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(libc.info)Cleaning Streams

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Cleaning Streams
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   On the GNU system, you can clean up any stream with `fclean':

 - Function: int fclean (FILE *STREAM)
     Clean up the stream STREAM so that its buffer is empty.  If STREAM
     is doing output, force it out.  If STREAM is doing input, give the
     data in the buffer back to the system, arranging to reread it.

   On other systems, you can use `fflush' to clean a stream in most
cases.

   You can skip the `fclean' or `fflush' if you know the stream is
already clean.  A stream is clean whenever its buffer is empty.  For
example, an unbuffered stream is always clean.  An input stream that is
at end-of-file is clean.  A line-buffered stream is clean when the last
character output was a newline.

   There is one case in which cleaning a stream is impossible on most
systems.  This is when the stream is doing input from a file that is not
random-access.  Such streams typically read ahead, and when the file is
not random access, there is no way to give back the excess data already
read.  When an input stream reads from a random-access file, `fflush'
does clean the stream, but leaves the file pointer at an unpredictable
place; you must set the file pointer before doing any further I/O.  On
the GNU system, using `fclean' avoids both of these problems.

   Closing an output-only stream also does `fflush', so this is a valid
way of cleaning an output stream.  On the GNU system, closing an input
stream does `fclean'.

   You need not clean a stream before using its descriptor for control
operations such as setting terminal modes; these operations don't affect
the file position and are not affected by it.  You can use any
descriptor for these operations, and all channels are affected
simultaneously.  However, text already "output" to a stream but still
buffered by the stream will be subject to the new terminal modes when
subsequently flushed.  To make sure "past" output is covered by the
terminal settings that were in effect at the time, flush the output
streams for that terminal before setting the modes.  Note: Terminal
Modes.