Postfix CLI Calculator in C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#define STACKSIZE 250

typedef struct{
        int top;
        long long int items[STACKSIZE];
        }STACK;


void push(long long int x, STACK *ps);
int pop(STACK *ps);
int ispositivedigit(char* input);
int isnegativedigit(char* input);  // function for detection negative numbers
void printtop(STACK *ps);  // bonus



int main()
{
    char input[1024];  // array for loaded values

    int end = 1;  // detects end of input
    int count = 0;  // variable for detection if no arguments were enetred
    STACK ps = {0, {0}};
    long long int number, a, b, result, printresult;


    while (end != EOF) {

        // Read next token
        end = scanf("%s", input);
        count += 1;
        if(end == EOF) {
            count -= 1;
            break;
        }

        // If it's a numebr
        if (ispositivedigit(input) || isnegativedigit(input)) {
            number = atoll(input);  // converts string to long long int
            push(number, &ps);  // add a number to the top of the stack
        }
        // It it's an operator
        else if (isoperator(input)) {
            // p
            if (strcmp(input, "p") == 0) {
                printtop(&ps);
            }
            // +
            else if (strcmp(input, "+") == 0) {
                a=pop(&ps); b=pop(&ps);
                result = a + b;
                push(result, &ps);
            }
            // -
            else if (strcmp(input, "-") == 0) {
                a=pop(&ps); b=pop(&ps);
                result = a - b;
                push(result, &ps);
            }
            // *
            else if (strcmp(input, "*") == 0) {
                a=pop(&ps); b=pop(&ps);
                result = a * b;
                push(result, &ps);
            }
            // /
            else if (strcmp(input, "/") == 0) {
                a=pop(&ps); b=pop(&ps);

                if (a == 0) {
                    fprintf(stderr, "Error: Division by zero!\n");
                    exit(EXIT_FAILURE);
                }
                else result = b / a;

                push(result, &ps);
            }
        }
        // Detect invalid input
        else {
            fprintf(stderr, "Error: Invalid input!\n");
            exit(EXIT_FAILURE);
        }
    }

    // Returns error when no arguments are entered
    if (count == 0) {
        fprintf(stderr, "Error: No arguments!\n");
        exit(EXIT_FAILURE);
    }


    // If the last character isn't "p", the program print the result clasically
    if (strcmp(input, "p") != 0) {
        printresult = pop(&ps);
        printf("%lld\n", printresult);
    }

    return 0;
}



void push(long long int x, STACK *ps)
{
    if (ps->top == STACKSIZE-1){
        fprintf(stderr, "Error: Stack overflow!\n");
        exit(EXIT_FAILURE);
    } else
        ps->items[++(ps->top)] = x;

    return;
}



int pop(STACK *ps)
{
    if (ps->top == 0) {
        fprintf(stderr, "Error: Stack underflow!\n");
        exit(EXIT_FAILURE);
    }
    else
        return(ps->items[(ps->top)--]);
}



bool ispositivedigit(char* input)
{
    // cycle which determines if each character from input is a number or not
    for (unsigned int i=0; i<strlen(input); i++) {
        if (isdigit(input[i]) == 0) {}
            return false;
        }
    }

    return true;
}



bool isnegativedigit(char* input)
{
    if (strlen(input) > 1 && strncmp(input, "-", 1) == 0) {
        for (unsigned int i = 1; i < strlen(input); i++) {
            if (isdigit(input[i]) == 0) {
                return false;  // if a character different from number appears, function returns false
            }
        }
    }
    else
        return false;  // if the firts character isn't "-", function returns false

    return true;  // it is a negative number
}



bool isoperator(char* input)
{
    if (strlen(input) != 1) return false;  // only one-char operators are allowed

    if (strncmp(input, "+", 1) == 0) return true;
    if (strncmp(input, "-", 1) == 0) return true;
    if (strncmp(input, "*", 1) == 0) return true;
    if (strncmp(input, "/", 1) == 0) return true;
    if (strncmp(input, "p", 1) == 0) return true;

    return false;
}



void printtop(STACK *ps)  // bonus
{
    // not empty stuck
    if (ps->top != 0)
        printf("%lld \n", ps->items[ps->top]);
    // empty stack
    else {
        fprintf(stderr, "Error: Attempt to call for an empty stack!\n");
        exit(EXIT_FAILURE);
    }
}