Question

I have am inside a callback that allows me to access the pointers passed to it as int (i.e. the Python type() function returns int).

What so I have to do if I want convert this into a pointer of a struct whose layout I know and can define as a ctypes class. Say I have this already:

class data_t(Structure):
  _fields = [ ("foo", c_int), ("bar", c_wchar_p), ("baz", c_char_p) ]

How can a variable x for which type(x) gives the output int now be cast into a pointer to the above struct type?

The gist: I have no influence on the declaration of the callback function or the declaration of the arguments I get to see, so I need some way to get from the Python int type to a ctypes pointer and access the struct members from there …

This is Python 2.6, but I reckon most 2.x will be similar enough at least. I can’t get rid of this version requirement as it is embedded into a product that requires this particular Python version.

Answer 1

You can use ctypes.cast, or just declare the callback to return a POINTER(data_t). Examples of both below and tested on Python 2.7 and Python 3.2:

test DLL code (Windows)

typedef struct data {
    int foo;
    wchar_t* bar;
    char* baz;
} data_t;

typedef void (*CALLBACK)(data_t* p);

CALLBACK g_callback = 0;

extern "C" __declspec(dllexport) void set_callback(CALLBACK f)
{
    g_callback = f;
}

extern "C" __declspec(dllexport) void call_callback()
{
    data_t data;
    data.foo = 123;
    data.bar = L"马克";
    data.baz = "Mark";
    g_callback(&data);
}

Python

from ctypes import *

class data_t(Structure):
    _fields_ = [
        ("foo", c_int),
        ("bar", c_wchar_p),
        ("baz", c_char_p)]

dll = CDLL('test')

@CFUNCTYPE(None,c_int)
def callback(n):
    p = cast(n,POINTER(data_t)).contents
    print(p.foo,p.bar,p.baz)

@CFUNCTYPE(None,POINTER(data_t))
def callback2(n):
    p = n.contents
    print(p.foo,p.bar,p.baz)

dll.set_callback(callback)
dll.call_callback()

dll.set_callback(callback2)
dll.call_callback()

Output

123 马克 b'Mark'
123 马克 b'Mark'