Program crashing with C++ DLL using OpenMP - c++

Question

I have a program using OpenMP on C++ and I need it to port into Dll so I can call it from Python. It returns an array of double values, which calculated using a lot of for loops with openmp pragma. I was doubtful if it is going to work, so I started from a little test program that calculates Pi value in a loop with different precision values, then I would measure performance and ensure that OpenMP works properly that way. Plain (w/o Omp) implementation works fine from Python and C++, however Omp variant gives a runtime error in Python (exception: access violation writing 0x000000000000A6C8) and crashes without an error in C++. Also Omp variant works fine if it is not a Dll and just a regular executable. The Dll is made with a makefile. App that uses the Dll built into an executable with g++ with no flags (source code is in UnitMain.cpp). All the relevant code and a Makefile below (I didn't include some files and functions for brevity).
UPD: I tried Microsoft compiler and it works, also I tested a linux dynamic library on WSL/g++ and it also works. Looks like it is Windows gcc specific, I'll try another version of gcc (btw my current version is this):
Thread model: posix gcc version 8.1.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)
UnitFunctions.cpp
#include "UnitFunctions.h"
#include <omp.h>
#include <stdio.h>
#include <string.h>
typedef long long int64_t;
double pi(int64_t n) {
double sum = 0.0;
int64_t sign = 1;
for (int64_t i = 0; i < n; ++i) {
sum += sign/(2.0*i+1.0);
sign *= -1;
}
return 4.0*sum;
}
void calcPiOmp(double* arr, int N) {
int64_t base = 10e5;
#pragma omp parallel for
for(int i = 0; i < N; ++i) {
arr[i] = pi(base+i);
}
}
UnitMain.cpp
#include <windows.h>
#include <iostream>
using namespace std;
struct DllHandle
{
DllHandle(const char * const filename)
: h(LoadLibrary(filename)) {}
~DllHandle() { if (h) FreeLibrary(h); }
const HINSTANCE Get() const { return h; }
private:
HINSTANCE h;
};
int main()
{
const DllHandle h("Functions.DLL");
if (!h.Get())
{
MessageBox(0,"Could not load DLL","UnitCallDll",MB_OK);
return 1;
}
typedef const void (*calcPiOmp_t) (double*, int);
const auto calcPiOmp = reinterpret_cast<calcPiOmp_t>(GetProcAddress(h.Get(), "calcPiOmp"));
double arr[80];
calcPiOmp(arr, 80);
cout << arr[0] << endl;
return 0;
}
Makefile
all: UnitEntryPoint.o UnitFunctions.o
g++ -m64 -fopenmp -s -o Functions.dll UnitEntryPoint.o UnitFunctions.o
UnitEntryPoint.o: UnitEntryPoint.cpp
g++ -m64 -fopenmp -c UnitEntryPoint.cpp
UnitFunctions.o: UnitFunctions.cpp
g++ -m64 -fopenmp -c UnitFunctions.cpp
A Python script
import numpy as np
import ctypes as ct
cpp_fun = ct.CDLL('./Functions.dll')
cpp_fun.calcPiNaive.argtypes = [np.ctypeslib.ndpointer(), ct.c_int]
cpp_fun.calcPiOmp.argtypes = [np.ctypeslib.ndpointer(), ct.c_int]
arrOmp = np.zeros(N).astype('float64')
cpp_fun.calcPiOmp(arrOmp, N)