Shared Pointer

shared_ptr.c

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

static inline void increment(unsigned *cnt)
{
	__atomic_fetch_add(cnt, 1, __ATOMIC_RELAXED);
}

static inline bool decrement(unsigned *cnt)
{
	return __atomic_fetch_sub(cnt, 1, __ATOMIC_ACQ_REL) == 1;
}

/*
static inline bool try_increment(unsigned *cnt)
{
	unsigned n = __atomic_load_n(cnt, __ATOMIC_RELAXED);
	while (n > 0) {
		if (__atomic_compare_exchange_n(cnt, &n, n+1, true, __ATOMIC_RELAXED, __ATOMIC_RELAXED)) {
			return true;
		}
	}
	return false;
}
*/

struct Shared {
	unsigned strong_ref;
	unsigned weak_ref;
	void (*final)(void *);
};

struct Thing {
	struct Shared shared;
	int id;
};

static void
final_thing(void *ptr)
{
	printf("final: %d\n", ((struct Thing *)ptr)->id);
}

static struct Thing *
make_thing(int id)
{
	struct Thing *t = malloc(sizeof(*t));
	t->shared.strong_ref = 1;
	t->shared.weak_ref = 1;
	t->shared.final = final_thing;
	t->id = id;
	return t;
}

static struct Thing *
weak_retain(struct Thing *t)
{
	if (t) {
		increment(&t->shared.weak_ref);
	}
	return t;
}

static struct Thing *
retain(struct Thing *t)
{
	if (t) {
		increment(&t->shared.strong_ref);
	}
	return t;
}

static void
weak_release(struct Thing *t)
{
	if (t && decrement(&t->shared.weak_ref)) {
		free(t);
	}
}

static void
release(struct Thing *t)
{
	if (t && decrement(&t->shared.strong_ref)) {
		t->shared.final(t);
		weak_release(t);
	}
}

int
main(void)
{
	struct Thing *t = make_thing(5);
	struct Thing *t2 = retain(t);
	struct Thing *t3 = weak_retain(t);
	release(t);
	weak_release(t3);
	release(t2);
}