How to pass python 2d matrix to C function ctypes

Question

I tried to pass my matrix from python to C++ based on ctypes to multiply by 2 but I could not get the result that I want because it says inf instead.

C code (DLL)

float mult(float *x,int rowLen,int colLen)
{
   int rows = rowLen;

   for (int i = 0; i < rows; i++)
   {
      for (int j = 0; j < rows; j++)
      {
         *(x+ i*rows + j) = 2*(x+ i*rows + j));
         cout << "Test : " << *(x+ i*rows + j);
      }
   }
   return *(x);
}

Python Code

import numpy as np
import ctypes
from ctypes.util import *

matrix = [
         [1, 0.23, 0.25],
         [4.34, 1, 1.11],
         [3.93, 0.90, 1],
         ]

ptr = (ct.c_double*3*3)()

for i in range(3):
    for j in range(3):
        ptr[i][j] = matrix[i][j]
        print(ptr[i][j])

c_lib = ctypes.CDLL('testDll.dll')
c_lib.mult.restype = ctypes.c_float
answer = c_lib.mult(ptr, 3, 3)
print(f"{answer}")

` I am not sure why the shell shows 'inf' and why C did not print the calculation in every iteration. Please advised me how to overcome this problem?

Answer 1

The main problem was the Python array used c_double instead of c_float. Additional changes to make it compile (please post a compilable example with your question next time):

test.c

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

API float mult(float *x,int rowLen,int colLen)
{
   int rows = rowLen;

   for (int i = 0; i < rows; i++) {
      for (int j = 0; j < colLen; j++) {
         x[i*rows + j] *= 2;
      }
   }
   return *x;
}

test.py

import ctypes as ct

matrix = [[1, 0.23, 0.25],
         [4.34, 1, 1.11],
         [3.93, 0.90, 1]]

ptr = (ct.c_float*3*3)()  # was incorrectly c_double

for i in range(3):
    for j in range(3):
        ptr[i][j] = matrix[i][j]
        #print(ptr[i][j])

c_lib = ct.CDLL('./test')
c_lib.mult.restype = ct.c_float
answer = c_lib.mult(ptr, 3, 3)
print(answer)

for row in ptr:
    print(' '.join([f'{x:.2f}' for x in row]))

Output:

2.0
2.00 0.46 0.50
8.68 2.00 2.22
7.86 1.80 2.00