How to access data from pointer in struct from Python with ctypes?

Question

I have the following C struct:

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

With ctypes, via a callback, I am able to obtain a pointer to such a struct in Python, let's call it ref. I can easily obtain a, b, c this way:

from ctypes import cast, c_uint8, c_uint32, POINTER

a = cast(ref, POINTER(c_uint8)).contents.value
b = cast(ref + 1, POINTER(c_uint8)).contents.value
c = cast(ref + 2, POINTER(c_uint32)).contents.value

but I can't read the bytes from d. I tried the following:

d_pointer = cast(ref + 6, POINTER(POINTER(c_uint8))).contents
first_byte_of_d = d_pointer.contents
print type(first_byte_of_d) # prints <class 'ctypes.c_ubyte'>
print first_byte_of_d

At this last line I encounter a SIGSEGV when debugging with gdb. So the question is, how should one access the first byte of a pointer from a struct in Python?

Are you sure your bug isn't in the callback (i.e., is it properly initializing the pointer)? — Karl Knechtel, Jul 23 '14 at 14:08
Yes, I'm positive about that, as I managed to retrieve data using Java. — adrian.nicolau, Jul 24 '14 at 06:26

score 9 · Accepted Answer · answered Jul 24 '14 at 00:48

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))

This saved me: "The proper way is to declare one-to-one mapping of your structure in ctypes". I knew about this, but for some reason I thought I couldn't cast `ref` to `object_t` (the docs say "The cast() function can be used to cast a **ctypes instance** into a pointer to a different ctypes data type"). Thank you! — adrian.nicolau, Jul 24 '14 at 06:57

How to access data from pointer in struct from Python with ctypes?

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