Question

While making an OpenGL toolkit using GLFW and Cython I stumbled upon a very, very weird issue. I created the following file pxd file (quite large, so I gisted it):

https://gist.github.com/1441970

Next I have this wrapping code (extremely simplified to show the core of the problem).

pygrafix.window module:

from pygrafix.c_headers.glfw cimport * # this is the pxd file

glfwInit()

_window = None

cdef void _mouse_scroll_callback_handler(int pos):
    if _window._mouse_scroll_callback:
        _window._mouse_scroll_callback(_window, pos)

cdef class Window:
    cdef public object _mouse_scroll_callback

    def __cinit__(self):
        global _window
        self._mouse_scroll_callback = None

        _window = self

    def __init__(self, int width = 0, int height = 0):
        glfwOpenWindow(width, height, 0, 0, 0, 0, 0, 0, GLFW_WINDOW)
        glfwSetMouseWheelCallback(<GLFWmousewheelfun> &_mouse_scroll_callback_handler)
        print("TEST LOCATION ONE")

    def is_open(self):
        return glfwGetWindowParam(GLFW_OPENED)

    def flip(self):
        glfwSwapBuffers()

    def set_mouse_scroll_callback(self, func):
        self._mouse_scroll_callback = func

And this main file:

from pygrafix import window

window = window.Window(800, 600)
print("TEST LOCATION TWO")

def on_scroll(window, pos):
    print(pos)

window.set_mouse_scroll_callback(on_scroll)

while window.is_open():
    window.flip()

And finally I compile it like this:

cython.py -o pygrafix/window.cy.c pygrafix/window.pyx
gcc -O3 -shared -DGLFW_DLL -IC:\Python27\include pygrafix/window.cy.c -o pygrafix/window.pyd -LC:\Python27\libs -lpython27 -lgfwldll

But it crashes (windows says there was an error in the program). When I comment out the call to glfwSetMouseWheelCallback it doesn’t crash.
The strange thing is, if I compile with -O0 it doesn’t crash and works as it should! I am completely baffled by this. I checked the C code that cython outputs, and it looks ok.
_mouse_scroll_callback_handler is of the type void (*)(int), and the pointer to it gets passed nicely into glfw.

Other oddities:

• This only happens with glfwSetMouseWheelCallback (or at least to me), not with glwSetMousePosCallback for example.
• If I pass NULL to glfwSetMouseWheelCallback, there is no problem.
• Even if there is a crash, TEST LOCATION ONE still gets printed, but TEST LOCATION TWO doesn’t.
• It also works if I compile with -O3 -pg

What could possibly be the cause of this, and what would be the correct fix (without having to compile at -O0)?

---

Other trivia:

I’m using Windows 7 64 bit, GLFW 2.7.2, Cython 0.15.1, GCC 4.6.1 under MinGW and CPython 2.7.2.

Answer 1

I finally found the solution. The problem was that I didn’t add __stdcall in Cython to the callback functions, nor did I know it supported this.