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ctype: WindowsError: exception: access violation reading

My environment: Windows Vista 64, Python3.1, Visual Studio 2008 sp1

Error Description: I am trying to wrap a dll (eg. Lib.dll) using ctype, when proceeding to one of the function (eg. func()) in Lib.dll, an error occurred: 

"WindowsError: exception: access violation reading"

I think the different between func() and previous functions of Lib.dll written in my '.py' file is that func() contains a DOUBLE type parameter, but I am sure that the parameter are passed correctly, since I have used c_double() to cast it. And it seems the error occurred when entering func(), since the first code (printf()) inside func() does not execute.

I also tried to run the same dll and its functions in C environment, it runs smoothly.

Further Information:

the lib.dll is compiled in MSVC (with extern "C"), and I am using CDLL for the calling type. The problem is actually happened on a function in another dll (lib2.dll). I use lib.dll only because the lib2.dll is written in C++ and I wrap the functions that I want in lib2. It looks like this:

///////////////////////////////////
// lib.cpp

lib2 l;

void func(char * c1, char * c2, double d) 
{
    printf("%s, %s, %f\n", c1, c2, d); // All the parameters are passed correctly
    l.func(c1, c2, d);                 // THIS IS where the error occurred
}
///////////////////////////////////
// lib2.cpp

void lib2::func(char * c1, char * c2, double d)
{
    printf();  // The error happened before this line being executed.
    ...
}
///////////////////////////////////
And python script looks like this:
// my.py

dll = cdll.LoadLibrary("lib.dll")

dll.some_func1(c_char_p('a'))
dll.some_func2(c_char_p('b'))

func(c_char_p('c1'), c_char_p('c2'), c_double(1.1))
////////////////////////////////////

it is also weird that lib2.dll cannot work when I use ctype to load it. It shows the function is not found. So I have to use lib.dll to call functions in lib2.dll.

Could anyone give me some hints? Thanks

Solution

  • ctypes is for C, but you can write a wrapper to expose a C++ class. Since you mentioned you use Python 3.1, I also noted you have c_char_p('c1') where 'c1' is a Unicode string. Since the example provided is not a complete example that can be used as is to reproduce the problem, it's difficult to tell what problem you are having.

    Below is a complete, working example. You can build it from a Visual Studio command prompt by running "nmake".

    lib1.cpp

    This wrapper "flattens" the C++ object into a C API.

    #include "lib2.h"
extern "C" {
__declspec(dllexport) lib2* lib2_new() { return new lib2; }
__declspec(dllexport) void lib2_delete(lib2* p) { delete p; }
__declspec(dllexport) void lib2_func(lib2* p, char* c1, char* c2, double d) {
    p->func(c1,c2,d);
}
}

    lib2.h

    #ifdef LIB2_EXPORTS
#   define LIB2_API __declspec(dllexport)
#else
#   define LIB2_API __declspec(dllimport)
#endif

class LIB2_API lib2
{
public:
    void func(char * c1, char * c2, double d);
};

    lib2.cpp

    #include <stdio.h>
#include "lib2.h"

void lib2::func(char * c1, char * c2, double d)
{
    printf("%s %s %f\n",c1,c2,d);
}

    makefile

    all: lib1.dll lib2.dll

lib1.dll: lib1.cpp lib2.dll
    cl /nologo /LD /W4 lib1.cpp -link lib2.lib

lib2.dll: lib2.cpp lib2.h
    cl /nologo /LD /W4 /D LIB2_EXPORTS lib2.cpp

    test.py

    #!python3
from ctypes import *

class lib2:

    lib1 = CDLL('lib1')
    # It's best to declare all arguments and types, so Python can typecheck.
    lib1.lib2_new.argtypes = []
    lib1.lib2_new.restype = c_void_p # Can use this for an opaque pointer.
    lib1.lib2_func.argtypes = [c_void_p,c_char_p,c_char_p,c_double]
    lib1.lib2_func.restype = None
    lib1.lib2_delete.argtypes = [c_void_p]
    lib1.lib2_delete.restype = None

    def __init__(self):
        self.obj = self.lib1.lib2_new()

    def __del__(self):
        self.lib1.lib2_delete(self.obj)

    def func(self,c1,c2,d):
        self.lib1.lib2_func(self.obj,c1,c2,d)

o = lib2()
o.func(b'abc',b'123',1.2) # Note byte strings

    Output

    C:\temp>nmake

Microsoft (R) Program Maintenance Utility Version 11.00.50727.1
Copyright (C) Microsoft Corporation.  All rights reserved.

        cl /nologo /LD /W4 /D LIB2_EXPORTS lib2.cpp
lib2.cpp
   Creating library lib2.lib and object lib2.exp
        cl /nologo /LD /W4 lib1.cpp -link lib2.lib
lib1.cpp
   Creating library lib1.lib and object lib1.exp

C:\temp>test.py
abc 123 1.200000

    Alternatives

    Since writing wrappers can be tedious, it is better to use something like boost::Python, Cython, or SWIG. I'm most familiar with SWIG, so here's another example:

    makefile

    all: _lib2.pyd lib2.dll

PYTHON_ROOT = c:\python33

lib2_wrap.cxx: lib2.i
    @echo Generating wrapper...
    swig -c++ -python lib2.i

_lib2.pyd: lib2_wrap.cxx lib2.dll
    cl /nologo /EHsc /MD /LD /W4 /I$(PYTHON_ROOT)\include lib2_wrap.cxx -link lib2.lib /LIBPATH:$(PYTHON_ROOT)\libs /OUT:_lib2.pyd

lib2.dll: lib2.cpp lib2.h
    cl /nologo /LD /W4 /D LIB2_EXPORTS lib2.cpp

    lib2.i

    %module lib2

%begin %{
#pragma warning(disable:4127 4211 4706)
%}

%{
#include "lib2.h"
%}

%include <windows.i>
%include "lib2.h"

    test.py

    #!python3
import lib2
o = lib2.lib2()
o.func('abc','123',1.2) #Note SWIG encodes Unicode strings by default

    Output

    C:\temp>nmake /las
Generating wrapper...
lib2.cpp
   Creating library lib2.lib and object lib2.exp
lib2_wrap.cxx
   Creating library lib2_wrap.lib and object lib2_wrap.exp    

C:\temp>test
abc 123 1.200000