runner% cat main.c
/* 
 * main.c 
 * fetch characters and push, then pop
 * and print (in reverse order).
 */
#include <stdio.h>
#include "stack.h"

void main(void)
{
        char c;
        while ((c = getchar()) != '\n' && !full())
                push(c);
        while (!empty())
                printf("%c", c = pop());
        printf("\n");
}
-------------------------------------------------
runner% cat stack.h
/* 
 * stack.h -- stack header file
 */
typedef char Stacktype;
Stacktype pop(void);
void push(Stacktype);
int empty(void);
int full(void);
-------------------------------------------------
runner% cat stack.c
/* 
 * stack.c -- stack implementation
 */
#include <stdio.h>
#include "stack.h"
#define S_SIZE 12
static Stacktype s[S_SIZE];
static int sp = 0;

/* pop: remove top element from stack */
Stacktype pop(void)
{
        if (!empty())
                return s[--sp];
        else {

                fprintf(stderr,
                        "Underflow\n");
                return 0;
        }
}

/* push: add new element to stacktop */
void push(Stacktype c)
{
        if (!full())
                s[sp++] = c;
        else
                fprintf(stderr,
                        "Overflow\n");
}

/* empty: check if stack is empty */
int empty(void)
{
        return sp == 0;
}

/* full: check if stack is full */
int full(void)
{
        return sp == S_SIZE;
}
-------------------------------------------------
runner% cat makefile
stack: main.c stack.c stack.h
        cc -g -o stack main.c stack.c
lint:
        lint -m -u main.c stack.c

runner% stack
missippippi
ippippissim
runner% stack
madamimadam
madamimadam
runner% stack
abcdefghijklmnop
lkjihgfedcba