int main(void)
{
    char array1[100];
    char array2[20];

    char* ptr = array2;
    sizeof(ptr) // you want 20 here, right?
    ptr -= 100;
    sizeof(ptr) // what now? 100? 20? 120? ptr points to the same place as array1.

    // The problem here is that any time you do pointer math you are going to have to check every array and see if a pointer lands on one

    return 0;
}