[英]Wrapping c-function in Python with ctypes
I have a c-function with the following signature 我有一个带有以下签名的C函数
__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. 其中para定义如下。 I want to build a dll with this function (and others) and access it from Python.
我想用此函数(和其他函数)构建一个dll并从Python访问它。 The important Python lines are
重要的Python行是
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)
. 其中
cf
, cs
, ci
, cr
是(python)ints, para
是如下定义的struct-type, c_y0=(ct_c_double * 2)()
(从python端和c端都应为长度2, )和c_reltol
和c_abstol
为ct.c_double,因为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) 当我尝试运行我的主应用程序时,出现
WindowsError: exception: access violation writing xxx
在lib.setup
调用中WindowsError: exception: access violation writing xxx
,我看不到原因...在调用lib.setup
之前进行打印lib.setup
给出以下输出值和类型传递给函数的参数(顺序)
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. 我一直在试图与调试这个和这个问题,但没有成功。 Since the call signature of the c-function is fairly simple I cant see why it should break.
由于C函数的调用签名非常简单,所以我看不到它为什么会中断。
PS It runs perfectly on Ubuntu, with __declspec...
replaced by extern
PS它可以在Ubuntu上完美运行,用
__declspec...
替换为extern
c-struct defined as c结构定义为
typedef struct para PARA;
struct para
{
double a;
double b;
double c;
};
and corresponding pystruct as 和相应的pystruct为
class para(ct.Structure):
_fields_ = [('a', ct.c_double),
('b', ct.c_double),
('c', ct.c_double)]
EDIT c_y0
is defined as 编辑
c_y0
定义为
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 所有“ cvode”函数和
N_Vector
都是Sundials套件的一部分, 用于求解非线性方程
__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;
}
Here's an MCVE. 这是MCVE。 It show's your declarations are correct so the problem is likely in the function implementation.
它表明您的声明是正确的,因此问题可能出在函数实现中。 If below doesn't work for you, update your question with a similar MCVE that reproduces your failure.
如果以下方法对您不起作用,请使用类似的MCVE更新您的问题,以重现您的失败。
test.c 测试
#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 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 输出量
1 2 3 4 1.500000 2.500000 3.500000 4.500000 5.500000 6.500000 7.500000
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