How to pass a 2d array from Python to C?

Question

I’m trying to pass a 2d array from Python to C, using ctypes.
The array dtype is uint16.
I wrote a simple code just to understand how it works:

C:

#include <stdint.h>

__declspec(dllexport) uint16_t Test(uint16_t **arr)
{
     return (arr[5][5]);
}

Python:

import numpy as np
from ctypes import cdll
import ctypes
from numpy.ctypeslib import ndpointer

_p = ndpointer(dtype=np.uint16, ndim=2, shape=(10, 10), flags='C')
mydll = cdll.LoadLibrary("mydll.dll")
_func = mydll.Test
_func.argtypes = [_p]
_func.restypes = ctypes.c_uint16

data = np.empty([10, 10], dtype=np.uint16)
data[5, 5] = 5
print(_func(data))

I get OSError: access violation reading 0xFFFFFFFFFFFFFFFFFFFFFFF
what am I doing wrong and how do I fix it?

Asked By: Dan Shorla Ki

||

Source

Answer 1

Listing [SciPy.Docs]: C-Types Foreign Function Interface (numpy.ctypeslib) (and [Python.Docs]: ctypes – A foreign function library for Python just in case).

This is a exactly like [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati’s answer) (a duplicate), it just happens to also involve NumPy.
In other words, you have Undefined Behavior, as argtypes must be CTypes types (not NumPy).

Below is a modified version of the your code, that works.

dll00.c:

#include <stdint.h>

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


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

DLL00_EXPORT_API uint16_t dll00Func00(uint16_t **ppArr);

#if defined(__cplusplus)
}
#endif


uint16_t dll00Func00(uint16_t **ppArr)
{
    return ppArr[5][5];
}

code00.py:

#!/usr/bin/env python

import ctypes as ct
import sys

import numpy as np


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


def main(*argv):
    UI16Ptr = ct.POINTER(ct.c_uint16)
    UI16PtrPtr = ct.POINTER(UI16Ptr)

    dll00 = ct.CDLL(DLL_NAME)
    dll00Func00 = dll00.dll00Func00
    dll00Func00.argtypes = [UI16PtrPtr]
    dll00Func00.restype = ct.c_uint16


    dim0 = 10
    dim1 = 10
    np_arr_2d = np.empty([dim0, dim1], dtype=np.uint16)

    np_arr_2d[5][5] = 5
    print(np_arr_2d)

    # The "magic" happens in the following lines of code
    ct_arr = np.ctypeslib.as_ctypes(np_arr_2d)
    UI16PtrArr = UI16Ptr * ct_arr._length_
    ct_ptr = ct.cast(UI16PtrArr(*(ct.cast(row, UI16Ptr) for row in ct_arr)), UI16PtrPtr)
    res = dll00Func00(ct_ptr)

    print("n{0:s} returned: {1:d}".format(dll00Func00.__name__, res))


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

Output:

[cfati@CFATI-5510-0:e:WorkDevStackOverflowq058727931]> sopr.bat
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[prompt]> "c:Installx86MicrosoftVisual Studio Community2017VCAuxiliaryBuildvcvarsall.bat" x64>nul

[prompt]> dir /b
code00.py
dll00.c

[prompt]> cl /nologo /DDLL dll00.c  /link /NOLOGO /DLL /OUT:dll00.dll
dll00.c
   Creating library dll00.lib and object dll00.exp

[prompt]> dir /b
code00.py
dll00.c
dll00.dll
dll00.exp
dll00.lib
dll00.obj

[prompt]> "e:WorkDevVEnvspy_064_03.07.03_test0Scriptspython.exe" code00.py
Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 22:22:05) [MSC v.1916 64 bit (AMD64)] 064bit on win32

NumPy: 1.16.2

[[19760  5277   632     0 32464  5280   632     0   111   114]
 [  107    92    68   101   118    92    86    69   110   118]
 [  115    92   112   121    95    48    54    52    95    48]
 [   51    46    48    55    46    48    51    95   116   101]
 [  115   116    48    92   108   105    98    92   115   105]
 [  116   101    45   112    97     5   107    97   103   101]
 [  115    92   110   117   109   112   121    92   116   101]
 [  115   116   105   110   103    92    95   112   114   105]
 [  118    97   116   101    92   110   111   115   101   116]
 [  101   115   116   101   114    46   112   121     0     0]]

dll00Func00 returned: 5

Done.

The explanation for all those funky conversions can be found at [SO]: C++ & Python: Pass and return a 2D double pointer array from python to c++ (@CristiFati’s answer) (and the referenced [SO]: Problems with passing and getting arrays for a C function using ctypes (@CristiFati’s answer)).

Answered By: CristiFati

How to pass a 2d array from Python to C?

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