Clion exit code -1073741571 (0xC00000FD)

I get a weird exit code in clion:

exit code -1073741571 (0xC00000FD)

This is my code:

int main()
{
    std::cin.sync_with_stdio(false);
    std::cin.tie(nullptr);

    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);

    int n = 0, i = 0, j = 0;
    int arr[30007][5];

    for (i = 1; i <= 30000; i++)
        arr[0][i] = 1;

    //...

    return 0;
}

I tested it and find out its because of this line:

int arr[30007][5];

I had no problem in declaration of array in size of less than 1.000.000 2 days ago and now I get this error. I changed nothing in Clion.

What should I do?

The error number 0xC00000FD stands for "stack overflow" (I suppose your platform is Windows). Under Windows local variables are allocated on the stack (as on most other platforms too) and int arr[30007][5] is pretty big (30007 * 5 * 4 = 600140 bytes) and stacks are usually rather small (typically around 1 Mb, again platform dependant)

You have many options:

1. use std::vector instead of raw arrays (preferred)
2. declare the array as static (static int arr[30007][5];), then it won't reside on the stack anymore
3. increase the stack size of your executable. This is highly platform/tool dependant.
4. allocate the array dynamically