Accessing value from callback in ctypes - c++

Question

I'm using Python/ctypes to write an app based on a commercial DLL. This DLL reads a flatfile and returns data through structs. The relevant C structs looks like this:
struct System{
unsigned short user;
unsigned short version;
};
struct Message {
unsigned short header;
unsigned short data;
unsigned int messageType;
};
The API that came with the DLL provides a pointer to a function with the following prototype. This function loops through the flatfile and calls Callback defined next:
typedef int (__stdcall *Process) (const char* filename, const char* base, unsigned int flags, int user, Callback stdcallback);
and the Callback prototype (the fifth argument above) is defined as:
typedef int (__stdcall *Callback) (const System* Sys, const Message* Msg);
In my python file:
from ctypes import *
# using WinDLL for stdcall
lib = WinDLL('CVO.dll')
class System(Structure):
_fields_ = [('user', c_ushort),
('version', c_ushort)]
class Message(Structure):
_fields_ = [('header', c_ushort),
('data', c_ushort),
('messageType', c_uint)]
PROTO = WINFUNCTYPE(c_int, POINTER(System), POINTER(Message))
def py_callbck(Sys, Msg):
print Msg._type_.messageType
# return 0 to read the next line in the flatfile
return 0
Callback = PROTO(py_callbck)
pProcess lib.Process
pProcess.argtypes = [c_char_p,c_char_p,c_uint,c_int,PROTO]
pProcess.restype = c_int
pProcess(c_char_p('flat.ccf'),c_char_p(None),c_uint(0),c_int(0),Callback)
My problem lies in accessing the value the DLL passes to the Msg struct. The code should be printing an c_int (0, 1, 2) but instead is returning a Field type like such:
<Field type=c_ulong, ofs=136, size=4>
I know the flatfile is being processed, because my Python code prints out hundreds of the <Field ...> statements, which it is supposed to do.
My question is how I can access the value the DLL is pushing to the Msg Structure instead of returning <Field ...> statements?
Note I'm relatively new to ctypes and have very little C so if you notice issues with defining anything, please let me know. For example, I know there is a difference between ctypes.pointer which returns and object and ctypes.POINTER which returns a new type. When I use ctypes.pointer the code errors out.

Answer 1

For being relatively new to ctypes you've done a pretty good job here. The _type_ attribute of a pointer is a reference to the pointed at data type. In this case it's Message. The class attribute messageType is the CField data descriptor used by an instance.
What you want to do instead is dereference the pointer to get a Message instance. You can either use a [0] subscript or the contents attribute:
def py_callbck(Sys, Msg):
print Msg[0].messageType # Msg.contents.messageType
# return 0 to read the next line in the flatfile
return 0
Remember to retain a reference to Callback to keep it from being garbage collected. Currently you reference it as a global. That's fine.
Regarding the way you're calling pProcess, it's generally unnecessary to manually create ctypes objects for simple types when you've defined argtypes. You can more simply use the following:
pProcess('flat.ccf', None, 0, 0, Callback)
But pay attention to whether the function needs a writable string buffer. In that case use create_string_buffer. It's unnecessary here since the char * parameters are both const.