ctypes structure arrays in python

Question

I'm trying to have a pointer created to nested structure array. But to c++ only first structure elements are passed...

C++ code:

typedef structure
{
int One;
int Two;
}nestedStru; 

typedef structure
{
int First;
nestedStru* Poniter; //Pointer to nested structure array
}mainStru;

Equivalent python code:

class nestedStru(Structure)
    _fields_ = [("One",c_uint8),          
        ("Two",c_uint8)]

class mainStru(Structure):
    _fields_ = [("First",c_uint8),
                ("PointerToNested",POINTER(nestedStru))] 

I tried creating an object of main class and cast pointer to array objects..

object = mainStru()
object.Second = cast((nestedStru * 2)(), POINTER(nestedStru)) 

Any suggestions would be welcome. Thanks in advance!

Answer 1

You use c_uint8, which is 8-bit, while your structure uses int, in ctypes c_int, usually 32-bit.

Your structures should be:

class nestedStru(Structure):
    _fields_ = [
      ("One", c_int),          
      ("Two", c_int)
    ]

class mainStru(Structure):
    _fields_ = [
      ("First", c_int),
      ("Poniter", POINTER(nestedStru))
    ]

This is a test library:

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

typedef struct
{
  int One;
  int Two;
} nestedStru; 

typedef struct
{
  int First;
  nestedStru* Poniter; // Pointer to nested structure array
} mainStru;

void
func(const mainStru *obj, size_t s)
{
  size_t i;

  for( i = 0 ; i < s ; i++ )
  {
    printf("%d, %d\n", obj->Poniter[i].One, obj->Poniter[i].Two);
  }
}

Python client:

#!python
from ctypes import *

class nestedStru(Structure):
    _fields_ = [
      ("One", c_int),          
      ("Two", c_int)
    ]

class mainStru(Structure):
    _fields_ = [
      ("First", c_int),
      ("Poniter", POINTER(nestedStru))
    ]

if __name__ == '__main__':
    obj = mainStru()
    obj.First = 0
    obj.Poniter = (nestedStru * 3)((1, 11), (2, 22), (3, 33))

    func = CDLL('./lib.dll').func
    func.argtypes = [POINTER(mainStru), c_size_t]
    func.restype = None

    func(obj, 3)

now it works fine:

> gcc -Wall lib.c -o lib.dll -shared
> python file.py
1, 11
2, 22
3, 33
>