Обратная польская запись - C (СИ)

int main(){
        double s[STACK_SIZE];
        int s_top = STACK_EMPTY;
        int i;
        printf("Enter mathematical expression: ");
        while(!feof(stdin)){
                int c = getchar();
                double x;
                switch (c) {
                        case '+': push(s, (pop(s, &s_top) + pop(s, &s_top)), &s_top, STACK_SIZE);break;
                        case '\n': printf("Result = %lf\n", pop(s, &s_top)); break;
                        default:
                                ungetc(c, stdin);
                                if(scanf("%lf", &x) != 1){
                                        fprintf(stderr, "You enter incorrect expression\n ") ;
                                        return -1;
                                }
                                else{
                                        push(s, x, &s_top, STACK_SIZE);
                                }
                                break;
                }
        }
        return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#define STACK_SIZE      10
#define STACK_FULL      -2
#define STACK_EMPTY     -1
#define NORMAL          0
 
int myerror = NORMAL;
 
void push(double [], double, int *, int);
double pop(double [], int *);
 
void push(double stack[], double item, int *top, int max_size){
        if(*top < max_size-1){
                ++(*top);
                stack[*top] = item;
                myerror = NORMAL;
        }
        else
                myerror = STACK_FULL;
}
 
double pop(double stack[], int*top){
        double item;
        if (*top >= 0){
                item = stack[*top];
                --(*top);
                myerror = NORMAL;
        }
        else{
                item = STACK_EMPTY;
                myerror = STACK_EMPTY;
        }
        return item;
}