Pass C struct as "deep" const

Question

I have a struct which holds some pointers to data it does not own, e.g.:

struct s {
    int* x;
    int* y;
};

I can then pass it as a const pointer itself:

int foo(const struct s* s) {
    return *s->x + *s->y;
}

This however is not as safe as I want, because while foo is not allowed to modify the pointers s->x and s->y, it is allowed to modify the ints they point to:

int foo(const struct s* s) {
    // s->x = s->y;  // compiler error: assignment of member in read-only object
    *s->x = *s->y;  // allowed!
    return *s->x + *s->y;
}

Is there any way to change the signature of foo so that it treats the passed struct as "deeply" const, including all its pointers (possibly nested)?

In C++ I would solve it by making the pointers private and overloading getters:

class foo {
    private:
    int *_x;
    int *_y;

    public:
    s(int* x, int* y) : _x{ x }, _y{ y } {}
    int* x() { return _x; }
    const int* x() const { return _x; }
    int* y() { return _y; }
    const int* y() const { return _y; }
};

Answer 1

Is there any way to change the signature of foo so that it treats the passed struct as "deeply" const, including all its pointers (possibly nested)?

No. Because the members of s are declared as int * which means you are allowed to change what they point to. I can't see a way around that short of changing the declaration of s to make the pointers point to const integers. This, for example errors:

struct SConst 
{
    const int* x;
    const int* y;
};

int foo(const struct SConst* s) 
{
    *s->x = *s->y; // Error! 
    return *s->x + *s->y;
}

This is what I see if I compile the above (with clang)

foo.c:19:8: error: read-only variable is not assignable
   19 |         *s->x = *s->y;
      |         ~~~~~ ^

Of course, it does mean you can't assign to the things the members point to.

    int x = 1;
    int y = 2;
    struct SConst baz = { .x = &x, .y = &y };
    *baz.x = 5; // error!

But then, maybe that's what you want since you can still assign to the thing pointed to.

int main()
{
    int x = 1;
    int y = 2;
    struct SConst baz = { .x = &x, .y = &y };
    x = 5;
    printf("%d\n", *baz.x); // prints 5
}