using http://cast.rubyforge.org to generate https://github.com/ffi/ffi decls

cast_ffi.rb

#only dealing with opaque types here; no struct gen
require 'cast'

CallbackTypes = {}

def ffi_type(t)
  if t.Pointer?
    if t.type.Char?
      :string
    elsif t.type.CustomType? and CallbackTypes[t.type.name] 
      t.type.name
    else
      :pointer
    end
  elsif t.Char?
    :string
  elsif t.CustomType? and t.name == "Bool"
    :bool
  else
    t
  end
end

header = ARGV[0]
libname = ARGV[1] || File.basename(header.gsub(".h", ""))

code = IO.popen("cpp #{header}").readlines.select { |line| not line.match(/^#/) }
tree = C::Parser.new.parse(code)

indent = " " * 4
ffi = { :functions => [], :enums => [], :constants => [] }

tree.entities.select {|n| n.Declaration? }.each do |node|
  if node.type.Enum?
    if typedef_node = [node, node.prev].select { |n| n.typedef? }.first
      name = typedef_node.declarators.first.name
      ffi[:enums] << "#{indent}enum :#{name}, ["
      node.type.members.each do |enum|
        e = "#{indent}  :#{enum.name},"
        e << " #{enum.val}," unless enum.val.nil?
        ffi[:enums] << e
      end
      ffi[:enums] << "#{indent}]"
    else
      node.type.members.each do |enum|
        ffi[:constants] << "#{indent}#{enum.name} = #{enum.val}"
      end
    end
  elsif node.typedef?
    node.declarators.each do |decl|
      CallbackTypes[decl.name] = true if decl.type.Function?
    end
  else
    node.declarators.each do |decl|
      if decl.type.Function?
        params = decl.type.params.collect { |p| ":#{ffi_type(p.type)}" }
        if decl.type.var_args?
          params << ':varargs'
        end
        ffi[:functions] << "#{indent}attach_function :#{decl.name}, [#{params.join(", ")}], :#{ffi_type(node.type)}"
      end
    end
  end
end

ffi.each do |name,code|
  #... echo code > name.rb
end