为什么我的PyArrayObject *数据被截断了？

Question

Why do i get truncated array inside my C function?

In C:

#include <Python.h>
#include <arrayobject.h>

PyObject *edge(PyObject *self, PyObject *args) {
    int *ptr;
    unsigned char *charPtr;
    PyArrayObject *arr;
    PyObject *back;
    int ctr = 0;
    int size = 500 * 500;

    if (!PyArg_ParseTuple(args, "O", &arr))
        return NULL;

    charPtr = (char*)arr->data;
    printf("\n strlen of charPtr is ---> %d \n", strlen(arr->data)); // --->> 25313 
    printf("\n strlen of charPtr is ---> %d \n", strlen(charPtr));  //--->> also 25313 

    back = Py_BuildValue("s", "Nice");
    return back;
}

In Python:

import ImageProc
import cv2
import numpy
import matplotlib.pyplot as plt

img = cv2.imread("C:/Users/srlatch/Documents/Visual Studio 2015/Projects/PythonImage/andrew.jpg", cv2.IMREAD_GRAYSCALE)
np =  cv2.resize(img, (500,500))

for i in np:
  for k in i:
      count += 1

print("Size before passing to edge " + str(count) ) // --->>  250000 

result = ImageProc.edge(np)
cv2.imshow("image", np)
cv2.waitKey()

When I tried this with differently sized image I get the same result (9/10 of data is removed).

Answer 1

strlen counts as far as the first 0 in your data (it's designed for null terminated text strings). Also, if the first 0 it encounters is after your data has finished then it'll return a number that's too big meaning you might try to write to data you don't own.

To work out the size of the pyarrayobject you need to use arr->nd to work out the number of dimensions then arr->dimensions (an array) to work out how big each dimension is. You should also use arr->descr to work out what data type your array is rather than just testing it as char.