Simple stack implementation in C

stack.c

#include<stdlib.h>
#include "stack.h"

stack make_stack() {
    stack s = {
        (int*)malloc(BUFSIZE*sizeof(int)),
        BUFSIZE,
        0
    };
    return s;
}

void reallocate(stack* s) {
    int* new_buf = (int*)malloc(s->bufsize*2);
    for (int i = 0; i < s->bufsize; i++) {
        new_buf[i] = s->buffer[i];
    }
    free(s->buffer);
    s->buffer = new_buf;
    s->bufsize *= 2;
}

void push(stack* s, int el) {
    s->buffer[s->ptr++] = el;
    if (s->bufsize == s->ptr)
        reallocate(s);
}

int pop(stack* s) {
    return s->buffer[--s->ptr];
}

int size(stack* s) {
    return s->ptr;
}

int empty(stack* s) {
    return size(s) == 0;
}

int peek(stack* s) {
    return s->buffer[s->ptr-1];
}

stack.h

#define BUFSIZE 4

typedef struct
{
    int *buffer;
    int bufsize;
    int ptr;
} stack;

stack make_stack();
void push(stack* s, int el);
int pop(stack* s);
int peek(stack* s);
int size(stack* s);
int empty(stack* s);