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<a name="fflush_002c-fflush_005funlocked_002d_002d_002dflush-buffered-file-output"></a>
<h3 class="section">4.9 <code>fflush</code>, <code>fflush_unlocked</code>&mdash;flush buffered file output</h3>
<a name="index-fflush"></a>
<a name="index-fflush_005funlocked"></a>
<a name="index-_005ffflush_005fr"></a>
<a name="index-_005ffflush_005funlocked_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
int fflush(FILE *<var>fp</var>);

#define _BSD_SOURCE
#include &lt;stdio.h&gt;
int fflush_unlocked(FILE *<var>fp</var>);

#include &lt;stdio.h&gt;
int _fflush_r(struct _reent *<var>reent</var>, FILE *<var>fp</var>);

#define _BSD_SOURCE
#include &lt;stdio.h&gt;
int _fflush_unlocked_r(struct _reent *<var>reent</var>, FILE *<var>fp</var>);

</pre></div>
<p><strong>Description</strong><br>
The <code>stdio</code> output functions can buffer output before delivering it
to the host system, in order to minimize the overhead of system calls.
</p>
<p>Use <code>fflush</code> to deliver any such pending output (for the file
or stream identified by <var>fp</var>) to the host system.
</p>
<p>If <var>fp</var> is <code>NULL</code>, <code>fflush</code> delivers pending output from all
open files.
</p>
<p>Additionally, if <var>fp</var> is a seekable input stream visiting a file
descriptor, set the position of the file descriptor to match next
unread byte, useful for obeying POSIX semantics when ending a process
without consuming all input from the stream.
</p>
<p><code>fflush_unlocked</code> is a non-thread-safe version of <code>fflush</code>.
<code>fflush_unlocked</code> may only safely be used within a scope
protected by flockfile() (or ftrylockfile()) and funlockfile().  This
function may safely be used in a multi-threaded program if and only
if they are called while the invoking thread owns the (FILE *)
object, as is the case after a successful call to the flockfile() or
ftrylockfile() functions.  If threads are disabled, then
<code>fflush_unlocked</code> is equivalent to <code>fflush</code>.
</p>
<p>The alternate functions <code>_fflush_r</code> and <code>_fflush_unlocked_r</code> are
reentrant versions, where the extra argument <var>reent</var> is a pointer to
a reentrancy structure, and <var>fp</var> must not be NULL.
</p>
<br>
<p><strong>Returns</strong><br>
<code>fflush</code> returns <code>0</code> unless it encounters a write error; in that
situation, it returns <code>EOF</code>.
</p>
<br>
<p><strong>Portability</strong><br>
ANSI C requires <code>fflush</code>.  The behavior on input streams is only
specified by POSIX, and not all implementations follow POSIX rules.
</p>
<p><code>fflush_unlocked</code> is a BSD extension also provided by GNU libc.
</p>
<p>No supporting OS subroutines are required.
</p>
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