CODEWITHSUNDEEP
pythoncdllctypes
Budelon
Budelon

Reputation: 53

Error while calling C DLL function from Python

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

Upvotes: 2

Views: 337

Answers (1)

CristiFati
CristiFati

Reputation: 41167

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).

Upvotes: 1

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