清除这个资料流的缓冲区，让所有缓冲资料全部写入档案。

Clears buffers for this stream and causes any buffered data to be written to the file.

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当所有数据全部写入之后，就需要设置eof(文件结束参数)。

When all the data have been written, set the eof (end-of-file) argument.

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注意：要确定您可以找到指定术语的所有匹配项，并且由于这些术语对于底层字典是未知的，需要全部使用小写字母写入术语。

Note: To make sure, that you can find all occurrences of the specified terms, and because these terms are not known by the underlying dictionary, write the terms in all lower-case letters.

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