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| <h4 class="subsection">13.5.3 Cleaning Streams</h4> |
| |
| <p>On the GNU system, you can clean up any stream with <code>fclean</code>: |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>fclean</b> (<var>FILE *stream</var>)<var><a name="index-fclean-1249"></a></var><br> |
| <blockquote><p>Clean up the stream <var>stream</var> so that its buffer is empty. If |
| <var>stream</var> is doing output, force it out. If <var>stream</var> is doing |
| input, give the data in the buffer back to the system, arranging to |
| reread it. |
| </p></blockquote></div> |
| |
| <p>On other systems, you can use <code>fflush</code> to clean a stream in most |
| cases. |
| |
| <p>You can skip the <code>fclean</code> or <code>fflush</code> 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. However, a just-opened input stream |
| might not be clean, as its input buffer might not be empty. |
| |
| <p>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, |
| <code>fflush</code> 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 <code>fclean</code> avoids both of |
| these problems. |
| |
| <p>Closing an output-only stream also does <code>fflush</code>, so this is a |
| valid way of cleaning an output stream. On the GNU system, closing an |
| input stream does <code>fclean</code>. |
| |
| <p>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. See <a href="Terminal-Modes.html#Terminal-Modes">Terminal Modes</a>. |
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