尽管 const 限定符声明，为什么嵌套结构成员会改变？

Question

I observed nested structure member get alter despite my const qualifier declaration, may I know any method for me to avoid nested structure member get alter inside the function?

Code

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

struct middle_data
{
    struct base_data *int_data_ptr;
};

struct base_data
{
    int value;
};

void function( const struct  middle_data const *middle_ptr)
{

    middle_ptr->int_data_ptr->value= 2; // how to forbid value get alter?

    struct base_data *ptr= middle_ptr->int_data_ptr;

    printf("ptr->value = %d\n",  ptr->value);
}

int main(int argc, char **argv)
{

   struct  middle_data *middle_data_ptr;

    middle_data_ptr = (struct  middle_data*)malloc(sizeof(*(middle_data_ptr)));

    middle_data_ptr->int_data_ptr =malloc(sizeof(*(middle_data_ptr->int_data_ptr)));

    middle_data_ptr->int_data_ptr->value=3;

    function(middle_data_ptr);

    printf("  middle_data_ptr->int_data_ptr->value = %d\n",    middle_data_ptr->int_data_ptr->value);

    free(middle_data_ptr);
    free(middle_data_ptr->int_data_ptr);
    return 0;
}

Answer 1

唯一的方法是将 struct base_data的成员设为const

Answer 2

For starters in this function declaration

void function( const struct  middle_data const *middle_ptr)

one qualifier const is redundant. They both mean that the pointed object is constant not the pointer itself.

So this qualifier const means that data members of the pointed object are also constant that is in fact you have

struct base_data * const int_data_ptr;

That is you have a constant pointer that points to a non-constant object that may be changed.

If you do not want that the pointed object would be changed then you have to declare the structure like

struct middle_data
{
    const struct base_data *int_data_ptr;
};