Error in returning pointer of array to Python

Question

My function purpose is to make custom sized array and pass it to python. Problem is everytime I try to do so, I get

Error in `python3': double free or corruption (fasttop)

or something similar.

I have produced a minimal example of such bug (I built tst.so shared library):

for c++:

int * lala = new int[1];
void def()
{
    delete[] lala;
    int * lala = new int[100];
}
extern "C" int * abc()
{
    def();
    return lala;
}

for python:

import ctypes
import numpy as np
from numpy.ctypeslib import ndpointer
import inspect
from os.path import abspath, dirname, join
fname = abspath(inspect.getfile(inspect.currentframe()))
libIII = ctypes.cdll.LoadLibrary(join(dirname(fname), 'tst.so'))
abc = libIII.abc
abc.restype = ndpointer(dtype=ctypes.c_int, shape=(100,))
abc.argtypes= None
asd = np.reshape(np.frombuffer(abc(), dtype = np.uint16), (100))
asd = np.reshape(np.frombuffer(abc(), dtype = np.uint16), (100))
asd = np.reshape(np.frombuffer(abc(), dtype = np.uint16), (100))

if I make int * lala = new int[100]; (same size as it should be), everything works. Am I doing something wrong? How should I delete old array and make other with different size?

cigien · Accepted Answer · 2020-05-04 17:02:56Z

1

You are declaring a local lala inside def. Allocating memory for that will not change the global lala. Instead, do this:

void def()
{
    delete[] lala;
    lala = new int[100];   // use global lala
}

answered May 4, 2020 at 17:02

cigien

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1 Comment

cigien Over a year ago

No worries, happens to all of us. Learn how to use a debugger, it'll help you catch these bugs yourself.

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