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dup.html
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<!-- Creator : groff version 1.22.4 -->
<!-- CreationDate: Wed Jan 29 11:26:23 2020 -->
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta name="generator" content="groff -Thtml, see www.gnu.org">
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<meta name="Content-Style" content="text/css">
<style type="text/css">
p { margin-top: 0; margin-bottom: 0; vertical-align: top }
pre { margin-top: 0; margin-bottom: 0; vertical-align: top }
table { margin-top: 0; margin-bottom: 0; vertical-align: top }
h1 { text-align: center }
</style>
<title>DUP</title>
</head>
<body>
<h1 align="center">DUP</h1>
<a href="#NAME">NAME</a><br>
<a href="#SYNOPSIS">SYNOPSIS</a><br>
<a href="#DESCRIPTION">DESCRIPTION</a><br>
<a href="#RETURN VALUE">RETURN VALUE</a><br>
<a href="#ERRORS">ERRORS</a><br>
<a href="#VERSIONS">VERSIONS</a><br>
<a href="#CONFORMING TO">CONFORMING TO</a><br>
<a href="#NOTES">NOTES</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<a href="#COLOPHON">COLOPHON</a><br>
<hr>
<h2>NAME
<a name="NAME"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">dup, dup2, dup3
- duplicate a file descriptor</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>#include
<unistd.h></b></p>
<p style="margin-left:11%; margin-top: 1em"><b>int
dup(int</b> <i>oldfd</i><b>); <br>
int dup2(int</b> <i>oldfd</i><b>, int</b>
<i>newfd</i><b>);</b></p>
<p style="margin-left:11%; margin-top: 1em"><b>#define
_GNU_SOURCE</b> /* See feature_test_macros(7) */ <b><br>
#include <fcntl.h></b> /* Obtain O_* constant
definitions */ <b><br>
#include <unistd.h></b></p>
<p style="margin-left:11%; margin-top: 1em"><b>int
dup3(int</b> <i>oldfd</i><b>, int</b> <i>newfd</i><b>,
int</b> <i>flags</i><b>);</b></p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">The
<b>dup</b>() system call creates a copy of the file
descriptor <i>oldfd</i>, using the lowest-numbered unused
file descriptor for the new descriptor.</p>
<p style="margin-left:11%; margin-top: 1em">After a
successful return, the old and new file descriptors may be
used interchangeably. They refer to the same open file
description (see <b>open</b>(2)) and thus share file offset
and file status flags; for example, if the file offset is
modified by using <b>lseek</b>(2) on one of the file
descriptors, the offset is also changed for the other.</p>
<p style="margin-left:11%; margin-top: 1em">The two file
descriptors do not share file descriptor flags (the
close-on-exec flag). The close-on-exec flag
(<b>FD_CLOEXEC</b>; see <b>fcntl</b>(2)) for the duplicate
descriptor is off.</p>
<p style="margin-left:11%; margin-top: 1em"><b>dup2()</b>
<br>
The <b>dup2</b>() system call performs the same task as
<b>dup</b>(), but instead of using the lowest-numbered
unused file descriptor, it uses the file descriptor number
specified in <i>newfd</i>. If the file descriptor
<i>newfd</i> was previously open, it is silently closed
before being reused.</p>
<p style="margin-left:11%; margin-top: 1em">The steps of
closing and reusing the file descriptor <i>newfd</i> are
performed <i>atomically</i>. This is important, because
trying to implement equivalent functionality using
<b>close</b>(2) and <b>dup</b>() would be subject to race
conditions, whereby <i>newfd</i> might be reused between the
two steps. Such reuse could happen because the main program
is interrupted by a signal handler that allocates a file
descriptor, or because a parallel thread allocates a file
descriptor.</p>
<p style="margin-left:11%; margin-top: 1em">Note the
following points:</p>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="1%">
<p>*</p></td>
<td width="3%"></td>
<td width="85%">
<p>If <i>oldfd</i> is not a valid file descriptor, then the
call fails, and <i>newfd</i> is not closed.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="1%">
<p>*</p></td>
<td width="3%"></td>
<td width="85%">
<p>If <i>oldfd</i> is a valid file descriptor, and
<i>newfd</i> has the same value as <i>oldfd</i>, then
<b>dup2</b>() does nothing, and returns <i>newfd</i>.</p></td></tr>
</table>
<p style="margin-left:11%; margin-top: 1em"><b>dup3() <br>
dup3</b>() is the same as <b>dup2</b>(), except that:</p>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="1%">
<p style="margin-top: 1em">*</p></td>
<td width="3%"></td>
<td width="85%">
<p style="margin-top: 1em">The caller can force the
close-on-exec flag to be set for the new file descriptor by
specifying <b>O_CLOEXEC</b> in <i>flags</i>. See the
description of the same flag in <b>open</b>(2) for reasons
