Buffering
Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Flushing output on a buffered stream means transmitting all accumulated characters. There are many circumstances when buffered output on a stream is flushed automatically:
exit.If you want to flush the buffered output at another time, call fflush, which is declared in the header file stdio.h.
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