Generic stack in ANSI C

stack.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct stack {
	size_t capacity;
	size_t size;
	size_t pos;
	size_t us;
	void *container;
} stack;

stack *newStack(size_t capacity, size_t unitSize)
{
	stack *s = (stack *) malloc(sizeof(stack));
	s->capacity = capacity;
	s->pos = 0;
	s->us = unitSize;
	s->container = malloc(unitSize * capacity);
	return s;
}

int push(stack * s, void *v)
{
	if (!s)
		return -1;
	if (s->pos == s->capacity) {
		return -1;
	}
	memcpy(s->container + s->us * s->pos, v, s->us);
	s->pos++;
	return 0;
}

void pop(stack * s)
{
	if (!s || !s->pos)
		return;
	s->pos--;
}

void *top(stack * s)
{
	if (!s || !s->pos)
		return NULL;
	return (char *)(s->container + (s->pos - 1) * s->us);
}

int main()
{
	stack *s = newStack(10, sizeof(int));
	for (int i = 0; i < 10; i++) {
		int ii = i * 10000;
		int res = push(s, &ii);
		printf("Pushed %d (res=%d)\n", ii, res);
	}

	int x = 10;
	while (x--) {
		printf("%d\n", *(int *)top(s));
		pop(s);
	}

	stack *s2 = newStack(10, sizeof(char **));

	const char *marek = "marek";
	const char *ania = "anna";

	push(s2, (void *)&marek);
	push(s2, (void *)&ania);

	char *t1 = *(char **)top(s2);
	printf("Got: %s\n", *(char **)top(s2));
	pop(s2);

	printf("Got: %s\n", *(char **)top(s2));
	pop(s2);

	return 0;
}