opengl-bindings : glDebugMessageCallback のコールバック関数としてrubyのブロックを渡す例。Fiddle::Closure::BlockCaller を使うと手短に書けました。

debug_output.rb

require 'fiddle'

=begin
# https://www.opengl.org/sdk/docs/man4/html/glDebugMessageCallback.xhtml
typedef void (APIENTRY *DEBUGPROC)(GLenum source,
            GLenum type,
            GLuint id,
            GLenum severity,
            GLsizei length,
            const GLchar *message,
            void *userParam);
=end

cb_args = [-Fiddle::TYPE_INT, -Fiddle::TYPE_INT, -Fiddle::TYPE_INT, -Fiddle::TYPE_INT, Fiddle::TYPE_INT, Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP]
cb_retval = Fiddle::TYPE_VOID

$debug_log_callback = Fiddle::Closure::BlockCaller.new( cb_retval, cb_args, Fiddle::Function::STDCALL ) { |source, type, id, severity, length, message, userParam|
  p source, type, id, severity, length, message.to_s, userParam
}

#
# ...
#

glfwInit()
glfwWindowHint( GLFW_OPENGL_DEBUG_CONTEXT, GL_TRUE )
window = glfwCreateWindow( 640, 480, "OpenGL Debug Context Test", nil, nil )

#
# ...
#

# for OpenGL 4.3
glDebugMessageCallback( $debug_log_callback, nil )
glEnable( GL_DEBUG_OUTPUT_SYNCHRONOUS )

# for OpenGL 3
=begin
if OpenGL.check_extension( 'GL_ARB_debug_output' )
  OpenGL.setup_extension( 'GL_ARB_debug_output' )
  glDebugMessageCallbackARB( $debug_log_callback, nil )
  glEnable( GL_DEBUG_OUTPUT_SYNCHRONOUS_ARB )
end
=end

# for OpenGL ES
=begin
if OpenGL.check_extension('GL_KHR_debug')
  OpenGL.setup_extension('GL_KHR_debug')
  glDebugMessageCallback( $debug_log_callback, nil )
  glEnable( GL_DEBUG_OUTPUT_SYNCHRONOUS )
end
=end