A stack that grows with realloc() in C Solution, output & explanation

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

struct stack {
    int *items;
    size_t len, cap;
};

bool push(struct stack *s, int v) {
    if (s->len == s->cap) {
        size_t cap = s->cap ? 2 * s->cap : 2;
        int *tmp = realloc(s->items, cap * sizeof *tmp);
        if (tmp == NULL)
            return false;
        s->items = tmp;
        s->cap = cap;
    }
    s->items[s->len++] = v;
    return true;
}

bool pop(struct stack *s, int *v) {
    if (s->len == 0)
        return false;
    *v = s->items[--s->len];
    return true;
}

int main(void) {
    struct stack s = {0};
    for (int v = 1; v <= 10; v++)
        if (!push(&s, v))
            return 1;
    int v;
    printf("popped:");
    for (int i = 0; i < 3 && pop(&s, &v); i++)
        printf(" %d", v);
    printf("\nlen %zu, cap %zu\n", s.len, s.cap);
    free(s.items);
    return 0;
}

popped: 10 9 8
len 7, cap 16