K&R C Programs Exercise 4-8 - C Program Examples

Exercise 4-8. Suppose that there will never be more than one character of pushback. Modify getch and ungetch accordingly.

The original K&R implementation uses an array buf[BUFSIZE] and an index bufp to support multiple pushed-back characters. If only one character of pushback is ever needed, the array becomes unnecessary. A single static int buf suffices — initialized to EOF to mean “empty”. This is simpler, uses less memory, and makes the constraint explicit in the code.

Solution

/* K&R Exercise 4-8 — getch/ungetch with at most one character of pushback
 * Compile: gcc -ansi -Wall ex4-8.c -o ex4-8 */
#include <stdio.h>

/* buf == EOF means the buffer is empty */
static int buf = EOF;

int getch(void)
{
    int c = buf;
    buf = EOF;
    return (c != EOF) ? c : getchar();
}

void ungetch(int c)
{
    buf = c; /* no bounds check needed — only one slot */
}

int main(void)
{
    int c;
    ungetch('X');
    c = getch(); putchar(c);         /* X */
    c = getch(); putchar(c);         /* reads from stdin */
    putchar('\n');
    return 0;
}

Before and After Comparison

Version	Buffer	Capacity	Overflow handling
Original	`char buf[100]; int bufp`	100 chars	Error message on overflow
Exercise 4-8	`int buf = EOF`	1 char	Silently overwrites (one-slot constraint is a precondition)

Compile and Run

echo "hello" | gcc -ansi -Wall ex4-8.c -o ex4-8 && ./ex4-8

Sample Output

Xh

X comes from the pushed-back character; h is the first character from stdin (hello).

What This Exercise Teaches

Right-sizing state — when you know a constraint will never be violated, you don’t need infrastructure to handle its violation; simpler code is often better code
EOF as sentinel — using EOF (-1) to mean “empty” works because all valid characters fit in unsigned char, which is why buf must be int, not char
Same interface, simpler internals — getch and ungetch have identical signatures; callers need not change

Set Up Your C Environment

← Exercise 4-7 |
Chapter 4 Solutions |
Exercise 4-9 →

Book: The C Programming Language, 2nd Ed — Kernighan & Ritchie