You are assuming that c directly follows b which is not the case. The compiler will pad a few bytes, 2 on x86, in that structure to align c.

The proper way is to declare one-to-one mapping of your structure in ctypes:

from ctypes import *

class object_t(Structure):
    _fields_ = [
        ('a', c_uint8),
        ('b', c_uint8),
        ('c', c_uint32),
        ('d', POINTER(c_uint8)),
    ]

No you can obtain the value of any member thought this type.

C example library:

#include <stdint.h>
#include <stdlib.h>

struct object_t {
  uint8_t a;
  uint8_t b;
  uint32_t c;
  uint8_t* d;
};

static struct object_t object = {'a', 'b', 12345, NULL};

struct object_t * func1(void)
{
  return &object;
}

void func2(void(*callback)(struct object_t *))
{
  callback(&object);
}

Using it from Python:

from ctypes import *

class object_t(Structure):
    _fields_ = [
        ('a', c_uint8),
        ('b', c_uint8),
        ('c', c_uint32),
        ('d', POINTER(c_uint8)),
    ]

callback_t = CFUNCTYPE(None, POINTER(object_t))

lib = CDLL('./file.dll')

func1 = lib.func1
func1.argtypes = None
func1.restype  = POINTER(object_t)

func2 = lib.func2
func2.argtypes = [callback_t]
func2.restype   = None

ret = func1()

a = ret.contents.a
b = ret.contents.b
c = ret.contents.c
d = ret.contents.d

def mycallback(obj):
    a = obj.contents.a
    b = obj.contents.b
    c = obj.contents.c
    d = obj.contents.d

func2(callback_t(mycallback))

Change

foo = POINTER(temp_foo)

to

foo = pointer(temp_foo)

can solve the problem.

Please see http://docs.python.org/library/ctypes.html#ctypes-pointers for more information.

I think what you may be missing is knowing exactly where you want the struct memory to be allocated. The c code below provides a function that allocates memory for the struct and returns a pointer to it (new_struct()).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    int a;
    int b;
} my_struct;

my_struct *new_struct()
{
    my_struct *struct_instance = (my_struct *)malloc(sizeof(my_struct));
    memset(struct_instance, 0, sizeof(my_struct));
    return struct_instance;
}

int modify_struct(my_struct *ms) {
    ms->a = 1;
    ms->b = 2;
    return 0;
}

void print_struct_c(my_struct *ms) {
    printf("my_struct {\n"
           "    a = %d\n"
           "    b = %d\n"
           "}\n", ms->a, ms->b);
}

From Python, to get the pointer, call the C function that does the allocation, then you can pass that to the other C functions that take it as a parameter.

import ctypes

lib_file_path = <<< path to lib file >>>

# Very simple example of how to declare a ctypes structure to twin the
# C library's declaration. This doesn't need to be declared if the Python
# code isn't going to need access to the struct's data members.
class MyStruct(ctypes.Structure):
    _fields_ = [('a', ctypes.c_int),
                ('b', ctypes.c_int)]

def print_struct(s):
    # Print struct that was allocated via Python ctypes.
    print("my_struct.a = %d, my_struct.b = %d" % (s.a, s.b))

def print_struct_ptr(sptr):
    # Print pointer to struct. Note the data members of the pointer are 
    # accessed via 'contents'.
    print("my_struct_ptr.contents.a = %d, my_struct_ptr.contents.b = %d" 
          % (sptr.contents.a, sptr.contents.b))


my_c_lib = ctypes.cdll.LoadLibrary(lib_file_path)

# If you don't need to access the struct's data members from Python, then 
# it's not necessary to declare MyStruct above. Also, in that case,
# 'restype' and 'argtypes' (below) can be set to ctypes.c_void_p instead.
my_c_lib.new_struct.restype     =  ctypes.POINTER(MyStruct)
my_c_lib.modify_struct.argtypes = [ctypes.POINTER(MyStruct)]

# Call C function to create struct instance.
my_struct_c_ptr = my_c_lib.new_struct()
print_struct_ptr(my_struct_c_ptr)

my_c_lib.modify_struct(my_struct_c_ptr)
print_struct_ptr(my_struct_c_ptr)

# Allocating struct instance from Python, then passing to C function.
my_struct_py = MyStruct(0, 0)
print_struct(my_struct_py)

my_c_lib.modify_struct(ctypes.byref(my_struct_py))
print_struct(my_struct_py)

# Data members of Python allocated struct can be acessed directly.
my_struct_py.a = 555

my_c_lib.print_struct_c(ctypes.byref(my_struct_py)) # Note use of 'byref()'
                                                    # to invoke c function.

The code above has been updated to include an example of how to allocate a struct instance via Python, and how to access the data members of either the C allocated or Python allocated struct (note differences in the print functions).

The problem is that the higher level "POINTER" from ctypes is, in Python, a different object than "a generic pointer" (ctypes.CArgObject by ctypes.byref)which is returned or a single number representing a memory address (which is what is returned by ctype's adrresof) - you can either annotate your function to receive a `ctypes.c_voidp and call it with _lib.mymethod(ctypes.addressof(a)) instead -

Or if you want to work on the stronged-typed side to avoid errors that would crash Python (a type error raises a Python exception instead - a wrong parameter passed to a C unction would cause a segmentation fault on the Python interpreter itself), you have to create a variable to hold the new "type" which is a POINTER to your structure - and then create an instance of this type with the address of your structure:

mystruct_pointer = ctypes.POINTER(mystruct)
_lib.mymethod.argtypes = (mystruct_pointer,)
_lib.mymethod.restype = ctypes.c_int

s = mystruct()

_lib.mymethod(mystruct_pointer.from_address(ctypes.addressof(s)))

Here's a working example. It looks like you are passing the wrong type to the function and the structure was declared incorrectly for the Data member. Note below how to declare the type of a fixed-size array when declaring the Python structure.

test.c ("cl /W4 /LD .c" on Windows):

#include <stdio.h>

#ifdef _WIN32
#   define SMBUS_API __declspec(dllexport)
#else
#   define SMBUS_API
#endif

#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) {
    for(unsigned char i = 0; i < request->BlockLength; ++i)
        request->Data[i] = i;
    return request->BlockLength;
}

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

test.py:

import ctypes as ct

SMB_MAX_DATA_SIZE = 5
ARRAY = ct.c_ubyte * SMB_MAX_DATA_SIZE

class SMB_REQUEST(ct.Structure):
    _fields_ = (('Address', ct.c_ubyte),
                ('Command', ct.c_ubyte),
                ('BlockLength', ct.c_ubyte),
                ('Data', ARRAY))

dll = ct.CDLL('./test')
smbus_read_byte = dll.SmBusReadByte
smbus_read_byte.argtypes = ct.c_void_p, ct.POINTER(SMB_REQUEST)
smbus_read_byte.restype = ct.c_int
open_smbus = dll.OpenSmbus
open_smbus.argtypes = ()
open_smbus.restype = ct.c_void_p

handle = open_smbus()
print(f'{handle = :08X}h')

smb_request = SMB_REQUEST(1, 2, 5)

print(f'returned = {smbus_read_byte(handle, smb_request)}')
print(f'Address = {smb_request.Address}')
print(f'Command = {smb_request.Command}')
print(f'BlockLength = {smb_request.BlockLength}')
for i, b in enumerate(smb_request.Data):
    print(f'Data[{i}] = {b:02X}h')

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