Question

目标：使用Cython从2D C ++ char数组获取Memoryview。

一些背景知识：

我有一个本地C ++库，该库生成一些数据并通过char**返回到Cython世界。数组被初始化并在库中操作如下：

struct Result_buffer{
    char** data_pointer;
    int length = 0;

    Result_buffer( int row_capacity) {
        data_pointer; = new char*[row_capacity];
        return arr;
    }

    // the actual data is appended row by row
    void append_row(char* row_data) {
         data_pointer[length] = row_data;
         length++;
    }     
}

因此，我们基本上得到了一个嵌套子数组的数组。

注意事项：
-每行具有相同的列数
-行可以共享内存，即指向同一row_data

目标是最好将此数组与memoryview一起使用，而无需进行昂贵的内存复制。

第一种方法（无效）：

使用Cython阵列和内存视图：

这是.pyx文件，应该使用生成的数据

from cython cimport view
cimport numpy as np
import numpy as np

[...]

def raw_data_to_numpy(self):

    # Dimensions of the source array
    cdef int ROWS = self._row_count
    cdef int COLS = self._col_count

    # This is the array from the C++ library and is created by 'create_buffer()'
    cdef char** raw_data_pointer = self._raw_data

    # It only works with a pointer to the first nested array
    cdef char* pointer_to_0 = raw_data_pointer[0]

    # Now create a 2D Cython array
    cdef view.array cy_array = <char[:ROWS, :COLS]> pointer_to_0

    # With this we can finally create our NumPy array:
    return np.asarray(cy_array)

这实际上可以正常编译并且运行时不会崩溃，但是结果与我预期的不太一样。如果我打印出NumPy数组的值，我会得到：

000: [1, 2, 3, 4, 5, 6, 7, 8, 9]
001: [1, 0, 0, 0, 0, 0, 0, 113, 6]
002: [32, 32, 32, 32, 96, 96, 91, 91, 97]
[...]

事实证明，第一行已正确映射，但其他行看起来很像未初始化的内存。因此，char**的内存布局和2D内存视图的默认模式可能不匹配。

编辑＃1 ：我从other question中学到的是内置cython数组不支持间接内存布局，因此我必须为unsigned char**创建一个cython包装器，以暴露buffer-protocol

Answer 1

解决方案：

手动实施缓冲协议：

包装Sub WorksheetsForNext() ' Exceptions Comma-Separated List Const cExceptions As String = "Sheet1,Sheet2" Dim vntExceptions As Variant ' Exceptions Array Dim i As Integer ' Worksheets Counter Dim j As Integer ' Exceptions Counter vntExceptions = Split(cExceptions, ",") For i = 1 To Worksheets.Count With Worksheets(i) For j = 0 To UBound(vntExceptions) If .Name = vntExceptions(j) Then Exit For End If Next If j > UBound(vntExceptions) Then ' Code in here e.g. Debug.Print .Name End If End With Next End Sub并实现缓冲区协议（ Indirect2DArray.pyx ）的包装器类：

unsigned char**

注意：我无法通过包装的构造函数传递原始指针，因此我不得不使用单独的cdef函数来设置指针设置

这是它的用法：

cdef class Indirect2DArray:
    cdef Py_ssize_t len
    cdef unsigned char** raw_data
    cdef ndim
    cdef Py_ssize_t item_size
    cdef Py_ssize_t strides[2]
    cdef Py_ssize_t shape[2]
    cdef Py_ssize_t suboffsets[2]


    def __cinit__(self,int nrows,int ncols):
        self.ndim = 2
        self.len = nrows * ncols
        self.item_size = sizeof(unsigned char)

        self.shape[0] = nrows
        self.shape[1] = ncols

        self.strides[0] = sizeof(void*)
        self.strides[1] = sizeof(unsigned char)

        self.suboffsets[0] = 0
        self.suboffsets[1] = -1


    cdef set_raw_data(self, unsigned char** raw_data):
        self.raw_data = raw_data        

    def __getbuffer__(self,Py_buffer * buffer, int flags):
        if self.raw_data is NULL:
            raise Exception("raw_data was NULL when calling __getbuffer__ Use set_raw_data(...) before the buffer is requested!")

        buffer.buf = <void*> self.raw_data
        buffer.obj = self
        buffer.ndim = self.ndim
        buffer.len = self.len
        buffer.itemsize = self.item_size
        buffer.shape = self.shape
        buffer.strides = self.strides
        buffer.suboffsets = self.suboffsets
        buffer.format = "B" # unsigbed bytes


    def __releasebuffer__(self, Py_buffer * buffer):
        print("CALL TO __releasebuffer__")

产生以下输出：

def test_wrapper(self):
    cdef nrows= 10000
    cdef ncols = 81    

    cdef unsigned char** raw_pointer = self.raw_data
    wrapper = Indirect2DArray(nrows,ncols)    
    wrapper.set_raw_data(raw_pointer)

    # now create the memoryview:
    cdef unsigned char[::view.indirect_contiguous, ::1] view = wrapper

    # print some slices 
    print(list(view[0,0:30]))
    print(list(view[1,0:30]))
    print(list(view[2,0:30]))

这正是我所期望的。感谢所有帮助我的人

Cython-动态2D C ++数组的Memoryview

第一种方法（无效）：

1 个答案:

解决方案：