int create_user(const char *name, User **user_ptr_add) {
    // Check username length
    if (strlen(name) > MAX_NAME - 1) {
        return 2;
    }
    User * curr = *user_ptr_add; 
    // Check empty list
    if (curr == NULL) {
        User *new_user = malloc(sizeof(User));
        strncpy(new_user->name, name, MAX_NAME);
        new_user->name[MAX_NAME-1] = '\0';
        * user_ptr_add = new_user;
        return 0;
    }
    do { // Guarantee check first element
        if (strcmp(curr->name, name) == 0) {
            return 1;
        }
        curr = curr->next;
    } while (curr != NULL);
    // Add new User
    User *new_user = malloc(sizeof(User));
    strncpy(new_user->name, name, MAX_NAME);
    new_user->name[MAX_NAME-1] = '\0';
    curr->next = new_user;
    return 0;
}

accepts a string up to length MAX_NAME