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| <h4 class="subsection">12.20.2 Flushing Buffers</h4> |
| |
| <p><a name="index-flushing-a-stream-1135"></a><dfn>Flushing</dfn> output on a buffered stream means transmitting all |
| accumulated characters to the file. There are many circumstances when |
| buffered output on a stream is flushed automatically: |
| |
| <ul> |
| <li>When you try to do output and the output buffer is full. |
| |
| <li>When the stream is closed. See <a href="Closing-Streams.html#Closing-Streams">Closing Streams</a>. |
| |
| <li>When the program terminates by calling <code>exit</code>. |
| See <a href="Normal-Termination.html#Normal-Termination">Normal Termination</a>. |
| |
| <li>When a newline is written, if the stream is line buffered. |
| |
| <li>Whenever an input operation on <em>any</em> stream actually reads data |
| from its file. |
| </ul> |
| |
| <p>If you want to flush the buffered output at another time, call |
| <code>fflush</code>, which is declared in the header file <samp><span class="file">stdio.h</span></samp>. |
| <a name="index-stdio_002eh-1136"></a> |
| <!-- stdio.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: int <b>fflush</b> (<var>FILE *stream</var>)<var><a name="index-fflush-1137"></a></var><br> |
| <blockquote><p>This function causes any buffered output on <var>stream</var> to be delivered |
| to the file. If <var>stream</var> is a null pointer, then |
| <code>fflush</code> causes buffered output on <em>all</em> open output streams |
| to be flushed. |
| |
| <p>This function returns <code>EOF</code> if a write error occurs, or zero |
| otherwise. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- POSIX --> |
| <div class="defun"> |
| — Function: int <b>fflush_unlocked</b> (<var>FILE *stream</var>)<var><a name="index-fflush_005funlocked-1138"></a></var><br> |
| <blockquote><p>The <code>fflush_unlocked</code> function is equivalent to the <code>fflush</code> |
| function except that it does not implicitly lock the stream. |
| </p></blockquote></div> |
| |
| <p>The <code>fflush</code> function can be used to flush all streams currently |
| opened. While this is useful in some situations it does often more than |
| necessary since it might be done in situations when terminal input is |
| required and the program wants to be sure that all output is visible on |
| the terminal. But this means that only line buffered streams have to be |
| flushed. Solaris introduced a function especially for this. It was |
| always available in the GNU C library in some form but never officially |
| exported. |
| |
| <!-- stdio_ext.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>_flushlbf</b> (<var>void</var>)<var><a name="index-g_t_005fflushlbf-1139"></a></var><br> |
| <blockquote><p>The <code>_flushlbf</code> function flushes all line buffered streams |
| currently opened. |
| |
| <p>This function is declared in the <samp><span class="file">stdio_ext.h</span></samp> header. |
| </p></blockquote></div> |
| |
| <p><strong>Compatibility Note:</strong> Some brain-damaged operating systems have |
| been known to be so thoroughly fixated on line-oriented input and output |
| that flushing a line buffered stream causes a newline to be written! |
| Fortunately, this “feature” seems to be becoming less common. You do |
| not need to worry about this in the GNU system. |
| |
| <p>In some situations it might be useful to not flush the output pending |
| for a stream but instead simply forget it. If transmission is costly |
| and the output is not needed anymore this is valid reasoning. In this |
| situation a non-standard function introduced in Solaris and available in |
| the GNU C library can be used. |
| |
| <!-- stdio_ext.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>__fpurge</b> (<var>FILE *stream</var>)<var><a name="index-g_t_005f_005ffpurge-1140"></a></var><br> |
| <blockquote><p>The <code>__fpurge</code> function causes the buffer of the stream |
| <var>stream</var> to be emptied. If the stream is currently in read mode all |
| input in the buffer is lost. If the stream is in output mode the |
| buffered output is not written to the device (or whatever other |
| underlying storage) and the buffer the cleared. |
| |
| <p>This function is declared in <samp><span class="file">stdio_ext.h</span></samp>. |
| </p></blockquote></div> |
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