implement getch and ungetch to handle pushed-back EOF correctly.

#include <stdio.h>

#define BUFSIZE  100

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

/*getch: get a (possibly pushed back) character    */
int getch(void){
    return (bufp > 0) ? buf[--bufp): getchar();
}
/*ungetch: push character back onto the input */
void ungetch(int c){
     if (bufp >= BUFSIZE)
          printf("ungetch: too many characters\n");
     else
          buf[bufp++] = c;
}