRef], CFIndex
    extern :CFDataGetBytePtr, [CFTypeRef], VoidPointer
    def self.CFStringToRubyString(string)
    data = CFStringCreateExternalRepresentation(NULL,
    string,
    KCFStringEncodingUTF8,
    0)
    if data.null?
    raise TypeError, 'Unable to convert CFStringRef.'
    end
    bytes_ptr = CFDataGetBytePtr(data)
    result = bytes_ptr.to_str(CFDataGetLength(data))
    result.force_encoding(Encoding::UTF_8)
    result
    end
    

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23. let's look at some code
    Using private Xcode functionality
    

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24. def self.objc_msgSend(args, return_type = CoreFoundation::VoidPointer)
    arguments = [CoreFoundation::VoidPointer, CoreFoundation::VoidPointer] + args
    Fiddle::Function.new(image['objc_msgSend'], arguments, return_type)
    end
    class CFDictionary < NSObject
    public
    def initialize(dictionary)
    @dictionary = dictionary
    end
    def plistDescriptionUTF8Data
    selector = 'plistDescriptionUTF8Data'
    return nil unless NSObject.respondsToSelector(@dictionary, selector)
    plistDescriptionUTF8Data = CFDictionary.objc_msgSend([])
    plistDescriptionUTF8Data.call(
    @dictionary,
    CoreFoundation.NSSelectorFromString(CoreFoundation.RubyStringToCFString(selector)))
    end
    end
    

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25. Recap
    • C extensions have a bad UX
    • Fiddle provides a way to use native code dynamically
    • Which eliminates all the compilation hassle
    • We can even call Objective-C if we want
    

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26. @NeoNacho
    boris@contentful.com
    http://buegling.com/talks
    

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