knr/5.1.1.c

#include <stdio.h>
#define BUFSIZE 100
#define SIZE 16

   char buf[BUFSIZE];    /* buffer for ungetch */
   int bufp1 = 0;         /* next free position in buf */

   int getch(void)  /* get a (possibly pushed-back) character */
   {
       return (bufp1 > 0) ? buf[--bufp1] : getchar();
   }

   void ungetch(int c)   /* push character back on input */
   {
       if (bufp1 >= BUFSIZE)
           printf("ungetch: too many characters\n");
       else
           buf[bufp1++] = c;
   }