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

Question

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

Answer 1

Here's a working example. It looks like you are passing the wrong type to the function.

Test DLL Code ("cl /W4 /LD x.c" on Windows)

#include <stdio.h>

#define SMBUS_API __declspec(dllexport)
#define SMB_MAX_DATA_SIZE 5

typedef void* SMBUS_HANDLE;

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

SMBUS_API int SmBusReadByte(SMBUS_HANDLE handle,SMB_REQUEST *request)
{
    unsigned char i;
    for(i = 0; i < request->BlockLength; i++)
        request->Data[i] = i;
    return request->BlockLength;
}

SMBUS_API SMBUS_HANDLE OpenSmbus(void)
{
    return (void*)0x12345678;
}

Python code

from ctypes import *
SMB_MAX_DATA_SIZE = 5
ARRAY5 = c_ubyte * SMB_MAX_DATA_SIZE

class SMB_REQUEST(Structure):
    _fields_ = [
        ("Address", c_ubyte),
        ("Command", c_ubyte),
        ("BlockLength", c_ubyte),
        ("Data", ARRAY5)]

smbus_read_byte = CDLL('x').SmBusReadByte
smbus_read_byte.argtypes = [c_void_p,POINTER(SMB_REQUEST)]
smbus_read_byte.restype = c_int
open_smbus = CDLL('x').OpenSmbus
open_smbus.argtypes = []
open_smbus.restype = c_void_p

handle = open_smbus()
print 'handle = %08Xh' % handle

smb_request = SMB_REQUEST(1,2,5)

print 'returned =',smbus_read_byte(handle,byref(smb_request))
print 'Address =',smb_request.Address
print 'Command =',smb_request.Command
print 'BlockLength =',smb_request.BlockLength
for i,b in enumerate(smb_request.Data):
    print 'Data[%d] = %02Xh' % (i,b)

Output

handle = 12345678h
returned = 5
Address = 1
Command = 2
BlockLength = 5
Data[0] = 00h
Data[1] = 01h
Data[2] = 02h
Data[3] = 03h
Data[4] = 04h