Simple shared pointer lib in C

shared_ptr.h

#ifndef SHARED_PTR_H
#define SHARED_PTR_H

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

#ifdef TEST
#include <assert.h>
#include <string.h>
#define streq(a, b) (!strcmp((a), (b)))
#endif

/* API */

typedef struct shared_ptr shared_ptr;

shared_ptr*	sp_new(void **obj, void (*free_fn)(void *));
shared_ptr*	sp_copy(const shared_ptr *sp);
void	sp_free(shared_ptr *sp);
void*	sp_get(const shared_ptr *sp);

/* Implementation */

#ifdef TEST
int malloc_count = 0;
int free_count = 0;

void *_malloc(size_t size) {
	malloc_count++;
	return malloc(size);
}

void _free(void *ptr) {
	free_count++;
	free(ptr);
}

#define malloc _malloc
#define free _free
#endif

typedef struct obj {
	char *content;
} obj;

typedef struct inner_ptr {
	_Atomic int count;
	void *obj;
	void (*free_fn)(void *);
} inner_ptr;

struct shared_ptr {
	inner_ptr *ptr;
};

shared_ptr* sp_new(void **obj, void (*free_fn)(void *)) {
	shared_ptr *sp = (shared_ptr*)malloc(sizeof(shared_ptr));
	sp->ptr = (inner_ptr*)malloc(sizeof(inner_ptr));
	sp->ptr->count = 1;
	sp->ptr->obj = *obj;
	sp->ptr->free_fn = free_fn;
	*obj = NULL;
	return sp;
}

shared_ptr* sp_copy(const shared_ptr *sp) {
	atomic_fetch_add(&sp->ptr->count, 1);
	shared_ptr *new_sp = (shared_ptr*)malloc(sizeof(shared_ptr));
	new_sp->ptr = sp->ptr;
	return new_sp;
}

void sp_free(shared_ptr *sp) {
	if (atomic_fetch_sub(&sp->ptr->count, 1) == 1) {
		sp->ptr->free_fn(sp->ptr->obj);
		free(sp->ptr);
	}
	free(sp);
}

void* sp_get(const shared_ptr *sp) {
	return sp->ptr->obj;
}

/* Example */

#ifdef TEST
int main(int argc, char const *argv[])
{
	obj *o = malloc(sizeof(struct obj));
	char *str = "Hello World";
	o->content = str;

	shared_ptr *sp = sp_new((void **)&o, free);
	assert(o == NULL);
	assert(streq(str, ((obj*)(sp_get(sp)))->content));

	shared_ptr *sp2 = sp_copy(sp);

	assert(streq(str, ((obj*)(sp_get(sp2)))->content));
	assert(streq(str, ((obj*)(sp_get(sp)))->content));

	sp_free(sp2);

	assert(streq(str, ((obj*)(sp_get(sp)))->content));

	sp_free(sp);

	assert(malloc_count == free_count);

	printf("tests passed\n");

	return 0;
}
#endif

#endif // SHARED_PTR_H