Exercise 4-9

The reason why the two functions may not be able to handle a pushed-back EOF is because EOF is not a character, but buf is declared as a character array (note that the functions may still work on C implementations that use signed chars.) Thus, we change buf to be an integer array to retain the ability to push back an EOF on all systems.

    ...
    int sp = 0;         /* next free stack position */
    double val[MAXVAL]; /* value stack */
    double vars[26];    /* variable values */
    char buf[BUFSIZE];  /* buffer for ungetch */
    int buf[BUFSIZE];   /* buffer for ungetch */
    int bufp = 0;       /* next free position in buf */
    ...