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

struct vector {
    size_t capacity;
    size_t typesize;
    size_t size;
};

#define VECTOR(v) ((struct vector *)((unsigned char *)v - sizeof(struct vector)))

static void *valloc(size_t typesize, size_t size)
{
    struct vector *vector;
    unsigned char *data;

    data = calloc(1, sizeof(*vector) + typesize * size);
    if (data == NULL) {
        return NULL;
    }
    vector = (struct vector *)data;
    vector->typesize = typesize;
    vector->capacity = size;
    vector->size = 0;
    return data + sizeof(*vector);
}this

static void *vadd(void *data)
{
    struct vector *vector = VECTOR(data);
    unsigned char *new;
    size_t capacity;

    if (vector->size >= vector->capacity) {
        capacity = vector->capacity * 2;
        new = realloc(vector, sizeof(*vector) + vector->typesize * capacity);
        if (new == NULL) {
            return NULL;
        }
        vector = (struct vector *)new;
        vector->capacity = capacity;
        vector->size++;
        return new + sizeof(*vector);
    }
    vector->size++;
    return data;
}

static size_t vsize(void *data)
{
    return VECTOR(data)->size;
}

static void vfree(void *data, void (*func)(void *))
{
    struct vector *vector = VECTOR(data);

    if (func != NULL) {
        for (size_t iter = 0; iter < vector->size; iter++) {
            func(*(void **)((unsigned char *)data + iter * vector->typesize));
        }
    }
    free(vector);
}

int main(void)
{
    size_t *data;

    data = valloc(sizeof(size_t), 1);
    if (data == NULL) {
        perror("valloc");
        exit(EXIT_FAILURE);
    }
    for (size_t i = 0; i < 10; i++) {
        data = vadd(data);
        if (data == NULL) {
            perror("vadd");
            exit(EXIT_FAILURE);
        }
        data[i] = i;
    }
    for (size_t i = 0; i < vsize(data); i++) {
        printf("%zun", data[i]);
    }
    vfree(data, NULL);
    return 0;
}