fifo-8.c

#include <stdio.h>

#define FIFO_SIZE 10

int fifo[FIFO_SIZE];
int fifo_read = 0;
int fifo_write = 0;

void push (int x)
{
  if ((fifo_write + 1) % FIFO_SIZE == fifo_read)
    fprintf (stderr, "push(): FIFO is full\n");
  else
    {
      fifo[fifo_write++] = x;
      if (fifo_write >= FIFO_SIZE)
        fifo_write = 0;
    }
}

int pop (void)
{
  if (fifo_read == fifo_write)
    {
      fprintf (stderr, "pop(): FIFO is empty\n");
      return -1;
    }
  else
    {
      int result = fifo[fifo_read++];
      if (fifo_read >= FIFO_SIZE)
        fifo_read = 0;
      return result;
    }
}

int main (void)
{
  push (3);
  push (7);
  push (137);
  printf ("%d\n", pop ());
  printf ("%d\n", pop ());
  printf ("%d\n", pop ());
  printf ("%d\n", pop ());
  for (int i = 0; i < 15; i++)
    push (i);
  return 0;
}