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

Answer 1

Try changing

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

to

("Data", (c_char * SMB_MAX_DATA_SIZE)]

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];
} SMB_REQUEST;

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 test.py
import ctypes

SMB_MAX_DATA_SIZE = 16

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 test.py
SmBusReadByte: handle=0x12345678 request=[1 2 3 test]
result: 13

---

UPDATE

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).

Answer 3

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