From clc-wiki

The C Programming Language, 2nd Edition, by Kernighan and Ritchie
Exercise 5.01 on page 97

As written, getint treats a + or - not followed by a digit as a valid representation of zero. Fix it to push such a character back on the input.

Solution by Gregory Pietsch

Here is Greg's solution:

#include <ctype.h>
#include <stdio.h> /* for EOF */

int getch(void);
void ungetch(int);

/* getint:  get next integer from input into *pn 
 * As written, getint treats a + or - not followed by a digit as a valid 
 * representation of zero. Fix it to push such a character back on the input.
 */
int getint(int *pn)
{
    int c, sign, sawsign;

    while (isspace(c = getch()))   /* skip white space */
        ;
    if (!isdigit(c) && c != EOF && c != '+' && c != '-') {
        ungetch(c);    /* it's not a number */
        return 0;
    }
    sign = (c == '-') ? -1 : 1;
    if (sawsign = (c == '+' || c == '-'))
        c = getch();
    if (!isdigit(c)) {
        ungetch(c);
        if (sawsign)
            ungetch((sign == -1) ? '-' : '+');
        return 0;
    }
    for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');
    *pn *= sign;
    if (c != EOF)
        ungetch(c);
    return c;
}