ctypes struct containing arrays

Question

I’m trying to use ctypes. I’m interested in manipulating C structs containing arrays. Consider the following my_library.c

#include <stdio.h>


typedef struct {

    double first_array[10];
    double second_array[10];

} ArrayStruct;


void print_array_struct(ArrayStruct array_struct){

    for (int i = 0; i < 10; i++){
        printf("%fn",array_struct.first_array[i]);
    }

}

and suppose I’ve compiled it in a shared library my_so_object.so From Python I can do something like this

import ctypes
from ctypes import *

myLib = CDLL("c/bin/my_so_object.so")


class ArrayStruct(ctypes.Structure):
    _fields_ = [('first_array', ctypes.c_int * 10), ('second_array', ctypes.c_int * 10)]

    def __repr__(self):
        return 'ciaone'


myLib.print_array_struct.restype = None
myLib.print_array_struct.argtype = ArrayStruct

my_array_type = ctypes.c_int * 10
x1 = my_array_type()
x2 = my_array_type()

a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

x1[0:9] = a[0:9]

a = [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

x2[0:9] = a[0:9]

print(my_array_type)
>>> <class '__main__.c_int_Array_10'>

print(x1[2])
>>> 3

print(x2[2])
>>> 13

x = ArrayStruct(x1, x2)

print(x.first_array[0:9])
>>> [1, 2, 3, 4, 5, 6, 7, 8, 9]

So far so good: I’ve created the correct types and everything seems working fine. But then:

myLib.print_array_struct(x)
>>> 0.000000
>>> 0.000000 
>>> 0.000000
>>> 0.000000
>>> 0.000000
>>> 0.000000
>>> 0.000000
>>> 0.000000
>>> 0.000000
>>> 0.000000

I’m clearly missing something. The ArrayStruct type is recognized (otherwise the call myLib.print_array_struct(x) would throw an error) but not correctly initialized.

Asked By: Gabriele

||

Source

Answer 1

There were 2 problems with the code (as I stated in the comment):

print_array_struct.argtype – which is incorrect (and might have disastrous effects). Check (newer) [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati’s answer) for more details
In C the arrays are double based, while in Python they are ctypes.c_int (int) based

For more details, check [Python.Docs]: ctypes – A foreign function library for Python.
I modified your Python code, to correct the above mistakes (and some other minor stuff).

code00.py:

#!/usr/bin/env python

import ctypes as ct
import sys


DLL_NAME = DLL_NAME = "./dll00.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so")

DOUBLE_10 = ct.c_double * 10


class ArrayStruct(ct.Structure):
    _fields_ = (
        ("first_array", DOUBLE_10),
        ("second_array", DOUBLE_10),
    )


def main(*argv):
    dll_handle = ct.CDLL(DLL_NAME)
    print_array_struct = dll_handle.print_array_struct
    print_array_struct.argtypes = (ArrayStruct,)
    print_array_struct.restype = None

    x1 = DOUBLE_10()
    x2 = DOUBLE_10()
    x1[:] = range(1, 11)
    x2[:] = range(11, 21)
    print([item for item in x1])
    print([item for item in x2])
    arg = ArrayStruct(x1, x2)
    print_array_struct(arg)


if __name__ == "__main__":
    print("Python {:s} {:03d}bit on {:s}n".format(" ".join(elem.strip() for elem in sys.version.split("n")),
                                                   64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("nDone.")
    sys.exit(rc)

dll00.c (dummy):

#include <stdio.h>

#if defined(_WIN32)
#  define DLL00_EXPORT_API __declspec(dllexport)
#else
#  define DLL00_EXPORT_API
#endif

#define DIM 10


typedef struct {
    double first_array[DIM];
    double second_array[DIM];
} ArrayStruct;


#if defined(__cplusplus)
extern "C" {
#endif

DLL00_EXPORT_API void print_array_struct(ArrayStruct array_struct);

#if defined(__cplusplus)
}
#endif


void print_array_struct(ArrayStruct array_struct)
{
    printf("From C:n");
    for (int i = 0; i < DIM; ++i) {
        printf("  %2.1lfn", array_struct.first_array[i]);
    }
}

Output:

(qaic-env) [cfati@cfati-5510-0:/mnt/e/Work/Dev/StackOverflow/q050447199]> ~/sopr.sh
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[064bit prompt]> ls
code00.py  dll00.c
[064bit prompt]> gcc -fPIC -shared -o dll00.so dll00.c
[064bit prompt]> ls
code00.py  dll00.c  dll00.so
[064bit prompt]>
[064bit prompt]> python3.5 ./code00.py
Python 3.5.2 (default, Nov 23 2017, 16:37:01) [GCC 5.4.0 20160609] 064bit on linux

[1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0]
[11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0]
From C:
  1.0
  2.0
  3.0
  4.0
  5.0
  6.0
  7.0
  8.0
  9.0
  10.0

Done.

Answered By: CristiFati

ctypes struct containing arrays

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