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

Posted 2023-02-23

【中文标题】PyObject_CallMethod 在调用 python 方法时有时会出现段错误

【英文标题】：PyObject_CallMethod sometimes seg fault when calling python method

【发布时间】：2020-09-18 02:21:03

【问题描述】：

我正在测试一个场景，当 C++ 将函数指针设置为 python 类变量，然后使用 PyObject_CallMethod 运行另一个包含该类变量的 python 方法。

整个过程都会这样。

(1)。 PyCFunction_NewEx() 做一个 py 函数 -> (2)。 PyDict_SetItemString() 分配给 __dict__ -> (3) 下的类变量。 PyObject_CallMethod()调用python方法巫婆包含（1）。

当我将所有代码放入main()函数（whitout void setCallback()和void setCallback()中的所有代码都放入main()），它运行得非常好。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。

我该如何解决这个问题？

C++ 代码：ma​​in.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")

【参考方案1】：

这可能是因为PyMethodDef 是在setCallback 的堆栈上创建的

可以在cpythonhere的源码中验证。

PyMethodDef 未被复制，而是被引用。

【讨论】：

我将其更改为PyMethodDef *methd = new PyMethodDef"methd",pyCallFunc,METH_VARARGS,"py call func"; 并解决了问题。谢谢。