为什么这个 `ctypes` 包装函数的 return 值是 `c_long(0)` 而不是 `c_long(3)`？

Question

为什么这个 ctypes 包装函数的 return 值是 c_long(0) 而不是 c_long(3)？

// main.cpp

#include <iostream>

class AComplicatedCPPObj {

    int *d_;

public:

    explicit AComplicatedCPPObj(int *d)
            : d_(d) {
    }

    int *getD() const {
        return d_;
    }

    void setD(int *d) {
        d_ = d;
    }
};


#ifdef CTYPESTEST_EXPORTS
#if defined(_WIN64)
#define CTYPESTEST_API __declspec(dllexport)
#else
#define CTYPESTEST_API __declspec(dllimport)
#endif

#endif

extern "C" {
CTYPESTEST_API AComplicatedCPPObj *AComplicatedCPPObj_new(int *d) {
    return new AComplicatedCPPObj(d);
}

CTYPESTEST_API int *AComplicatedCPPObj_getD(AComplicatedCPPObj *aComplicatedCppObj) {
    return aComplicatedCppObj->getD();
}

CTYPESTEST_API void AComplicatedCPPObj_setD(AComplicatedCPPObj *aComplicatedCppObj, int *d) {
    aComplicatedCppObj->setD(d);
}

}

编译成共享库的cmake脚本

cmake_minimum_required(VERSION 3.15)
project(ctypesTest)

set(CMAKE_CXX_STANDARD 14)

add_library(ctypesTest SHARED main.cpp)
set(CMAKE_INSTALL_PREFIX ${CMAKE_SOURCE_DIR}/INSTALL)
add_definitions("-DCTYPESTEST_EXPORTS=TRUE")
install(TARGETS ctypesTest)

在 python 端：

import ctypes as ct
import os, glob, sys

WORKING_DIRECTORY = os.path.dirname(__file__)

# find the shared library
if sys.platform == "linux":
    so_path = os.path.join(WORKING_DIRECTORY, "INSTALL/lib/libctypesTest.so")
else:
    so_path = os.path.join(WORKING_DIRECTORY, "INSTALL/bin/ctypesTest.dll")

if not os.path.isfile(so_path):
    raise FileNotFoundError(so_path)

# load sharedd library into python
lib = ct.CDLL(so_path)


class AComplicatedCPPObj:

    def __init__(self, d):
        self.set_function_types("AComplicatedCPPObj_new", [ct.POINTER(ct.c_int)], ct.c_void_p)
        # checking some ctyeps functions
        print("d", d, ct.c_int(d), ct.pointer(ct.c_int(d)), ct.pointer(ct.c_int(d)).contents)

        self.obj = lib.AComplicatedCPPObj_new(ct.pointer(ct.c_int(d)))

        self.set_function_types("AComplicatedCPPObj_getD", [ct.c_void_p], ct.POINTER(ct.c_long))
        self.set_function_types("AComplicatedCPPObj_setD", [ct.c_void_p, ct.POINTER(ct.c_long)], None)

    def set_function_types(self, func_name, argtypes, restype):
        f = lib.__getattr__(func_name)
        f.argtypes = argtypes
        f.restype = restype

    def getD(self):
        return lib.AComplicatedCPPObj_getD(self.obj).contents

    def setD(self, d):
        if isinstance(d, str):
            d = d.encode()

        return lib.AComplicatedCPPObj_setD(self.obj, d)


if __name__ == '__main__':
    o = AComplicatedCPPObj(3)
    print(o.getD())

这个returns:

d 3 c_long(3) <__main__.LP_c_long object at 0x000001F66F0E8248> c_long(3)
c_long(0)

我期待的时候

d 3 c_long(3) <__main__.LP_c_long object at 0x000001F66F0E8248> c_long(3)
c_long(3)

编辑 - 一个错误

我不确定我现在做了什么不同的事情，但现在看来我在重新运行此代码时得到的输出是这样的：

d 3 c_long(3) <__main__.LP_c_long object at 0x000002320B8A52C8> c_long(3)
c_long(193626992)

此外，根据评论中的一个问题，当我运行:

x = 3
o = AComplicatedCPPObj(x)
print(o.getD())

输出为

d 3 c_long(3) <__main__.LP_c_long object at 0x000002320B8A5248> c_long(3)
c_long(193627312)

还不是

d 3 c_long(3) <__main__.LP_c_long object at 0x000002320B8A52C8> c_long(3)
c_long(3)

?

