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