Pointer to a const structure, can members still be modified?

Question

I have a structure which I want to pass to some external c code via some callback function that they register with my program. However, I want to pass that structure as read-only. My concern is that they can still modify the structures which I have pointer to inside the original structure that I pass. Explained with small example below:

struct s1 {
    int a;
    int b;
};

struct s2 {
    int x;
    struct s1 *y;
};

void f(const struct s2 *o)
{
    //o->x=10; //error
    o->y->a=20; //no error
    o->y->b=30; //no error
}

int main()
{
    struct s1 o1 = {10, 20};
    struct s2 o2 = {30, &o1};
    f(&o2);
}

So, How do I improve my code design so that they cannot modify anything w.r.t the structure I pass?

Answer 1

To properly handle this situation, you can only use forward declaration to hide members together with getter and setter functions.

Focus on code below and check:

• struct s1 only has forward declaration so you can make a pointer to it in struct s2.
• Actual implementation of struct s1 is in mylib.c thus all members are only visible to your library and not to user.
• Getters and setters are implemented to set/read the value to these hidden members as only your library has access to members, making it completely hidden from user.
• This forces him to use your functions.

mylib.h:

#ifndef __MYLIB_H
#define __MYLIB_H

//Create forward declaration only
//Implementation is in .c file
struct s1;

//Create user structure
struct s2 {
  int x;
  struct s1* y;
};

int get_a_from_s1(struct s2* s);
void set_a_to_s1(struct s2* s, int a);

#endif /* __MYLIB_H */

mylib.c:

#include "mylib.h"

//Now implement structure
struct s1 {
  int a, b;
};

//Make getter
int
get_a_from_s1(struct s2* s) {
  return s->y->a;
}

//Make setter
void
set_a_to_s1(struct s2* s, int a) {
  s->y->a = a;
}

main.c:

#include <stdio.h>
#include "mylib.h"

int main(void) {
  struct s2 s;
  int a;

  ....

  s.y->a = 5; //error

  //Set s1.a value from s2 structure
  set_a_to_s1(&s, 10); //OK

  //To view members of s1 inside s2, create member functions
  a = get_a_from_s1(&s); //OK

  printf("a: %d\r\n", a);

  return 0;
}

Of course, please make sure that ->y is not NULL or you have undefined behavior.