LuaJIT code exhibiting black window

local ffi = require( "ffi" )
ffi.cdef[[
enum { GLFW_WINDOWED = 0x00010001 };
typedef struct _GLFWwindow* GLFWwindow;
int glfwInit( void );
GLFWwindow  glfwOpenWindow( int width, int height, int mode, const char* title, GLFWwindow share );
void glfwSwapBuffers( void );
]]

ffi.cdef[[
enum { GL_COLOR_BUFFER_BIT = 0x00004000 };
typedef unsigned int GLbitfield;
typedef float GLclampf;
void glClear (GLbitfield mask);
void glClearColor (GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
]]

glfw = ffi.load('./libglfw.so')
gl = ffi.load('libGL.so')

glfw.glfwInit()
win = glfw.glfwOpenWindow(640, 480, glfw.GLFW_WINDOWED, 'testing', nil)

gl.glClearColor(1, 1, 1, 0)
while true do
   gl.glClear(gl.GL_COLOR_BUFFER_BIT)
   glfw.glfwSwapBuffers()
end