why this may be useful.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="1%">
<p>*</p></td>
<td width="3%"></td>
<td width="85%">
<p>If <i>oldfd</i> equals <i>newfd</i>, then <b>dup3</b>()
fails with the error <b>EINVAL</b>.</p></td></tr>
</table>
<h2>RETURN VALUE
<a name="RETURN VALUE"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">On success,
these system calls return the new file descriptor. On error,
-1 is returned, and <i>errno</i> is set appropriately.</p>
<h2>ERRORS
<a name="ERRORS"></a>
</h2>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p style="margin-top: 1em"><b>EBADF</b></p></td>
<td width="2%"></td>
<td width="78%">
<p style="margin-top: 1em"><i>oldfd</i> isn’t an open
file descriptor.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>EBADF</b></p></td>
<td width="2%"></td>
<td width="78%">
<p><i>newfd</i> is out of the allowed range for file
descriptors (see the discussion of <b>RLIMIT_NOFILE</b> in
<b>getrlimit</b>(2)).</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>EBUSY</b></p></td>
<td width="2%"></td>
<td width="78%">
<p>(Linux only) This may be returned by <b>dup2</b>() or
<b>dup3</b>() during a race condition with <b>open</b>(2)
and <b>dup</b>().</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>EINTR</b></p></td>
<td width="2%"></td>
<td width="78%">
<p>The <b>dup2</b>() or <b>dup3</b>() call was interrupted
by a signal; see <b>signal</b>(7).</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>EINVAL</b></p></td>
<td width="2%"></td>
<td width="78%">
<p>(<b>dup3</b>()) <i>flags</i> contain an invalid
value.</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>EINVAL</b></p></td>
<td width="2%"></td>
<td width="78%">
<p>(<b>dup3</b>()) <i>oldfd</i> was equal to
<i>newfd</i>.</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>EMFILE</b></p></td>
<td width="2%"></td>
<td width="78%">
<p>The per-process limit on the number of open file
descriptors has been reached (see the discussion of
<b>RLIMIT_NOFILE</b> in <b>getrlimit</b>(2)).</p></td></tr>
</table>
<h2>VERSIONS
<a name="VERSIONS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>dup3</b>()
was added to Linux in version 2.6.27; glibc support is
available starting with version 2.9.</p>
<h2>CONFORMING TO
<a name="CONFORMING TO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>dup</b>(),
<b>dup2</b>(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.</p>
<p style="margin-left:11%; margin-top: 1em"><b>dup3</b>()
is Linux-specific.</p>
<h2>NOTES
<a name="NOTES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">The error
returned by <b>dup2</b>() is different from that returned by
<b>fcntl(</b>..., <b>F_DUPFD</b>, ...<b>)</b> when
<i>newfd</i> is out of range. On some systems, <b>dup2</b>()
also sometimes returns <b>EINVAL</b> like
<b>F_DUPFD</b>.</p>
<p style="margin-left:11%; margin-top: 1em">If <i>newfd</i>
was open, any errors that would have been reported at
<b>close</b>(2) time are lost. If this is of concern,
then—unless the program is single-threaded and does
not allocate file descriptors in signal handlers—the
correct approach is <i>not</i> to close <i>newfd</i> before
calling <b>dup2</b>(), because of the race condition
described above. Instead, code something like the following
could be used:</p>
<p style="margin-left:11%; margin-top: 1em">/* Obtain a
duplicate of ’newfd’ that can subsequently <br>
be used to check for close() errors; an EBADF error <br>
means that ’newfd’ was not open. */</p>
<p style="margin-left:11%; margin-top: 1em">tmpfd =
dup(newfd); <br>
if (tmpfd == -1 && errno != EBADF) { <br>
/* Handle unexpected dup() error */ <br>
}</p>
<p style="margin-left:11%; margin-top: 1em">/* Atomically
duplicate ’oldfd’ on ’newfd’ */</p>
<p style="margin-left:11%; margin-top: 1em">if (dup2(oldfd,
newfd) == -1) { <br>
/* Handle dup2() error */ <br>
}</p>
<p style="margin-left:11%; margin-top: 1em">/* Now check
for close() errors on the file originally <br>
referred to by ’newfd’ */</p>
<p style="margin-left:11%; margin-top: 1em">if (tmpfd !=
-1) { <br>
if (close(tmpfd) == -1) { <br>
/* Handle errors from close */ <br>
} <br>
}</p>
<h2>SEE ALSO
<a name="SEE ALSO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>close</b>(2),
<b>fcntl</b>(2), <b>open</b>(2)</p>
<h2>COLOPHON
<a name="COLOPHON"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">This page is
part of release 5.02 of the Linux <i>man-pages</i> project.
A description of the project, information about reporting
bugs, and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.</p>
<hr>
</body>
</html>