Error while calling C DLL function from Python

Question

I'm trying to execute my Python Code that must call a C function, which execute some calculation and save the value in a pointer that must be accessible from Python code. I'd like to do that because i'm constructing a DLL and i want to validate the algebra inside the DLL function, therefore i'd like to use a python code to validate the DLL. The Python Code

from ctypes import *


if __name__ == '__main__':

    mydll = cdll.LoadLibrary("./dll_simples.dll")
    funcao = mydll.simuser
    funcao.argtypes = c_double,c_double,POINTER(c_double),POINTER(c_double)



    a = 0
    b = 0
    input_1 = (c_double * 1)()
    input_1[0] = 5
    output_1  = (c_double * 1)()
    funcao(a,b,input_1,output_1)

and my DLL

__declspec(dllexport) void simuser(double t, double delt, double* in, double* out)
{



out[0] = 2 * in[0];



}

after executing this code, i have the error

funcao(a,b,input_1,output_1)

OSError: exception: access violation reading 0x0000000000000018

Just a wild guess: have you tried funcao(a,b,byref(input_1),byref(output_1)) — PaulMcG
– PaulMcG, Commented Jan 24, 2020 at 22:27
Is python a requirement for some reason? Why not test from C and save all the DLL hassles. If you needed exceptions and such then use C++ or C#. — Frank Merrow
– Frank Merrow, Commented Jan 25, 2020 at 1:00

CristiFati · Accepted Answer · 2020-01-27 07:48:27Z

1

Listing [Python 3.Docs]: ctypes - A foreign function library for Python.

So, you want to pass an array to a function that expects a pointer. For that case, ctypes.cast is required:

So, instead of:

funcao(a, b, input_1, output_1)

use:

funcao(a, b, cast(input_1, POINTER(c_double)), cast(output_1, POINTER(c_double)))

Looking at the existing C code, it only uses one value for the 2 pointers, case in which you won't be needing arrays at all (but I doubt that's the intent because then the input value shouldn't be a pointer):

# ...

input_1 = c_double(5)
output_1 = c_double(0)

funcao(a, b, byref(input_1), byref(output_1))

A working example: [SO]: Pointer from Python (ctypes) to C to save function output (@CristiFati's answer).

edited Jan 27, 2020 at 7:48

answered Jan 25, 2020 at 2:52

CristiFati

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9 Comments

Budelon Over a year ago

i tried this approach in the simplified code with only 1 argument

CristiFati Over a year ago

I really don;t understand what you said. What exactly did you try? Did it work?

Budelon Over a year ago

i'm sorry. The comment was incomplete unintentionally. I'd like to say that i've tried your approach but it didn't work. So i've simplified the code in order to have only 1 pointer array using your approach ('cast' function). But i'm still struggling to make this piece of code work well. I'll post here the solution when i find it.

Budelon Over a year ago

i must use array because in the future it'll be an array that will be sent to a C dll which will calculate some stuff and save in another array (output_1).

Budelon Over a year ago

i'm sorry, your answer is correct. The error is inside my C code. When i have Py_Initialize(); inside the DLL nothing works. This DLL must initialize a Python interpreter that works fine when i call DLL from C code, but when is Python code calling the DLL it cracks with the error "OSError: exception: access violation reading 0x0000000000000025".

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