PyObject_CallMethod 调用 python 方法时有时会出现段错误

Question

我正在测试一个场景，当 C++ 将函数指针设置为 python class 变量，然后使用 PyObject_CallMethod 到运行另一个 python 方法，其中包含 class 变量。

整个过程是这样的

(1)。 PyCFunction_NewEx() 创建一个 py 函数 -> (2)。 PyDict_SetItemString() 分配给 __dict__ -> (3) 下的 class 变量。 PyObject_CallMethod() 调用 python 方法包含 (1).

当我将所有代码放在 main() 函数 中时（whitout void setCallback() 和 void setCallback() 中的所有代码都放在 main() 中），它运行非常好。但是，在我将一些代码放入函数后，有时会出现段错误，有时不会在 python 中调用函数指针，有时会得到正确的答案。

如何解决这个问题？

C++代码：main.cpp

#include <python3.7/Python.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <python3.7/methodobject.h>

// func ptr.
PyObject *myCallFunc(PyObject *self,PyObject *args) {
    printf(" aaaaaaaaaaaaaaaaaaaaaaa\n");
    return NULL;
}

// def func ptr
typedef PyObject *(*PyCallFunc)(PyObject *self,PyObject *arg);

// set func ptr into python member var
void setCallback(PyObject *ClassObj){
    PyCallFunc pyCallFunc = myCallFunc;

    PyMethodDef methd = {"methd",pyCallFunc,METH_VARARGS,"py call func"};
    PyObject *fName = PyUnicode_FromString(methd.ml_name);
    if(fName == NULL) {
        printf(" fName\n");
        exit(0);
    }
    PyObject *pyRunFunc = PyCFunction_NewEx(&methd,NULL,fName);
    if(pyRunFunc == NULL){
        printf(" can not create py function. exit.");
        exit(0);
    }
    Py_DECREF(fName);

    PyObject* classAttrDict = PyObject_GetAttrString(ClassObj, "__dict__");     // extract instance Dictionary.
    if(classAttrDict == NULL) {
        printf(" classAttrDict\n");
        exit(0);
    }

    int pRetSetCurrPrice = PyDict_SetItemString(classAttrDict, "callFunc", pyRunFunc);
    if(pRetSetCurrPrice != 0){
        printf(" set error. exit.");
        exit(0);
    }
}

int main(int argc,char **argv){

    Py_SetProgramName((wchar_t *)argv[0]);
    void *pyMem = PyMem_Malloc(sizeof(wchar_t*)*argc);
    wchar_t** _argv = (wchar_t**)&pyMem;
    for (int i=0; i<argc; i++) {
        wchar_t* arg = Py_DecodeLocale(argv[i], NULL);
        _argv[i] = arg;
    }
    Py_Initialize();
    PySys_SetArgv(argc, _argv);


    PyObject* programName = PyUnicode_FromString("test");
    if(programName == NULL) {
        printf(" programName\n");
        exit(0);
    }

    PyObject* pCustomFunc = PyImport_Import(programName);   // import test
    Py_DECREF(programName);
    if(pCustomFunc == NULL) {
        printf(" pCustomFunc\n");
        exit(0);
    }
    PyObject* pClass = PyObject_GetAttrString(pCustomFunc, "Test");  // pClass = test.Test
    if(pClass == NULL) {
        printf(" pClass\n");
        exit(0);
    }    
    PyObject* pNewInstance = PyObject_CallObject(pClass,NULL);  // pNewInstance = test.Test()
    if(pNewInstance == NULL) {
        printf(" pNewInstance\n");
        exit(0);
    }

    setCallback(pNewInstance);

    PyObject* pCallRet = PyObject_CallMethod(pNewInstance, "runCustomFunc",NULL); // pCallRet = pNewInstance.callFunc()
    if(pCallRet == NULL) {
        printf(" pCallRet\n");
        //exit(0);
    }

    sleep(2);

    printf(" \n\nend\n\n");
    Py_Finalize();
    return 0;
}

Python代码：test.py

import sys

def dummyFunc():
    pass

class Test:
    def __init__(self):
        self.aaa = 0
        self.callFunc = dummyFunc

    def runCustomFunc(self):
        print(" print from python.")
        print(" ref count of self.callFunc 1 is %d" %(sys.getrefcount(self.callFunc)))
        self.callFunc()
        print(" ref count of self.callFunc 2 is %d" %(sys.getrefcount(self.callFunc)))
        return 1

此测试项目的cmake：CMakeLists.txt

# set cmake and compiler.
cmake_minimum_required(VERSION 3.12...3.15)
set(CMAKE_CXX_FLAGS -std=c++17)

# set variable
set(CMAKE_POSITION_INDEPENDENT_CODE ON)    # test if this can resolve the problem
set(THREADS_PREFER_PTHREAD_FLAG ON)

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release)
endif()

set(CMAKE_CXX_FLAGS "-Wall -Wextra")    # test if optimize cause the problem
set(CMAKE_CXX_FLAGS_DEBUG "-g")         # test if optimize cause the problem
set(CMAKE_CXX_FLAGS_RELEASE "-O0")      # test if optimize cause the problem

set(LINK_LIB "/usr/local/lib")

set(PYTHON3_LINKER "-lpython3.7")
#set(PTHREAD "-lpthread")
set(PYTHON3_HEADER "/usr/include/python3.7")
set(PYTHON3_LIB "/usr/lib/python3.7/config-3.7m-x86_64-linux-gnu")

set(CPP_FILE_LIST "main.cpp")



include_directories( ${PYTHON3_HEADER})
link_directories( ${PYTHON3_LIB} ${LINK_LIB})

add_executable(pyEmbedFunPtrTest ${CPP_FILE_LIST})

target_link_libraries(pyEmbedFunPtrTest ${PYTHON3_LINKER})

find_package(Threads REQUIRED)
target_link_libraries(pyEmbedFunPtrTest Threads::Threads)

#target_compile_options(pyEmbedFunPtrTest PUBLIC "-pthread")

Answer 1

可能是因为 PyMethodDef 是在 setCallback

的堆栈上创建的

你可以在cpython的源码中验证here。

PyMethodDef 未被复制，而是被引用。

PyObject_CallMethod 调用 python 方法时有时会出现段错误

PyObject_CallMethod sometimes seg fault when calling python method

c++

python

segmentation-fault