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134 lines
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134 lines
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<a name="fflush"></a>
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<div class="header">
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<p>
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Next: <a href="fgetc.html#fgetc" accesskey="n" rel="next">fgetc</a>, Previous: <a href="ferror.html#ferror" accesskey="p" rel="prev">ferror</a>, Up: <a href="Stdio.html#Stdio" accesskey="u" rel="up">Stdio</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="fflush_002c-fflush_005funlocked_002d_002d_002dflush-buffered-file-output"></a>
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<h3 class="section">4.9 <code>fflush</code>, <code>fflush_unlocked</code>—flush buffered file output</h3>
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<a name="index-fflush"></a>
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<a name="index-fflush_005funlocked"></a>
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<a name="index-_005ffflush_005fr"></a>
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<a name="index-_005ffflush_005funlocked_005fr"></a>
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<p><strong>Synopsis</strong>
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</p><div class="example">
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<pre class="example">#include <stdio.h>
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int fflush(FILE *<var>fp</var>);
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#define _BSD_SOURCE
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#include <stdio.h>
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int fflush_unlocked(FILE *<var>fp</var>);
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#include <stdio.h>
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int _fflush_r(struct _reent *<var>reent</var>, FILE *<var>fp</var>);
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#define _BSD_SOURCE
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#include <stdio.h>
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int _fflush_unlocked_r(struct _reent *<var>reent</var>, FILE *<var>fp</var>);
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</pre></div>
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<p><strong>Description</strong><br>
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The <code>stdio</code> output functions can buffer output before delivering it
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to the host system, in order to minimize the overhead of system calls.
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</p>
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<p>Use <code>fflush</code> to deliver any such pending output (for the file
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or stream identified by <var>fp</var>) to the host system.
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</p>
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<p>If <var>fp</var> is <code>NULL</code>, <code>fflush</code> delivers pending output from all
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open files.
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</p>
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<p>Additionally, if <var>fp</var> is a seekable input stream visiting a file
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descriptor, set the position of the file descriptor to match next
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unread byte, useful for obeying POSIX semantics when ending a process
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without consuming all input from the stream.
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</p>
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<p><code>fflush_unlocked</code> is a non-thread-safe version of <code>fflush</code>.
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<code>fflush_unlocked</code> may only safely be used within a scope
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protected by flockfile() (or ftrylockfile()) and funlockfile(). This
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function may safely be used in a multi-threaded program if and only
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if they are called while the invoking thread owns the (FILE *)
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object, as is the case after a successful call to the flockfile() or
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ftrylockfile() functions. If threads are disabled, then
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<code>fflush_unlocked</code> is equivalent to <code>fflush</code>.
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</p>
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<p>The alternate functions <code>_fflush_r</code> and <code>_fflush_unlocked_r</code> are
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reentrant versions, where the extra argument <var>reent</var> is a pointer to
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a reentrancy structure, and <var>fp</var> must not be NULL.
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</p>
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<br>
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<p><strong>Returns</strong><br>
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<code>fflush</code> returns <code>0</code> unless it encounters a write error; in that
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situation, it returns <code>EOF</code>.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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ANSI C requires <code>fflush</code>. The behavior on input streams is only
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specified by POSIX, and not all implementations follow POSIX rules.
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</p>
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<p><code>fflush_unlocked</code> is a BSD extension also provided by GNU libc.
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</p>
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<p>No supporting OS subroutines are required.
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</p>
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<br>
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<hr>
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<div class="header">
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<p>
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Next: <a href="fgetc.html#fgetc" accesskey="n" rel="next">fgetc</a>, Previous: <a href="ferror.html#ferror" accesskey="p" rel="prev">ferror</a>, Up: <a href="Stdio.html#Stdio" accesskey="u" rel="up">Stdio</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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