python cffi crashes after callback

Question

I have a proprietary dll which interfaces with usb device, ctypes interface for it is working fine, but cffi one is crashing after calling a callback. function SwitchOn(6) should return pointer to a struct if it finds a device and NULL if it doesnt, also if it does not find the device error callback gets called with errno=10.

I am using python27, py33 behaves the same(need to remove 'import thread' to run)

Am I using it right? How do I debug it?

Trying to adapt doc's example for my needs as abarnert suggested. It still crashes. Am I doing it right?

>>> cffi.__version__
'0.7.2'

ctypes example outputs:

10288
(10288, 10L, 1L)
0

cffi example outputs:

4504
(4504, 10L, 1L)

and crashes

cffi_crash.py

import thread
def error(errno, critical):
    print(thread.get_ident(), errno, critical)

from cffi import FFI
ffi = FFI()
ffi.cdef('''
void* SwitchOn(int FPort);
typedef void(*type_func_user_error)(unsigned int, unsigned int);
void SetErrorFunction(type_func_user_error);
''')
eeg_dll = ffi.dlopen("EEG4DLL.dll")
err_cb = ffi.callback('type_func_user_error', error)

eeg_dll.SetErrorFunction(err_cb)
print(thread.get_ident())
x = eeg_dll.SwitchOn(6)
print(x)

ctypes_no_crash.py

import thread

def error(errno, critical):
    print(thread.get_ident(), errno, critical)

import ctypes
from ctypes import c_uint, WINFUNCTYPE

eeg_dll = ctypes.windll.EEG4DLL
func_user_error = WINFUNCTYPE(None, c_uint, c_uint)

SetErrorFunction = eeg_dll.SetErrorFunction
SetErrorFunction.argtypes = [func_user_error]
SetErrorFunction.restype = None

err_cb = func_user_error(error)

eeg_dll.SetErrorFunction(err_cb)
print(thread.get_ident())
x = eeg_dll.SwitchOn(6)
print(x)

cffi_indirection.py

def error(errno, critical):
    print(errno, critical)

from cffi import FFI
ffi2 = FFI()
ffi2.cdef('''
void (*python_callback)(unsigned int, unsigned int);
void *const c_callback;
''')
wr = ffi2.verify('''
    static void(*python_callback)(unsigned int x, unsigned int y);
    static void c_callback(unsigned int x, unsigned int y) {
        python_callback(x, y);
    }
''')
err_cb = ffi2.callback('void(unsigned int, unsigned int)', error)
wr.python_callback = err_cb

ffi = FFI()
ffi.cdef('''
void* SwitchOn(int FPort);
typedef void(*type_func_user_error)(unsigned int, unsigned int);
void SetErrorFunction(type_func_user_error);
''')
eeg_dll = ffi.dlopen("EEG4DLL.dll")
eeg_dll.SetErrorFunction(wr.c_callback)
x = eeg_dll.SwitchOn(6)
print(x)

Answer 1

According to the docs say:

Windows: you can’t yet specify the calling convention of callbacks… Use an indirection…

And your crash (happening immediately after return from your function) looks like exactly what you'd get by passing a cdecl function and having it called as a stdcall function: The caller (presumably the SwitchOn function in the C library) expects the callee (CFFI's wrapper around your error function) to clean up the stack; the callee expects the caller to clean up the stack… so nobody cleans up the stack, so when SwitchOn tries to return, it's returning to one of your arguments or a local variable or some other garbage instead of to its caller.

Immediately above that, the docs show how to "use an indirection", by which they mean writing a C wrapper that you ffi.verify. (They're showing how to do it for passing a varargs callback, but it's the same idea.)