fat.c

#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <assert.h>

struct Array {
    size_t type_size, element_count;
    uint8_t data[];
};

static void *create_array(size_t type_size, size_t element_count) {
    struct Array *array = calloc(1, sizeof(struct Array) + type_size * element_count);

    array->type_size = type_size;
    array->element_count = element_count;

    return array->data;
}

static void free_array(void *array)
{ free(((struct Array *)array) - 1); }

static size_t type_size(void *array)
{ return ((struct Array *)array - 1)->type_size; }

static size_t element_count(void *array)
{ return ((struct Array *)array - 1)->element_count; }

typeof(int[2]) *pair(int i, int x)
{
    int (*array)[2] = create_array(sizeof(int), 2);
    (*array)[0] = i;
    (*array)[1] = x;
    return array;
}


int main()
{
    int (*array)[3] = create_array(sizeof(int), 3);
    (*array)[0] = 1;
    (*array)[1] = 2;
    (*array)[2] = 3;

    assert(type_size(array) == sizeof(int));
    assert(element_count(array) == 3);
    int *headptr = (int *)&array[0];
    int *plainify = &(*array)[0];
    assert(headptr == plainify);
  
    free_array(array);
}