#define RESZ_AMT 0x100

typedef struct {
    void **item;
    size_t sz, rsz;
} stack_t;

stack_t *stack_init()
{
    stack_t this = malloc(sizeof(stack_t));
    this->item = malloc(this->rsz = RESZ_AMT);
    this->sz = 0;
    return this;
}

void stack_push(stack_t *this, void *item)
{
    if (this->sz == this->rsz) {
        this->item = realloc(this->item, (this->rsz += RESZ_AMT)
                                         * sizeof(void *));
    }
    this->item[this->sz++] = item;
}

void *stack_pop(stack_t *this)
{
    if (this->sz == 0) return NULL;

    if (--this->sz && this->sz = this->rsz - RESZ_AMT) {
        this->item = realloc(this->item, (this->rsz -= RESZ_AMT)
                                         * sizeof(void *));
    }
    return this->item[this->sz];
}

void stack_free(stack_t *this)
{
    free(this->item);
    free(this);
}