C DLL crack when called from Python

Question

I\'ve a DLL that embeds Python interpreter using the C/Python API. The DLL works fine if called one time, but if the DLL is called twice, the code cracks and my program catch me

Answer 1

Listing [Python 3.Docs]: ctypes - A foreign function library for Python.

dll00.h:

#pragma once

#if defined(_WIN32)
#  if defined DLL0_EXPORTS
#    define DLL00_EXPORT_API __declspec(dllexport)
#  else
#    define DLL00_EXPORT_API __declspec(dllimport)
#  endif
#else
#  define DLL00_EXPORT_API
#endif


#if defined(__cplusplus)
extern "C" {
#endif

DLL00_EXPORT_API int dll00Func00(double t, double delta, unsigned int size, const double *pIn, double *pOut);

#if defined(__cplusplus)
}
#endif

dll00.c:

#include 
#include "Python.h"
#define DLL0_EXPORTS
#include "dll00.h"

#define C_TAG "From C .dll"


int dll00Func00(double t, double delta, unsigned int size, const double *pIn, double *pOut) {
    int res = 0;
    printf("%s: in function\n", C_TAG);
    const int isInit = Py_IsInitialized();
    // Modify array calling Python functions
    if (!isInit) {
        printf("%s: initializing Python interpreter\n", C_TAG);
        Py_Initialize();
    }
    res = PyRun_SimpleString("print(\"From Python (within C .dll): test\")");
    for (unsigned  int i = 0; i < size; i++) {
        pOut[i] = pIn[i] * t + delta;
    }
    if (!isInit) {
        printf("%s: uninitializing Python interpreter\n", C_TAG);
        Py_Finalize();
    }
    return 0;
}

main00.c:

#include 

#include "dll00.h"
#define SIZE 4


int main() {
    int res = 0;
    double in[SIZE] = { 10.0, 11.0, 12.0, 13.0 };
    double out[SIZE] = { 0 };
    res = dll00Func00(2, 0.5, SIZE, in, out);
    printf("Output array:\n");
    for (unsigned int i = 0; i < SIZE; i++) {
        printf("%.03f ", out[i]);
    }
    printf("\n");
    return 0;
}

code00.py:

#!/usr/bin/env python

import sys
import ctypes as ct


DLL_NAME = "./dll00.dll"


def main(*argv):
    DblPtr = ct.POINTER(ct.c_double)
    size = 5
    DblArr = ct.c_double * size

    dll00 = ct.PyDLL(DLL_NAME)
    dll00Func00 = dll00.dll00Func00
    dll00Func00.argtypes = (ct.c_double, ct.c_double, ct.c_uint, DblPtr, DblPtr)
    dll00Func00.restype = ct.c_int

    in_arr = DblArr(*range(size))
    out_arr = DblArr()
    print("Output array:")
    for i in range(size):
        print("{:.3f}".format(out_arr[i]), end=" ")
    print("\n")
    res = dll00Func00(2, 2.5, size, in_arr, out_arr)
    print("Output array:")
    for i in range(size):
        print("{:.3f}".format(out_arr[i]), end=" ")
    print()



if __name__ == "__main__":
    print("Python {0:s} {1:d}bit on {2:s}\n".format(" ".join(item.strip() for item in sys.version.split("\n")), 64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    main(*sys.argv[1:])
    print("\nDone.")

Notes:

• In Python, use ctypes.PyDLL as you're (indirectly) calling Python API functions
• In the .dll, use [Python 3.Docs]: Initialization, Finalization, and Threads - int Py_IsInitialized()
  • As a side note, the if test is not needed in Py_Initialize's case as Py_Initialize simply returns if the interpreter is already initialized (so I left it there just for consistency), but it is needed for Py_Finalize as one wouldn't want to uninitialize the interpreter while still in Python.
    So Py_Initialize / Py_Finalize pair doesn't work on "reference count" (every Py_Initialize call requires an Py_Finalize one)

• Calling Py_Initialize / Py_Finalize in the function, seems like an overkill (if the function is being called multiple times). I'd do 2 wrapper functions in the .dll and call:

  • one at the beginning
  • the other at the end

  of the (C) program

Output:

e:\Work\Dev\StackOverflow\q059937552>sopr.bat
*** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ***

[prompt]> "c:\Install\pc032\Microsoft\VisualStudioCommunity\2017\VC\Auxiliary\Build\vcvarsall.bat" x64
**********************************************************************
** Visual Studio 2017 Developer Command Prompt v15.9.19
** Copyright (c) 2017 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x64'

[prompt]> dir /b
code00.py
dll00.c
dll00.h
main00.c

[prompt]> cl /nologo /MD /DDLL /I"c:\Install\pc064\Python\Python\03.07.06\include" dll00.c  /link /NOLOGO /DLL /OUT:dll00.dll /LIBPATH:"c:\Install\pc064\Python\Python\03.07.06\libs"
dll00.c
   Creating library dll00.lib and object dll00.exp

[prompt]> cl /nologo /MD /W0 main00.c  /link /NOLOGO /OUT:main00_064.exe dll00.lib
main00.c

[prompt]> dir /b
code00.py
dll00.c
dll00.dll
dll00.exp
dll00.h
dll00.lib
dll00.obj
main00.c
main00.obj
main00_064.exe

[prompt]> "e:\Work\Dev\VEnvs\py_pc064_03.07.06_test0\Scripts\python.exe" code00.py
Python 3.7.6 (tags/v3.7.6:43364a7ae0, Dec 19 2019, 00:42:30) [MSC v.1916 64 bit (AMD64)] 64bit on win32

Output array:
0.000 0.000 0.000 0.000 0.000

From C .dll: in function
From Python (within C .dll): test
Output array:
2.500 4.500 6.500 8.500 10.500

Done.

[prompt]> set PATH=%PATH%;c:\Install\pc064\Python\Python\03.07.06

[prompt]> main00_064.exe
From C .dll: in function
From C .dll: initializing Python interpreter
From Python (within C .dll): test
From C .dll: uninitializing Python interpreter
Output array:
20.500 22.500 24.500 26.500