Stack infix in C

Based on the post of "Stack implementation in C", here's Stack infix implementation in C:

// the idea of this example from "Data Structures and Algorithms using C#"
// main_stack_infix.c

#include <stdio.h>
#include <ctype.h>
#include "stack.h"

#define CHAR_TO_INT(x) ((x)-48)

void calc(struct stack*, struct stack*);

int main(void)
{
    struct stack s1, s2;
    char *equation = "1 + 2 - 9 ";
    int count=0;

    init(&s1);
    init(&s2);

    while (*equation != '\0')
    {
        if (count == 2)
        {
            calc(&s1, &s2);
            count=1;
        }

        if (isdigit(*equation) != 0)
        {
            push(&s1, CHAR_TO_INT(*equation));
            count++;
        }else if (ispunct(*equation) != 0)
        {
            push(&s2, *equation);
        }
        equation++;     
    }
    printf("%i\n", peek(&s1));
    return 0;
}

void calc(struct stack *s1, struct stack *s2)
{
    int x = pop(s1);
    int y = pop(s1);
    int op = pop(s2);
    int ret = 0;
    
    switch (op)
    {
    case '+':
        ret = y+x;
        break;
    case '-':
        ret = y-x;
        break;
    case '*':
        ret = y*x;
        break;
    case '/':
        ret = y/x;
        break;
    }
    
    push(s1, ret);
}