Python C API: Access Violation when trying example module with MSVC

Question

I'm making a Python module in C++. Until now I used MingW to compile the module, which worked fine. But I want to switch to MSVC because an other library that I use is easier to use with MSVC.

But, I cannot get it to work. Compiling and linking works, but problems arise when I try to use the newly created module from Python. I get the error SystemError: Type does not define the tp_name field. when I call PyType_Ready. In the source code of Python I see that this error is reported when tp_name is null, but I checked that it is not null.

My module is too big to debug, so I tried the example module of the Python documentation instead, which gives a different problem. Here my files and the output of run.bat.

custom.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>

typedef struct {
    PyObject_HEAD
    /* Type-specific fields go here. */
} CustomObject;

static PyTypeObject CustomType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Custom",
    .tp_doc = "Custom objects",
    .tp_basicsize = sizeof(CustomObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT,
    .tp_new = PyType_GenericNew,
};

static PyModuleDef custommodule = {
    PyModuleDef_HEAD_INIT,
    .m_name = "custom",
    .m_doc = "Example module that creates an extension type.",
    .m_size = -1,
};

PyMODINIT_FUNC
PyInit_custom(void)
{
    PyObject *m;
    if (PyType_Ready(&CustomType) < 0)
        return NULL;

    m = PyModule_Create(&custommodule);
    if (m == NULL)
        return NULL;

    Py_INCREF(&CustomType);
    if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
        Py_DECREF(&CustomType);
        Py_DECREF(m);
        return NULL;
    }

    return m;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)

project(module)

add_library(module SHARED custom.c)

target_include_directories(module PRIVATE "C:/Program Files/Python37/include")
target_link_libraries(module "C:/Program Files/Python37/libs/python37_d.lib")

# Output name
set_target_properties(module PROPERTIES PREFIX "")
set_target_properties(module PROPERTIES OUTPUT_NAME "custom")
set_target_properties(module PROPERTIES SUFFIX ".pyd")

run.bat

@echo off
echo ----- Compiling -----
cmake .
cmake --build .
echo.
echo ----- Running -----
python -c "import sys; sys.path.append('Debug'); import custom; print('ok')"
echo Error code: %errorlevel%

Output

Microsoft Windows [Version 10.0.17763.914]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\x\pythontest>run
----- Compiling -----
-- Building for: Visual Studio 16 2019
-- Selecting Windows SDK version 10.0.18362.0 to target Windows 10.0.17763.
-- The C compiler identification is MSVC 19.24.28314.0
-- The CXX compiler identification is MSVC 19.24.28314.0
-- Check for working C compiler: H:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.24.28314/bin/Hostx64/x64/cl.exe
-- Check for working C compiler: H:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.24.28314/bin/Hostx64/x64/cl.exe -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: H:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.24.28314/bin/Hostx64/x64/cl.exe
-- Check for working CXX compiler: H:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.24.28314/bin/Hostx64/x64/cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: C:/x/pythontest
Microsoft (R) Build Engine version 16.4.0+e901037fe for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

  Checking Build System
  Building Custom Rule C:/x/pythontest/CMakeLists.txt
  custom.c
     Creating library C:/x/pythontest/Debug/custom.lib and object C:/x/pythontest/Debug/custom.exp
  module.vcxproj -> C:\x\pythontest\Debug\custom.pyd
  Building Custom Rule C:/x/pythontest/CMakeLists.txt

----- Running -----
Error code: -1073741819

Error code -1073741819 is 0xC0000005, which is Access Violation. So the example code crashes. This is a different error as with my own module, but it probably has the same cause. What am I doing wrong?

Answer 1

I found the solution: I need to build in release mode instead of debug mode and link to python37.lib instead of python37_d.lib.