fifo-1.c

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

#define FIFO_SIZE 42

int fifo[FIFO_SIZE];
int fifo_pointer = 0;

void fatal_error (char *msg)
{
  fprintf (stderr, "%s\n", msg);
  exit (1);
}

void push (int x)
{
  if (fifo_pointer < 0 || fifo_pointer >= FIFO_SIZE)
    fatal_error ("fifo overflow");
  else
    fifo[fifo_pointer++] = x;
}

int pop (void)
{
  if (fifo_pointer < 1 || fifo_pointer > FIFO_SIZE)
    {
      fatal_error ("fifo underflow");
      return 0;
    }
  else
    return fifo[--fifo_pointer];
}

void dump (void)
{
  if (fifo_pointer < 0)
    fatal_error ("fifo underflow");
  else if (fifo_pointer >= FIFO_SIZE)
    fatal_error ("fifo overflow");
  else if (fifo_pointer == 0)
    printf ("fifo is empty\n");
  else
    {
      printf ("fifo:");
      for (int i = 0; i < fifo_pointer; i++)
        printf (" %d", fifo[i]);
      printf ("\n");
    }
}

int main (void)
{
  dump ();
  push (7);
  dump ();
  push (13);
  dump ();
  push (42);
  dump ();
  printf ("%d\n", pop ());  /* 42 */
  dump ();
  printf ("%d\n", pop ());  /* 13 */
  dump ();
  printf ("%d\n", pop ());  /*  7 */
  dump ();
  return 0;
}