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<a name="freopen_002d_002d_002dopen-a-file-using-an-existing-file-descriptor"></a>
<h3 class="section">4.25 <code>freopen</code>&mdash;open a file using an existing file descriptor</h3>
<a name="index-freopen"></a>
<a name="index-_005ffreopen_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
FILE *freopen(const char *restrict <var>file</var>, const char *restrict <var>mode</var>,
    FILE *restrict <var>fp</var>);
FILE *_freopen_r(struct _reent *<var>ptr</var>, const char *restrict <var>file</var>,
    const char *restrict <var>mode</var>, FILE *restrict <var>fp</var>);

</pre></div>
<p><strong>Description</strong><br>
Use this variant of <code>fopen</code> if you wish to specify a particular file
descriptor <var>fp</var> (notably <code>stdin</code>, <code>stdout</code>, or <code>stderr</code>) for
the file.
</p>
<p>If <var>fp</var> was associated with another file or stream, <code>freopen</code>
closes that other file or stream (but ignores any errors while closing
it).
</p>
<p><var>file</var> and <var>mode</var> are used just as in <code>fopen</code>.
</p>
<p>If <var>file</var> is <code>NULL</code>, the underlying stream is modified rather than
closed.  The file cannot be given a more permissive access mode (for
example, a <var>mode</var> of &quot;w&quot; will fail on a read-only file descriptor),
but can change status such as append or binary mode.  If modification
is not possible, failure occurs.
</p>
<br>
<p><strong>Returns</strong><br>
If successful, the result is the same as the argument <var>fp</var>.  If the
file cannot be opened as specified, the result is <code>NULL</code>.
</p>
<br>
<p><strong>Portability</strong><br>
ANSI C requires <code>freopen</code>.
</p>
<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
<code>lseek</code>, <code>open</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
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