一系列控制杆连着一枚指针，在刻度盘上显示出压力数。
A series of levers joined to a pointer shows pressure on a dial.
如果函数需要在一系列调用期间维持某些数据的状态，比如工作缓存或指针，那么调用者应该提供此数据。
If a function needs to maintain the state of some data over successive calls, such as a working buffer or a pointer, the caller should provide this data.
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