[Python] Python & Ctypes: Passing a struct to a function as a pointer to get back data

I've looked through other answers but can't seem to get this to work. I'm trying to call a function within a DLL for communicating with SMBus devices. This function takes a pointer to a struct, which has an array as one of it's fields. so...

In C:

typedef struct _SMB_REQUEST
{
unsigned char Address;
unsigned char Command;
unsigned char BlockLength;
unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;


I think I have to set values for the Address, Command and BlockLength while the DLL fills the Data array. The function that requires this struct takes it as a pointer

SMBUS_API int SmBusReadByte( SMBUS_HANDLE handle, SMB_REQUEST *request );


So I've set up the struct in Python like so:

class SMB_REQUEST(ctypes.Structure):
_fields_ = [("Address", c_char),
("Command", c_char),
("BlockLength", c_char),
("Data", type(create_string_buffer(SMB_MAX_DATA_SIZE))]


*Note: I've also tried ctypes.c_char*SMB_MAX_DATA_SIZE for the data type*

To pass a pointer to a struct of this type to the function I have tried to initialise it first as follows:

data = create_string_buffer(SMB_MAX_DATA_SIZE)
smb_request = SMB_REQUEST('\x53', \x00', 1, data)


This responds with:

TypeError: expected string or Unicode object, c_char_Array_32 found


If I try leaving out the data array, like so:

smb_request = SMB_REQUEST('\x53', \x00', 1)


No, error. However, then when I try to pass this to the function:

int_response = smbus_read_byte(smbus_handle, smb_request))


I get:

ArgumentError: argument 2: <type 'exceptions.TypeError'>: expected LP_SMB_REQUES
T instance instead of SMB_REQUEST


I've tried passing it as a pointer:

int_response = smbus_read_byte(smbus_handle, ctypes.POINTER(smb_request))


and I get:

----> 1
2
3
4
5

TypeError: must be a ctypes type


Here's how I've set up the art types:

smbus_read_byte.argtypes = (ctypes.c_void_p, ctypes.POINTER(SMB_REQUEST))


I've tried casting but still no go. Can anyone shed some light on this for me?

Update:

If I first initialise the struct like so:

smb_request = SMB_REQUEST('\xA6', '\x00', chr(1), 'a test string')


and then bass by reference:

int_response = smbus_receive_byte(smbus_handle, ctypes.byref(smb_request))


I get no error. However, the function returns -1 when it should return '0' for success and non-zero for a fail. Checking the value of smb_request.Data gives back 'a test string' so no change there. Any suggestions as to what might be going on here would be greatly appreciated.

Thanks

UPDATE:

Since I've gotten a couple of enquiries about whether my handle is correct, here's how I'm using it. The header file for the DLL declares the following:

typedef void *SMBUS_HANDLE;

//
// This function call initializes the SMBus, opens the driver and
// allocates the resources associated with the SMBus.
// All SMBus API calls are valid
// after making this call except to re-open the SMBus.
//
SMBUS_API SMBUS_HANDLE OpenSmbus(void);


So here's how I'm doing this in python:

smbus_handle = c_void_p() # NOTE: I have also tried it without this line but same result

open_smbus = CDLL('smbus.dll').OpenSmbus
smbus_handle = open_smbus()
print 'SMBUS_API SMBUS_HANDLE OpenSmbus(void): ' + str(smbus_handle)


I call this before making the call to smbus_read_byte(). I have tried to set open_smbus.restype = c_void_p() but I get an error: TypeError: restype must be a type, a callable, or None

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