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

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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];

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

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?


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.



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.

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

Asked By: Jack


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)

typedef void* SMBUS_HANDLE;

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

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;

    return (void*)0x12345678;

Python code

from ctypes import *

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)


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
Answered By: Jack

Answer #2:

You’re almost there. You should use c_char * SMB_MAX_DATA_SIZE as the type for the definition of Data. This works for me on Mac OS X:

Shared library:

$ cat test.c
#include <stdio.h>

#define SMB_MAX_DATA_SIZE 16

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

int SmBusReadByte(void *handle, SMB_REQUEST *request)
  printf("SmBusReadByte: handle=%p request=[%d %d %d %s]n", handle, 
      request->Address, request->Command, request->BlockLength, request->Data);
  return 13;

$ gcc test.c -fPIC -shared -o libtest.dylib

Python driver:

$ cat
import ctypes


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

libtest = ctypes.cdll.LoadLibrary('libtest.dylib')

req = SMB_REQUEST(1, 2, 3, 'test')

result = libtest.SmBusReadByte(ctypes.c_voidp(0x12345678), ctypes.byref(req))

print 'result: %d' % result

$ python
SmBusReadByte: handle=0x12345678 request=[1 2 3 test]
result: 13


You’re having problems because you need to set the result type of open_smbus to void*. By default, ctypes assumes that functions return ints. You need to say this:

open_smbus.restype = ctypes.c_void_p

You were getting an error because you were using c_void_p() (note the extra parentheses). There’s an important distinction between c_void_p and c_void_p(). The former is a type, and the latter is an instance of a type. c_void_p represents the C type void*, whereas c_void_p() represents an actual pointer instance (with a default value of 0).

Answered By: Mark Tolonen

Answer #3:

Try changing

("Data", type(create_string_buffer(SMB_MAX_DATA_SIZE))


("Data", (c_char * SMB_MAX_DATA_SIZE)]
Answered By: Adam Rosenfield

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