Callback functions using ctypes

Question

I have the code in C:

typedef result function_callback(struct mes_t* message, void* data) 
struct mes_t
{
uint32_t field1
uint32_t field2
void* data
};
function_one(&function_callback, data)

The application calls the user-defined (in the function_one) callback function function_callback. In the callback function passed field1, field2 and data parameters (data is usually equal to 0)

Whether the code on a python for this example is correctly written?

class mes_t(ctypes.Structure):
    pass
mes_t._fields_ = [
    ('field1', ctypes.c_uint32),
    ('dfield2', ctypes.c_uint32),
    ('data', ctypes.POINTER(ctypes.c_void_p))]
data_t=ctypes.c_void_p
data=data_t()
CALLBACK=CFUNCTYPE(ccg_msg, data_t)
cb_func=CALLBACK()
result = function_one(ctypes.byref(cb_func), ctypes.byref(data))

It may help to fix your sample code; the C declarations are invalid (I am guessing you're just missing some semicolons and putting things in the wrong order) and it's not clear what ccg_msg is. — the paul
– the paul, Commented May 24, 2012 at 15:09

the paul · Accepted Answer · 2012-05-24 16:00:43Z

I've guessed here the right way to interpret your code. Adjusted sample snippets here:

typedef int /* or whatever */ result;

struct mes_t
{
    uint32_t field1;
    uint32_t field2;
    void* data;
};
typedef result function_callback(struct mes_t* message, void* data);
result function_one(function_callback fcb, void* data);

And here's some example ctypes Python for making use of function_one():

class mes_t(ctypes.Structure):
    _fields_ = (
        ('field1', ctypes.c_uint32),
        ('field2', ctypes.c_uint32),
        ('data', ctypes.c_void_p))

result_t = ctypes.c_int; # or whatever

callback_type = ctypes.CFUNCTYPE(result_t, ctypes.POINTER(mes_t), ctypes.c_void_p)
function_one.argtypes = (callback_type, ctypes.c_void_p)
function_one.restype = result_t

data_p = ctypes.c_char_p('whatever')

def the_callback(mes_p, data_p):
    my_mes = mes_p[0]
    my_data_p = ctypes.cast(data_p, ctypes.c_char_p)  # or whatever
    my_data = my_data_p.value
    print "I got a mes_t object! mes.field1=%r, mes.field2=%r, mes.data=%r, data=%r" \
          % (my_mes.field1, my_mes.field2, my_mes.data, my_data)
    return my_mes.field1

result = function_one(callback_type(the_callback), ctypes.cast(data_p, ctypes.c_void_p))

You'll see there are numerous differences between this and your code; probably too much to give full explanations of everything. I can, however, explain a few particular parts if there are some which seem especially confusing. In general, though, it's important to have a good understanding of how ctypes pointers work (for example, you probably don't want a pointer to a pointer to void, but that's what your python code did).

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