Exercise 4.9 - getch and ungetch handling EOF Character

Question

Our getch and ungetch do not handle a pushed-back EOF correctly. Decide what their properties ought to be if an EOF is pushed back, then implement your design.

/**
 * getch and ungetch to handle EOF Character. In all the ungetch and getch
 * functions written so far, the buf is declared as char buf[BUFSIZ].
 * Changing this to int buf[BUFSIZ] enable it to handle EOF.  As EOF is an
 * integer declared in stdio.h having the value -1
 **/

#include<stdio.h>

#define BUFSIZE 100

int getch(void);

void ungetch(int c);

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

int main(void) {
    int c;

    c = '*';

    ungetch(c);

    while ((c = getch()) != EOF)
        putchar(c);

    return 0;
}

/* getch: get a (possibly pushed back) character */

int getch(void) {
    return (bufp > 0) ? buf[--bufp] : getchar();
}

/* ungetch: push a character back onto the input */

void ungetch(int c) {
    if (bufp >= BUFSIZE)
        printf("ungetch: too many characters \n");
    else
        buf[bufp++] = c;
}

Explanation

The previous getch and ungetch functions declared buf as char buf[BUFSIZE]. This has a limitation wherein the when an EOF character is encountered, it wont be stored in the buffer. The EOF character is an integer type. This problem can be solved by declaring our buf to be of integer type, like int buf[BUFSIZE] and ungetch(c) will store the character c, including EOF, now in an integer array.

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