Puzzle SL37

What does the following project write to the monitor?

/* --- stack.h --- */
void push( int item );
int pop();
int empty();
int full();

/* --- stack.c --- */
#define stackSize 16

static int stack[stackSize];

/* top is the top of the stack */
static int top = -1;

void push( int item )
{
  top++ ;
  stack[top] = item;
}

int pop()
{
  int item;
  item = stack[top];
  top--;
  return item;
}

int empty()
{
  return top == -1;
}

int full()
{
  return top == stackSize;
}

/* --- mainStack.c  --- */
#include <stdio.h>
#include "stack.h"

int main()
{
  if ( !full() ) push( 13 );
  if ( !full() ) push( -1 );
  if ( !full() ) push( 8 );
  if ( !full() ) push( 12 );

  if ( !empty() ) printf("%d\n",  pop() );
  if ( !empty() ) printf("%d\n",  pop() );
  if ( !empty() ) printf("%d\n",  pop() );
}