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.
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。
2
This changes the state of the buffer to FULL.
这样就把缓冲的状态变成FULL。
3
Because the state of the internal buffer is FULL, the available data is returned and a notify issued.