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I am attempting to use ctypes to share a C++ object with Python, by creating the object in C++, and passing the pointer out to Python via a C wrapper. I want to be able to act on this object later using the other functions in the Python class, do_something in the code below.

I've tried the following code, but I get a segfault. I'm new to interfacing C with C++ and C with Python, so I'm not sure if I'm doing something fundamentally wrong when passing the pointer, or if the memory is being cleared/shifted by Python garbage-collection, after I create the object?

This question discusses a similar problem for boost, but the answer isn't very useful for ctypes.

object.h

class object { public: // constructor object() { pointer = nullptr; } // destructor virtual ~object() { delete pointer; pointer = nullptr; } // get member functions of object_pointer // from C++ double do_something(); protected: // pointer to the object object_pointer *pointer; }; extern "C" { object* object_new(); void object_delete(object *Ob); double object_do_something(object *Ob); }

object.cpp

#include "object.h" double object::do_something() { return pointer->do_something(); } extern "C" { object *object_new() { return new object(); } void object_delete(object *Ob) { delete Ob; } double object_do_something(object *Ob) { return Ob->do_something(); } }

object.py

from ctypes import * lib = cdll.LoadLibrary('./lib_object.so') lib.object_new.argtypes = () lib.object_new.restype = c_void_p lib.special_delete.argtypes = c_void_p, lib.special_delete.restype = None lib.object_pointer.argtypes = c_void_p lib.object_pointer.restype = c_void_p class Object: def __init__(self): self.obj = lib.object_new() print self.obj def __del__(self): lib.object_delete(self.obj) def do_something(self): lib.object_do_something(self.obj) s = Object() >> 94549743086144 s.do_something() >> Segfault

Any help would be greatly appreciated!

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  • What command did you use to compile object.cpp into lib_object.so? Commented Feb 8, 2019 at 14:24
  • 1
    Do you ever change pointer? Otherwise pointer->do_something() dereferences a nullpointer. Also, why is it S->do_something() and not Ob->do_something? Commented Feb 8, 2019 at 15:06
  • Sorry the S->do_something() is a typo, I've fixed it Commented Feb 8, 2019 at 15:52
  • The pointer does get changed: an object is reconstructed from a character buffer, and then that object is pointed to by pointer Commented Feb 8, 2019 at 15:53
  • argtypes should be a list (or tuple): lib.object_pointer.argtypes = [c_void_p]. Commented Feb 8, 2019 at 22:04

1 Answer 1

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Notes:

  • None of the files (.cpp, .py) from the question compiled. They contain syntax errors and also semantic errors

  • I don't know what the pointer role was intended to be (it generated syntax errors). I can only assume that a Singleton implementation was attempted

  • Considering the above, instead of pointing the errors (there are many of them) in the existing files, I'm creating a brand new basic example

  • Although CTypes is not the only area to improve, I'm also pointing it out: [Python.Docs]: ctypes - A foreign function library for Python

object.hpp:

#pragma once #if defined(_WIN32) # if defined(OBJECT_EXPORTS) # define OBJECT_EXPORT_API __declspec(dllexport) # else # define OBJECT_EXPORT_API __declspec(dllimport) # endif #else # define OBJECT_EXPORT_API #endif class Object { public: Object(); virtual ~Object(); virtual double doSomething(); private: double m_double; }; extern "C" { OBJECT_EXPORT_API void* objectNew(); OBJECT_EXPORT_API void objectDel(void *pObj); OBJECT_EXPORT_API double objectDoSomething(void *pObj); }

object.cpp:

#define OBJECT_EXPORTS #include "object.hpp" #include <iostream> #define DBG_MSG0() std::cout << "CPP - [" << __FILE__ << "] " << __LINE__ << " (" << __FUNCTION__ << ")\n" Object::Object(): m_double(2.718282) { DBG_MSG0(); } Object::~Object() { DBG_MSG0(); } double Object::doSomething() { DBG_MSG0(); return m_double; } extern "C" { void* objectNew() { DBG_MSG0(); return new Object(); } void objectDelete(void *pObj) { DBG_MSG0(); delete reinterpret_cast<Object*>(pObj); } double objectDoSomething(void *pObj) { DBG_MSG0(); if (pObj) { return (reinterpret_cast<Object*>(pObj))->doSomething(); } return 0.0; } }

code00py:

#!/usr/bin/env python import ctypes as cts import sys DLL_NAME = "./libobject.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so") dll = cts.CDLL(DLL_NAME) objectNew = dll.objectNew objectNew.argtypes = () objectNew.restype = cts.c_void_p objectDelete = dll.objectDelete objectDelete.argtypes = (cts.c_void_p,) objectDoSomething = dll.objectDoSomething objectDoSomething.argtypes = (cts.c_void_p,) objectDoSomething.restype = cts.c_double class ObjectWrapper: def __init__(self): self.obj = objectNew() print("`Object` instance (as a `void *`): 0x{:016X}".format(self.obj)) def __del__(self): print("Deleting instance") objectDelete(self.obj) self.obj = None def do_something(self): print("Doing something") return objectDoSomething(self.obj) def main(*argv): obj = ObjectWrapper() ret = obj.do_something() print("do_something() returned: {:.6f}".format(ret)) if __name__ == "__main__": print("Python {:s} {:03d}bit on {:s}\n".format(" ".join(elem.strip() for elem in sys.version.split("\n")), 64 if sys.maxsize > 0x100000000 else 32, sys.platform)) rc = main(*sys.argv[1:]) print("\nDone.\n") sys.exit(rc)

Output:

[cfati@cfati-5510-0:/mnt/e/Work/Dev/StackExchange/StackOverflow/q054594122]> . ~/sopr.sh ### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ### [064bit prompt]> ls code00.py object.cpp object.hpp [064bit prompt]> [064bit prompt]> gcc -shared -fPIC -o libobject.so object.cpp -lstdc++ [064bit prompt]> ls code00.py libobject.so object.cpp object.hpp [064bit prompt]> [064bit prompt]> python ./code00.py Python 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0] 064bit on linux CPP - [object.cpp] 30 (objectNew) CPP - [object.cpp] 11 (Object) `Object` instance (as a `void *`): 0x0000557F47601910 Doing something CPP - [object.cpp] 41 (objectDoSomething) CPP - [object.cpp] 21 (doSomething) do_something() returned: 2.718282 Deleting instance CPP - [object.cpp] 35 (objectDelete) CPP - [object.cpp] 16 (~Object) Done.

An alternative to manually creating and destroying the (C++) object, is making it static (automatically initialized then the .dll is loaded, and destroyed then the .dll is unloaded).

Might want to look over:

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2 Comments

Thank you, this is very helpful. I will try to provide a proper stand-alone example in the future, rather than pulling random bits from my code...
You're welcome! [SO]: How to create a Minimal, Complete, and Verifiable example (mcve).

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