简单计算器在C语言中不能处理大数字

我有这个密码

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

long result = 0;
long *resultPointer = &result;

long plus(long *current, long num) {
    return (*current + num);
}

long krat(long *current, long num) {
    return (*current * num);
}

int main() {
    char operator[4];
    long num;

  printf("%d\n", 0);
    while(scanf("%s %li", &operator[0], &num) != EOF){
        if (num > 0) {
            if (strcmp(operator, "krat") == 0) {
                *resultPointer = krat(&result, num);
            }

            if (strcmp(operator, "plus") == 0) {
                *resultPointer = plus(&result, num);
            }

            printf("%li\n", result);
        }
    }

    return 0;
}

plus 123456789
krat 123456789
plus 0
krat 2
krat 3
krat 4
krat 5
krat 6

这就是输出

0
123456789
15241578750190521
30483157500381042
91449472501143126
365797890004572504
1828989450022862520
-7472807373572376496

问题是,当数字越来越大时,就会变成负数。这是变量内存分配的问题吗?如何解决?