使用 ctypes 将数组从 Python 传递到 C++，无法完全处理它

Question

我已经编写了以下代码来使用 ctypes 库修改我的自定义 python class Point，遵循我在此 [=18= 中找到的方法]. wrap_function 只是一个易于使用的小帮手，因为这是一个来自更大项目的 MWE。

python 一方：

import ctypes
import numpy as np 
libc = ctypes.WinDLL(r'C:\Path\lib.dll')
 
def wrap_function(lib, funcname, restype, argtypes): 
    func = lib.__getattr__(funcname) 
    func.restype = restype 
    func.argtypes = argtypes 
    return func 
 
class Point(ctypes.Structure): 
    _fields_ = [('x', ctypes.c_int), ('xdata', ctypes.c_void_p)]
     
list_of_points = []                                #unused for now 
xdata = np.zeros((40000,), dtype=np.double)
a = Point(1,xdata.ctypes.data) 
b = Point(3,xdata.ctypes.data) 

change_data_for_point = wrap_function(libc,'change_data_for_point', None, [ctypes.POINTER(Point)])
change_data_for_point(a)

C端：

---header: 

const int N = 40000;
typedef struct {
    double x; 
    double xdata[N];
} Point;

extern "C" LIB_API void change_data_for_point(Point* p);


---source:

void change_data_for_point(Point* p) {
    p->x++; 
    for (int i = 0; i < 40000; i++) {
        p->xdata[i] = 2.0*i;
        if (i % 1000 == 0) printf("xdata at index %d is %f\n", i, p->xdata[i]);
    }
}

在Windows 7 cmd中执行python文件时，打印出如下输出：

xdata at index 0 is 0.000000
xdata at index 1000 is 2000.000000
 // ... some more ... 
xdata at index 17000 is 34000.000000
xdata at index 18000 is 36000.000000
Traceback (most recent call last):
   File "test.py", line 40, in <module>

为什么停在18.000？我试了几次，有时循环达到 19 或 20k，但它永远不会高于那个。跟C端的数组初始化有关系吗？我弄乱了 python 端传递的参数吗？

额外问题：如何使用 ctypes 将这些点的列表传递给 C 端？

Answer 1

虽然 NumPy 增加了额外的复杂度，但每条信息都可以在 [Python 3]: ctypes - A foreign function library for Python.

上找到

（主要）问题是 Point 结构在 C 和 [=64= 中的定义不同]。
此外，该函数需要一个 Point*，因此必须使用 byref（没有它也可以工作，我不知道这是不是 Undefined Behavior's happy case，或者 ctypes 默默地做 - 由于 argtypes).

我已经修改了您的代码以便正常工作。

dll.c:

#include <stdio.h>

#if defined(_WIN32)
#  define DLL_EXPORT __declspec(dllexport)
#else
#  define DLL_EXPORT
#endif


const int N = 40000;

typedef struct {
    double x;
    double xdata[N];
} Point;


#if defined(__cplusplus)
extern "C" {
#endif

    DLL_EXPORT void change_data_for_point(Point *p);

#if defined(__cplusplus)
}
#endif


void change_data_for_point(Point *p) {
    p->x++; 
    for (int i = 0; i < 40000; i++) {
        p->xdata[i] = 2.0 * i;
        if (i % 10000 == 9999)
            printf("xdata at index %d is %f\n", i, p->xdata[i]);
    }
}

code.py:

#!/usr/bin/env python3

import sys
import ctypes
import numpy as np


DLL_NAME = "./dll.dll"

xdata_dim = 40000  # !!! Must match N (from C) !!!
DoubleArr = ctypes.c_double * xdata_dim

class Point(ctypes.Structure): 
    _fields_ = [
        ("x", ctypes.c_int),
        ("xdata", DoubleArr),
    ]


def wrap_function(lib, funcname, restype, argtypes):
    func = lib.__getattr__(funcname)
    func.restype = restype
    func.argtypes = argtypes
    return func


def main():
    dll = ctypes.CDLL(DLL_NAME)
    #xdata_dim = ctypes.c_int.in_dll(dll, "N")

    xdata = np.zeros((xdata_dim,), dtype=np.double)
    a = Point(1, DoubleArr.from_address(xdata.ctypes.data))
    b = Point(3, DoubleArr.from_address(xdata.ctypes.data))
    change_data_for_point = wrap_function(dll,"change_data_for_point", None, [ctypes.POINTER(Point)])
    change_data_for_point(ctypes.byref(a))
    print(a.xdata[30000])


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

输出:

[cfati@CFATI-5510-0:e:\Work\Dev\Whosebug\q055124400]> sopr.bat
*** Set shorter prompt to better fit when pasted in Whosebug (or other) pages ***

[prompt]> "c:\Install\x86\Microsoft\Visual Studio Community15\vc\vcvarsall.bat" x64

[prompt]> dir /b
code.py
dll.c

[prompt]> cl /nologo /DDLL /MD /Tp dll.c  /link /NOLOGO /DLL /OUT:dll.dll
dll.c
   Creating library dll.lib and object dll.exp

[prompt]> dir /b
code.py
dll.c
dll.dll
dll.exp
dll.lib
dll.obj

[prompt]> "e:\Work\Dev\VEnvs\py_064_03.06.08_test0\Scripts\python.exe" code.py
Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32

xdata at index 9999 is 19998.000000
xdata at index 19999 is 39998.000000
xdata at index 29999 is 59998.000000
xdata at index 39999 is 79998.000000
60000.0

@EDIT0:

如果你想处理一个点的列表，你可以使用数组。类似于：

PointArr = Point * len(list_of_points)
point_arr = PointArr(*list_od_points)

使用 ctypes 将数组从 Python 传递到 C++，无法完全处理它

Passing array from Python to C++ using ctypes, unable to process it entirely

c++

python

ctypes