int create_user(const char *name, User **user_ptr_add) {
    if (strlen(name) > MAX_NAME - 1) {
        return 2;
    }

    User curr = **user_ptr_add;
    while (curr.next != NULL) {
        if(strcmp(curr.name, name) == 0) {
            return 1;
        }
        curr = *curr.next;
    }

    User user;
    strcpy(user.name, name);
    curr.next = &user;
    return 0;
}
Create a new user with the given name. Insert it at the tail of the list of users whose head is pointed to by *user_ptr_add. Return: - 0 if successful - 1 if a user by this name already exists in this list - 2 if the given name cannot fit in the 'name' array (don't forget about the null terminator)