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<a name="fileno_002c-fileno_005funlocked_002d_002d_002dreturn-file-descriptor-associated-with-stream"></a>
<h3 class="section">4.15 <code>fileno</code>, <code>fileno_unlocked</code>&mdash;return file descriptor associated with stream</h3>
<a name="index-fileno"></a>
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<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
int fileno(FILE *<var>fp</var>);

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

</pre></div>
<p><strong>Description</strong><br>
You can use <code>fileno</code> to return the file descriptor identified by <var>fp</var>.
</p>
<p><code>fileno_unlocked</code> is a non-thread-safe version of <code>fileno</code>.
<code>fileno_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>fileno_unlocked</code> is equivalent to <code>fileno</code>.
</p>
<br>
<p><strong>Returns</strong><br>
<code>fileno</code> returns a non-negative integer when successful.
If <var>fp</var> is not an open stream, <code>fileno</code> returns -1.
</p>
<br>
<p><strong>Portability</strong><br>
<code>fileno</code> is not part of ANSI C.
POSIX requires <code>fileno</code>.
</p>
<p><code>fileno_unlocked</code> is a BSD extension also provided by GNU libc.
</p>
<p>Supporting OS subroutines required: none.
</p>
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