shared_ptr.h

#pragma once
#include <stddef.h>
#include <stdlib.h>
#include <string.h>

/*  Usage : this is very simple. To create a shared_ptr, you just need to
 *  call the make_shared function with the type of your pointer, and the
 *  number of element you want to allocate.
 *  To pass a pointer from a function to another, there is to cases :
 *       - From caller to callee : you just need to pass the pointer as is.
 *       - From callee to caller : you need to return the pointer with
 *           reference(ptr)
 */

struct meta {
    size_t count;
};

void clean_this_shit(void *ptr)
{
    char *t = *(char **)ptr;
    struct meta *tmp = (struct meta *)(t - sizeof(*tmp));
    if ( !(--tmp->count) )
        free(tmp);
}

#define shared __attribute__((cleanup(clean_this_shit)))

#define make_shared(Type, elt) ({    \
    struct meta tmp_s = {           \
        .count = 1                  \
    };                              \
    char *res = malloc((sizeof(Type) * (elt)) + sizeof(struct meta)); \
    memcpy(res, &tmp_s, sizeof(tmp_s)); \
    (void *)(res + sizeof(tmp_s)); \
})

#define reference(ptr) ({\
    char *t = (char *)ptr; \
    struct meta *tmp = (struct meta *)(t - sizeof(struct meta)); \
    tmp->count++; \
    ptr; \
})