void get_new_file_path() {
    data_file_number++;
    data_filename = "data" + to_string(data_file_number);
    string path = user_path + data_filename + ".txt";
    full_path = path.c_str();
    //cout << full_path << endl;
}

if (data_file == NULL) {
            get_new_file_path();
            //cout << full_path;
            data_file = fopen(full_path, "a+b");
}