RPC ex 4.3 K&R

#include <mylib.c>
#include <math.h>

#define MAXOP 100
#define NUMBER '0'

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

int sp = 0;         /* next free stack position */

/* get a (possibly pushed back) character, */
int
getch(void)
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}

/* push character back to input */
void ungetch(int c)
{
    if(bufp >= BUFSIZE)
        printf("ungetch: to many characters\n");
    else
        buf[bufp++] = c;
}

// PUSH & POP

#define MAXVAL 100  /* maximum depth of val stack */
double val[MAXVAL]; /* value stack */

/* push on top of the stack*/

void
push(double f)
{
    if(sp < MAXVAL)
        val[sp++] = f;
    else
        printf("error: stack if full, can't push %g\n", f);
}

/* extract end return a value on top of the stack*/
double
pop(void)
{
    if(sp > 0)
        return val[--sp];
    else
    {
        printf("error: stack is empty\n");
        return 0.0;
    }

}

/*
  get next operator or numeric operand
 */

int
getop(char s[])
{
    int i, c;

    while((s[0] = c = getch()) == ' ' || c == '\t')
        ;

    s[1] = '\0';

    if(!isDigit(c) && c != '.' && c != '-')
        return c; // is not a number or a minus
    i = 0;

    if (c == '-')
    {
        if((!isdigit(c = getch()) && c != '.'))
        {
            ungetch(c);
            return '-';
        }
        else if (c == '.')
        {
            s[++i] = c;
        }
        else
            ungetch(c);
    }

    if(isDigit(c)) //  collect integer part
        while(isDigit(s[++i] = c = getch()))
            ;

    if(c == '.') // collect fraction part
        while(isDigit(s[++i] = c = getch()))
            ;

    s[i] = '\0';

    if(c != EOF)
        ungetch(c);

    return NUMBER;
}

/*
  reverse polish calculator
 */
main()
{
    int type;
    double op2;
    char s[MAXOP];

    while((type = getop(s)) != EOF)
    {
        switch(type)
        {
            case NUMBER:
                push(atof(s));
                break;

            case '+':
                push(pop() + pop());
                break;

            case '*':
                push(pop() * pop());
                break;

            case '-':
                op2 = pop();
                push(pop() - op2);
                break;

            case '/':
                op2 =  pop();

                if(op2 != 0.0)
                    push(pop() / op2);
                else
                    printf("error : division by zero\n");
                break;
            case '%':
                op2 = pop();

                if(op2 != 0.0)
                    push(fmod(pop() , op2));
                else
                    printf("error : division by zero\n");
                break;

            case '\n':
                    printf("\t%.8g\n", pop());
                break;
            default:
                printf("error: command unknown %s\n", s);
                break;
        }
    }
    return 0;
}