C/Python binding: pointer address modification

Question

Source

C++

extern "C"
{
    Service* create_service( int port )
    {
        Settings settings;
        settings.set_port( port );

        auto service = new Service( settings );

        std::cout << "create_service returning pointer address: " << service << std::endl;

        return service;
    }

    void release_service( Service* service )
    {
        std::cout << "release_service consuming pointer address: " << service << std::endl;
        delete service;
    }
}

Python

from ctypes import *

library = cdll.LoadLibrary('distribution/library/libhelpers.dylib')

class Service(object):
    def __init__(self, port):
        self.obj = library.create_service(port)
        print "__init__ address: ", self.obj

    def __del__(self):
        print "__del__", self.obj
        library.release_service(self.obj);

Console

create_service returning pointer address: 0x7fc3a0e330e0

init address: -1595723552

del address: -1595723552

release_service consuming pointer address: 0xffffffffa0e330e0

Segmentation fault: 11

Error

Exception Type: EXC_BAD_ACCESS (SIGSEGV)

Exception Codes: KERN_INVALID_ADDRESS at 0xffffffff914d37a0

Build (cmake)

set( CMAKE_CXX_COMPILER clang++ )

set( CMAKE_CXX_FLAGS "-stdlib=libc++ -std=c++11 -Wall -Wextra -Weffc++ -pedantic" )

add_library( helpers SHARED ${MANIFEST} )

target_link_libraries( helpers restbed )

Description

When returning a C++ class instance as a pointer. Python receives the correct address. However when using this address at a later date, it appears to have been modified.

It has been allocated on the heap. The assumption is this would be acceptable? — Ben Crowhurst
– Ben Crowhurst, Commented Dec 7, 2013 at 17:33
"Python receives the correct address" - you haven't demonstrated that in what you show above. — Mat
– Mat, Commented Dec 7, 2013 at 17:34
@Mat I've updated with additional logging showing the address status when in the Python environment. — Ben Crowhurst
– Ben Crowhurst, Commented Dec 7, 2013 at 17:43

Ben Crowhurst · Accepted Answer · 2013-12-08 10:34:46Z

2

It appears I was giving insufficient context to ctypes.

from ctypes import *

library = cdll.LoadLibrary('distribution/library/libhelpers.dylib')

class Service(object):
    def __init__(self, port):
        library.create_service.restype = c_void_p
        self.obj = library.create_service(port)

    def __del__(self):
        library.release_service.argtypes = [c_void_p]
        library.release_service(self.obj);

answered Dec 8, 2013 at 10:34

Ben Crowhurst

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2 Comments

Eryk Sun Over a year ago

ctypes defaults to C int for integers, which, as you discovered, truncates a 64-bit pointer. You can use CDLL directly instead of cdll.LoadLibrary. cdll is meant for Windows, such as cdll.msvcrt. BTW, the typical tag for Python ctypes is just "ctypes", not "python-ctypes".

Eryk Sun Over a year ago

You should move setting restype and argtypes out of the method definitions. There's no point in redefining them for the single library instance each time you init/deallocate a Service object.

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