Question

我已经按照下面在tutorial中找到的方法，使用Point库编写了以下代码来修改我的自定义python类ctypes。 wrap_function只是易于使用的小帮手，因为这是来自较大项目的MWE。

在python端：

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)

在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，它将输出以下输出：

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>

为什么它停在18.000？我尝试了几次，有时循环达到19或20k，但从来没有超过这个值。它与C端的数组初始化有关吗？我是否弄乱了在python端传递的参数？

奖金问题：如何使用ctypes将这些点的列表传递给C端？

Answer 1

尽管 NumPy 增加了一个额外的复杂性级别，但是每条信息都可以在[Python 3]: ctypes - A foreign function library for Python上找到。

（主要）问题是 Point 结构在 C 和 Python 中定义不同。
此外，该函数需要一个Point*，因此必须使用 byref （如果没有它，它也可以工作，我不知道这是否是 Undefined Behavior < / em>的情况很高兴，或者 ctypes 默默地做到了-由于 argtypes ）。

我已对您的代码进行了修改以使其正常工作。

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\StackOverflow\q055124400]> sopr.bat *** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages *** [prompt]> "c:\Install\x86\Microsoft\Visual Studio Community\2015\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 ：

如果要处理 Point 的列表，则可以使用数组。像这样：

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

使用ctypes将数组从Python传递到C ++，无法完全处理数组

1 个答案: