用ctypes包装简单的c ++示例;分段故障 [英] Wrapping simple c++ example with ctypes; segmentation fault

问题描述

我通过添加私有成员变量并将其打印在<$ c中，扩展了此答案中提供的示例$ c> bar（） function：

I extended the example provided in this answer by adding a private member variable, and printing it in the bar() function:

#include <iostream>
class Foo{
    private:
        double m;
    public:
        Foo() { m = 2.344; };
        void bar(){
            std::cout << "Hello, number is " << m << std::endl;
        }
};

extern "C" {
    Foo* Foo_new(){ return new Foo(); }
    void Foo_bar(Foo* foo){ foo->bar(); }
}

ctypes wrapper不变，为：

The ctypes wrapper was unchanged and is:

from ctypes import *
lib = cdll.LoadLibrary('./libfoo.so')

class Foo(object):
    def __init__(self):
        self.obj = lib.Foo_new()

    def bar(self):
        lib.Foo_bar(self.obj)



f = Foo()
f.bar()

当我运行python代码（早已经编译了C ++代码之后），我得到了一个分段错误，到 bar（）中的 m 的打印。

When I run the python code (after having already compiled the C++ code earlier), I am getting a segmentation fault that I have narrowed down to the printing of m in bar().

2. 如果我取消打印 m ，但将其保留为变量


3. $ c> m 与 bar（）中的任何固定数字。

1. in the original code
2. if i remove printing of m but keep it as a variable
3. if i substitute m with any fixed number in bar().

我真的很困惑为什么会发生这种情况。因为这是一个学习ctypes的实验，任何帮助将不胜感激。

I am really puzzled why this should be happening. As this is an experiment to learn ctypes, any help would be appreciated.

推荐答案

如果您使用64位Python，则需要定义 restype 和 argtypes 。否则ctypes默认将值转换为32位C int 。

If you're using 64-bit Python, you need to define the restype and argtypes. Otherwise ctypes defaults to casting the values to a 32-bit C int.

from ctypes import *

lib = CDLL('./libfoo.so')

lib.Foo_new.argtypes = []
lib.Foo_new.restype = c_void_p

lib.Foo_bar.argtypes = [c_void_p]
lib.Foo_bar.restype = None

以下是2.7.5，modules / _ctypes / callproc.c的源代码链接：

Here are source links for 2.7.5, Modules/_ctypes/callproc.c:

• a href =http://hg.python.org/cpython/file/ab05e7dd2788/Modules/_ctypes/callproc.c#l559 =nofollow> ConvParam （第645-663行）


• GetResult （第914-915行）

• ConvParam (Lines 645-663)
• GetResult (Lines 914-915)

对于64位Windows，C long 是32位，但在大多数其他64位平台上它是64位。通过强制 int ，结果至少是一致的。

For 64-bit Windows a C long is 32-bit, but it's 64-bit on most other 64-bit platforms. By forcing int the result is at least consistent.

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