ruby-ffi (https://github.com/ffi/ffi) を使ってDLLの関数を呼ぶテスト。 RMath3D (https://github.com/vaiorabbit/rmath3d) の .dylib をビルド→その中にある RVec4SetElements をRubyから呼ぶところまで。

rmath3d_ffi.rb

require 'ffi'

module RMath3D
  extend FFI::Library

  # FFI
  case FFI::Platform::OS
  when "darwin"
    # $ clang -dynamiclib RMtx3.c RMtx4.c RVec3.c RVec4.c RQuat.c -o libRMath3D.dylib
    ffi_lib "libRMath3D.dylib"
  when "linux"
    ffi_lib "libRMath3D.so"
  when "windows"
    ffi_lib "libRMath3D.dll"
  else
    raise RuntimeError, "RMath3D: Unknown OS"
  end

  # typedef
  typedef :double, :rmreal

  # Class
  class RVec4 < FFI::Struct
    layout  :x, :rmreal,
            :y, :rmreal,
            :z, :rmreal,
            :w, :rmreal

    def x() return self[:x] end
    def y() return self[:y] end
    def z() return self[:z] end
    def w() return self[:w] end

    def x=(value) self[:x] = value end
    def y=(value) self[:y] = value end
    def z=(value) self[:z] = value end
    def w=(value) self[:w] = value end
  end

  # method
  attach_function  :native_RVec4SetElements, :RVec4SetElements, [:pointer, :rmreal, :rmreal, :rmreal, :rmreal], :void

  # function
  module_function
  def rvec4_set_elements( vec4, x, y, z, w )
    native_RVec4SetElements( vec4.to_ptr, x, y, z, w )
  end

end


if __FILE__ == $0
  v = RMath3D::RVec4.new
  p v[:x], v.x
  v.x = 1.0
  p v[:x], v.x

  RMath3D::rvec4_set_elements( v, 2, 3, 4, 5 )
  p v.x, v.y, v.z, v.w
end