Integer Overflow when Passing Arguments in C

Question

I think I'm having an integer overflow issue and I'm not sure how to fix it. I just started C coming from Python and JS and this is all new to me.

I put a really simple example below of what is happening. I'm passing an argument from the main function to a different function to multiply by 3, but when it's passed, the number overflows. The math works in the main function.

#include <stdio.h>

long long calc(number) {
    return number * 3;
}

int main(void)
{
    long long digits = 1111111111111;
    long long result = calc(digits);
    printf("calc result: %lld\n", result);
    
    long long mainTimes3 = digits * 3;
    printf("main result: %lld\n", mainTimes3);

    return 0;
}

I'm getting the error message

main.c:3:11: warning: type of ‘number’ defaults to ‘int’ [-Wimplicit-int]`

The printf is showing

calc result: 438711637
main result: 3333333333333

Answer 1

The compiler warning hints at the problem - since you didn't explicitly define the type of the number parameter, the compiler assumes it's an int. Therefore number * 3 is also an int, and an integer overflow occurs.

To solve the issue, explicitly define it as the type you intended, long long:

long long calc(long long number){
    /* Here ---^ */
    return number * 3;
}