UNIX 求解 control.open

control.open("control.txt",ios::in | ios::out | ios::nocreate);
              int i;
           control>>i;
    control.close();
        if(i!=0)//不为0就初始化
              {
                     initial();
              }
    control.open("control.txt",ios::in | ios::out | ios::nocreate);
              control.seekp(0);
              control<<0;//默认是上次基础上继续下去不用再初始化
    control.close()
有大神能说说control.open("control.txt",ios::in | ios::out | ios::nocreate);
   control>>i
control.close()
新人  求解释