Wrapping c-function in Python with ctypes

Question

I have a c-function with the following signature

__declspec( dllexport ) void* setup(int c_force, int c_stepping, int c_iteration, int c_roots,
                                    struct para* c_userdata, double* c_y0,
                                    double c_reltol, double c_abstol)
{
    ....
    return "a pointer";
}

where para is defined as below. I want to build a dll with this function (and others) and access it from Python. The important Python lines are

import ctypes as ct
lib = ct.cdll.LoadLibrary('lib_path.dll')
getattr(lib, 'setup')
lib.setup.restype = ct.c_void_p
# pystruct as defined below to avoid clogging code
lib.setup.argtypes = [ct.c_int, ct.c_int, ct.c_int, ct.c_int,
                      ct.POINTER(para), ct.POINTER(ct.c_double),
                      ct.c_double, ct.c_double]
ptr = lib.setup(cf, cs, ci, cr, ct.byref(para), c_y0, c_reltol, c_abstol) 
myobj = ct.c_void_p(ptr)

where cf, cs, ci, cr are (python) ints, para is of struct-type as defined below, c_y0=(ct_c_double * 2)() (should be length 2, both from the python side and on the c-side) and c_reltol and c_abstol are cast to ct.c_double as c_reltol = ct.c_double(reltol).

When I try to run my main application I get WindowsError: exception: access violation writing xxx at the lib.setup-function call and I cant see why... Printing just before calling lib.setup gives the following output for the values and types of the parameters passed to the function (in order)

1 2 2 1 <cparam 'P' (0000000003C84EB0)> <cvode_library.c_double_Array_2 object at 0x0000000003F95BC8> c_double(1e-06) c_double(1e-08)

<type 'int'> <type 'int'> <type 'int'> <type 'int'> <type 'CArgObject'> <class 'cvode_library.c_double_Array_2'> <class 'ctypes.c_double'> <class 'ctypes.c_double'>

I have been trying to debug with this and this question, but without success. Since the call signature of the c-function is fairly simple I cant see why it should break.

P.S. It runs perfectly on Ubuntu, with __declspec... replaced by extern

c-struct defined as

typedef struct para PARA;
struct para
{
    double a;
    double b;
    double c;
};

and corresponding pystruct as

class para(ct.Structure):
    _fields_ = [('a', ct.c_double),
               ('b', ct.c_double),
               ('c', ct.c_double)]

EDIT c_y0 is defined as

y0 = np.array([0., 0.])
c_y0 = (ct.c_double * 2)()
c_y0[0] = y0[0]
c_y0[1] = y0[1]

All "cvode"-functions and the N_Vector are part of Sundials suite for solving nonlinear equations

__declspec( dllexport ) void* setup(int c_force, int c_stepping, int c_iteration, int c_roots,
                                    struct para* c_userdata, double* c_y0,
                                    double c_reltol, double c_abstol)
{
    int flag;
    N_Vector y;
    void* cvode_mem;
    PARA* ptr_para;
    ptr_para = c_userdata;

    // ****** Set up vector with initial conditions ******
    y = N_VNew_Serial(2);
    NV_Ith_S(y,0) = c_y0[0];
    NV_Ith_S(y,1) = c_y0[1];

    // ****** Create cvode object with stepping and iteration method ******
    if(c_iteration==CV_FUNCTIONAL)
        cvode_mem = CVodeCreate(c_stepping, 1);    // Functional iteration
    else
        cvode_mem = CVodeCreate(c_stepping, 2);    // Newton interation
    if(check_flag((void *)cvode_mem, "CVodeCreate", 0)) return(NULL);

    flag = CVodeInit(cvode_mem, ode, 0, y);

    if(check_flag(&flag, "CVodeInit", 1)) return(NULL);

    // ****** Specify integration tolerances ******
    flag = CVodeSStolerances(cvode_mem, c_reltol, c_abstol);
    if(check_flag(&flag, "CVodeSStolerances", 1)) return(NULL);

    // ****** Set up linear solver module if required ******
    if(c_iteration==CV_DENSE_USER)
    {
        printf("Dense user supplied Jacobian\n");
        // Dense user-supplied Jacobian
        flag = CVDense(cvode_mem, 2);
        if(check_flag(&flag, "CVDense", 1)) return(NULL);

        flag = CVDlsSetDenseJacFn(cvode_mem, jac);

        if(check_flag(&flag, "CVDlsSetDenseJacFn", 1)) return(NULL);
    }
    else if(c_iteration==CV_DENSE_DQ)
    {
        // Dense difference quotient Jacobian
        flag = CVDlsSetDenseJacFn(cvode_mem, NULL);
        if(check_flag(&flag, "CVDlsSetDenseJacFn", 1)) return(NULL);
    }


    // Set optional inputs
    flag = CVodeSetUserData(cvode_mem, c_userdata);
    if(check_flag(&flag, "CVodeSetUserData", 1)) return(NULL);

    // Attach linear solver module

    // Specify rootfinding problem
    if(c_roots!=ROOTS_OFF)
    {
         flag = CVodeRootInit(cvode_mem, 1, root_func);
    }

    return cvode_mem;
}

Answer 1

Here's an MCVE. It show's your declarations are correct so the problem is likely in the function implementation. If below doesn't work for you, update your question with a similar MCVE that reproduces your failure.

test.c

#include <stdio.h>

typedef struct para PARA;
struct para
{
    double a;
    double b;
    double c;
};

__declspec(dllexport) void* setup(int c_force, int c_stepping, int c_iteration, int c_roots,
                                  struct para* c_userdata, double* c_y0,
                                  double c_reltol, double c_abstol)
{
    printf("%d %d %d %d %lf %lf %lf %lf %lf %lf %lf\n",c_force,c_stepping,c_iteration,c_roots,c_userdata->a,c_userdata->b,c_userda
ta->c,c_y0[0],c_y0[1],c_reltol,c_abstol);
    return NULL;
}

test.py

import ctypes as ct

class para(ct.Structure):
    _fields_ = [('a', ct.c_double),
               ('b', ct.c_double),
               ('c', ct.c_double)]

lib = ct.CDLL('test')
lib.setup.restype = ct.c_void_p
# pystruct as defined below to avoid clogging code
lib.setup.argtypes = [ct.c_int, ct.c_int, ct.c_int, ct.c_int,
                      ct.POINTER(para), ct.POINTER(ct.c_double),
                      ct.c_double, ct.c_double]
p = para(1.5,2.5,3.5)
c_y0 = (ct.c_double * 2)(4.5,5.5)
ptr = lib.setup(1,2,3,4,p, c_y0,6.5,7.5) 
myobj = ct.c_void_p(ptr)

Output

1 2 3 4 1.500000 2.500000 3.500000 4.500000 5.500000 6.500000 7.500000