binding_of_caller, pure Fiddle version (no compiler needed)

binding_of_caller-fiddler.rb

#!/usr/bin/env ruby

# =============================================================================
# UPDATE as of June 15, 2015: See the bindings gem at:
#
# https://rubygems.org/gems/bindings
# =============================================================================

# =============================================================================
# Quick proof-of-concept of an Fiddle-based binding_of_caller (no compiling!)
#
# Author: Steve Shreeve <steve.shreeve@gmail.com>
#  Basis: binding_of_caller and ffi gems, along with the standard ruby fiddle source code
#   Date: 29 May 2015
#
#  Legal: MIT License
# =============================================================================

# NOTE: Fiddle requires a dynamic library, it cannot be a static library
#
# To install a shared libruby using ruby-install, use this:
#
#   ruby-install ruby 2.2.2 -- --enable-shared --disable-install-doc
#
# To nuke everything if you're using ruby-install, use this:
#
#   rm -rf ~/.gem/ruby/2.2.2 ~/.rubies/ruby-2.2.2 ~/src/ruby-2.2.2

require 'fiddle/import'

module Fiddle
  module Binding
    extend Fiddle::Importer

    dlload Fiddle.dlopen(nil)

    extern "void* rb_debug_inspector_open(void*, void*)"
    bind("void* callback(void*, void*)") {|ptr, _| DebugStruct.new(ptr).contexts }

    DebugStruct = struct [
      "void* thread",
      "void* frame",
      "void* backtrace",
      "void* contexts",
      "long  backtrace_size"
    ]

    class << self
      def callers
        list = rb_debug_inspector_open(self['callback'], nil).to_value
        list.drop(4).map {|ary| ary[2]} # grab proper bindings
      end
    end
  end
end

class ::Binding
  def of_caller(n)
    c = ::Fiddle::Binding.callers
    n < c.size or raise "No such frame, gone beyond end of stack!"
    c[n]
  end
end

# ==[ Show that it works... ]==

outer = 10

class Z
  def z
    u = 10
    A.new.a
  end
end

class A
  def a
    y = 10
    B.new.b
  end
end

class B
  def b
    x = 10
    puts binding.of_caller(0).eval('local_variables')
    puts binding.of_caller(1).eval('local_variables')
    puts binding.of_caller(2).eval('local_variables')
    puts binding.of_caller(3).eval('local_variables')
    # puts binding.of_caller(400).eval('local_variables') # should throw an Exception
  end
end

Z.new.z

__END__

# output:
# => x
# => y
# => u
# => outer
# Exception