Answer 1

您在这一行中创建了一个指向临时对象的临时指针：

self.obj = lib.AComplicatedCPPObj_new(ct.pointer(ct.c_int(d)))

这一行执行后，pointer和c_int对象都被释放了。所以你有未定义的行为。传递给函数的指针不再存在。

我不知道你为什么要传递指向周围对象的指针，但要修复你当前正在做的事情，你需要保留对 c_int 对象的引用，只要 C++ 代码包含一个指向它的指针：

import ctypes as ct

lib = ct.CDLL('./ctypesTest')

class AComplicatedCPPObj:

    def __init__(self, d):
        self.set_function_types("AComplicatedCPPObj_new", [ct.POINTER(ct.c_int)], ct.c_void_p)
        self.set_function_types("AComplicatedCPPObj_getD", [ct.c_void_p], ct.POINTER(ct.c_int))
        self.set_function_types("AComplicatedCPPObj_setD", [ct.c_void_p, ct.POINTER(ct.c_int)], None)

        # keep reference to object and pass by reference to C++
        self.param = ct.c_int(d)
        self.obj = lib.AComplicatedCPPObj_new(ct.byref(self.param))

    def set_function_types(self, func_name, argtypes, restype):
        f = lib.__getattr__(func_name)
        f.argtypes = argtypes
        f.restype = restype

    def getD(self):
        return lib.AComplicatedCPPObj_getD(self.obj).contents

    def setD(self, d):
        # update local object and pass again by reference
        self.param = ct.c_int(d)
        return lib.AComplicatedCPPObj_setD(self.obj, ct.byref(self.param))

if __name__ == '__main__':
    o = AComplicatedCPPObj(3)
    print(o.getD())

但为什么不直接按值传递整数，这样就不必跟踪内存了？？

已更新 .cpp：

#include <iostream>

class AComplicatedCPPObj {
    int d_;
public:
    explicit AComplicatedCPPObj(int d)
            : d_(d) {
    }
    int getD() const {
        return d_;
    }
    void setD(int d) {
        d_ = d;
    }
};

// FYI, _WIN32 is defined for both 32-bit and 64-bit windows, unlike _WIN64.
#if defined(_WIN32)
#   define CTYPESTEST_API __declspec(dllexport)
#else
#   define CTYPESTEST_API __declspec(dllimport)
#endif

extern "C" {
    CTYPESTEST_API AComplicatedCPPObj *AComplicatedCPPObj_new(int d) {
        return new AComplicatedCPPObj(d);
    }

    CTYPESTEST_API int AComplicatedCPPObj_getD(AComplicatedCPPObj *aComplicatedCppObj) {
        return aComplicatedCppObj->getD();
    }

    CTYPESTEST_API void AComplicatedCPPObj_setD(AComplicatedCPPObj *aComplicatedCppObj, int d) {
        aComplicatedCppObj->setD(d);
    }
}

已更新Python：

import ctypes as ct

lib = ct.CDLL('./ctypesTest')

def set_function_types(func_name, argtypes, restype):
    f = lib.__getattr__(func_name)
    f.argtypes = argtypes
    f.restype = restype

class AComplicatedCPPObj:

    def __init__(self, d):
        set_function_types("AComplicatedCPPObj_new", [ct.c_int], ct.c_void_p)
        set_function_types("AComplicatedCPPObj_getD", [ct.c_void_p], ct.c_int)
        set_function_types("AComplicatedCPPObj_setD", [ct.c_void_p, ct.c_int], None)
        self.obj = lib.AComplicatedCPPObj_new(d)

    def getD(self):
        return lib.AComplicatedCPPObj_getD(self.obj)

    def setD(self, d):
        return lib.AComplicatedCPPObj_setD(self.obj, d)

if __name__ == '__main__':
    o = AComplicatedCPPObj(3)
    print(o.getD())

为什么这个 `ctypes` 包装函数的 return 值是 `c_long(0)` 而不是 `c_long(3)`？

Why is the return value of this `ctypes` wrapper function `c_long(0)` and not `c_long(3)`?

c++

python

ctypes

language-interoperability

shared-libraries

编辑 - 一个错误