使用 ctypes 将 Struct 从 C 返回到 Python 的问题

Question

I am trying to get the values of C struct member variables from within python using ctypes. My expected return values for x and y are 10 and 20 respectively. I am thinking I might be neglecting something subtle but not sure what it is. The output I get is 0 for x and y as shown at the end of the post. Any pointers appreciated.

Python code:

import ctypes
import os

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

directory = os.path.dirname(os.path.realpath(__file__))
print(directory)
source = directory + "\\cstruct.so"
 
clibrary = ctypes.CDLL(source)

clibrary.getPoint.restype = ctypes.POINTER(Point)
p1 = clibrary.getPoint()
print(p1.contents.x, p1.contents.y)

C code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


struct Point
{
    int x;
    int y;
};


struct Point* getPoint()
{
    struct Point *p;
    p->x = 10;
    p->y = 20;
    return p;
}

C code is compiled into a shared library file cstruct.so which is called in the python file.

Python Output:

0 0

Answer 1

I found out what the issue is. I had to dynamically allocate the size of the struct Point in the C file. Previously, I had not done this. This solves the issue.

Just modified the first line in the struct Point* getPoint() function as shown below.

struct Point *p = malloc(sizeof(struct Point));

Also added a C function in the c file to free the memory from the struct pointer as shown below.

void free_mem(struct Point* p) 
{
    free(p);
}