The C Programming Language, 2nd Edition, by Kernighan and Ritchie
Base source code on which to build solutions to exercises 4-3 through 4-10.

This code is a near-direct copy of code from within the book referenced above and is not under the default public domain licence of the code in the rest of this wiki - its copyright remains with its original author and publisher who can be contacted for details. It's reproduced here assuming implicit permission given that direct permission was obtained for it to be included in the original site from which this part of the wiki was seeded.

/* this program forms the basis on which exercises 4-3 through 4-10 build */

#include <stdio.h>
#include <stdlib.h> /* for atof() - in K&R, math.h is referenced - this is an anachronism */

#define MAXOP 100 /* max size of operand or operator */
#define NUMBER '0' /* signal that a number was found */

int getop(char []);
void push(double);
double pop(void);

/* reverse Polish calculator */

int main(void)
{
  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: zero divisor\n");
        break;
      case '\n':
        printf("\t%.8g\n", pop());
        break;
      default:
        printf("error: unknown command %s\n", s);
        break;
    }
  }

  return 0;
}

#define MAXVAL  100 /* maximum depth of val stack */

int sp = 0; /* next free stack position */
double val[MAXVAL]; /* value stack */

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

/* pop: pop and return top value from stack */
double pop(void)
{
  if(sp > 0)
    return val[--sp];
  else
  {
    printf("error: stack empty\n");
    return 0.0;
  }
}

#include <ctype.h>

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

/* getop: 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 != '.')
    return c; /* not a number */
  i = 0;
  if(isdigit(c)) /* collect integer part */
    while(isdigit(s[++i] = c = getch()))
      ;
  if(c == '.')
    while(isdigit(s[++i] = c = getch()))
      ;
  s[i] = '\0';
  if(c != EOF)
    ungetch(c);
  return NUMBER;
}

#define BUFSIZE 100

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

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

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