How to pass buffer address to c?

Question

I want to pass the address of a numpy array buffer to c function, My C function looks like:

void print_float_buff(void *buff)
{
    float *b = (float *)buff;
    printf("Float Data: %f, %f, %f,\n", b[0], b[1], b[2]);
}

In python my code is:

import numpy as np
fun=ctypes.CDLL("./mylib.so")
l = np.array([10., 12.6, 13.5], dtype = 'float')
address, flag = l.__array_interface__['data']
fun.print_float_buff(ctypes.c_void_p(address))

A am getting completely different data in my c function. Address doesn't seem good. How can I pass correct address to my C function? Thanks.

Mark Tolonen · Accepted Answer · 2021-05-08 20:04:48Z

If you specify .argtypes correctly, ctypes will tell you the type is wrong. Below requires a one-dimensional array compatible with c_float:

test.c

#include <stdio.h>

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

API void print_float_buff(float *buff, size_t size)
{
    for(size_t i = 0; i < size; ++i)
        printf("buff[%zu] = %f\n",i,buff[i]);
}

test.py

from ctypes import *
import numpy as np

dll = CDLL('./test')
dll.print_float_buff.argtypes = np.ctypeslib.ndpointer(c_float,ndim=1),c_size_t
dll.print_float_buff.restype = None

a = np.array([10., 12.6, 13.5])
dll.print_float_buff(a,len(a))

Output:

Traceback (most recent call last):
  File "C:\test.py", line 9, in <module>
    dll.print_float_buff(a,len(a))
ctypes.ArgumentError: argument 1: <class 'TypeError'>: array must have data type float32

Changing the array as suggested:

a = np.array([10., 12.6, 13.5],dtypes='float32')

Output:

buff[0] = 10.000000
buff[1] = 12.600000
buff[2] = 13.500000

Cool Goose · Accepted Answer · 2021-05-08 07:01:35Z

0

I found the issue. In c, float size is four bytes while in python float size is 8 bytes. If I allocate array like this:

l = np.array([10., 12.6, 13.5], dtype = 'float32')

it works fine.

answered May 8, 2021 at 7:01

Cool Goose

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