226 lines
6.9 KiB
HTML
226 lines
6.9 KiB
HTML
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<link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 -->
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>dup</title>
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<body bgcolor="white">
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<basefont size="3"> <a name="dup"></a> <a name="tag_03_114"></a><!-- dup -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_114_01"></a>NAME</h4>
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<blockquote>dup, dup2 - duplicate an open file descriptor</blockquote>
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<h4><a name="tag_03_114_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/unistd.h.html">unistd.h</a>><br>
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<br>
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int dup(int</tt> <i>fildes</i><tt>);<br>
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int dup2(int</tt> <i>fildes</i><tt>, int</tt> <i>fildes2</i><tt>);<br>
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</tt></code></p>
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</blockquote>
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<h4><a name="tag_03_114_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>dup</i>() and <i>dup2</i>() functions provide an alternative interface to the service provided by <a href=
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"../functions/fcntl.html"><i>fcntl</i>()</a> using the F_DUPFD command. The call:</p>
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<pre>
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<tt>fid = dup(</tt><i>fildes</i><tt>);
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</tt>
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</pre>
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<p>shall be equivalent to:</p>
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<pre>
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<tt>fid = fcntl(</tt><i>fildes</i><tt>, F_DUPFD, 0);
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</tt>
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</pre>
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<p>The call:</p>
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<pre>
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<tt>fid = dup2(</tt><i>fildes</i><tt>,</tt> <i>fildes2</i><tt>);
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</tt>
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</pre>
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<p>shall be equivalent to:</p>
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<pre>
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<tt>close(</tt><i>fildes2</i><tt>);
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fid = fcntl(</tt><i>fildes</i><tt>, F_DUPFD,</tt> <i>fildes2</i><tt>);
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</tt>
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</pre>
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<p>except for the following:</p>
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<ul>
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<li>
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<p>If <i>fildes2</i> is less than 0 or greater than or equal to {OPEN_MAX}, <i>dup2</i>() shall return -1 with <i>errno</i> set to
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[EBADF].</p>
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</li>
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<li>
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<p>If <i>fildes</i> is a valid file descriptor and is equal to <i>fildes2</i>, <i>dup2</i>() shall return <i>fildes2</i> without
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closing it.</p>
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</li>
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<li>
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<p>If <i>fildes</i> is not a valid file descriptor, <i>dup2</i>() shall return -1 and shall not close <i>fildes2</i>.</p>
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</li>
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<li>
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<p>The value returned shall be equal to the value of <i>fildes2</i> upon successful completion, or -1 upon failure.</p>
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</li>
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</ul>
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</blockquote>
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<h4><a name="tag_03_114_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion a non-negative integer, namely the file descriptor, shall be returned; otherwise, -1 shall be
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returned and <i>errno</i> set to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_114_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>dup</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The <i>fildes</i> argument is not a valid open file descriptor.</dd>
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<dt>[EMFILE]</dt>
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<dd>The number of file descriptors in use by this process would exceed {OPEN_MAX}.</dd>
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</dl>
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<p>The <i>dup2</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The <i>fildes</i> argument is not a valid open file descriptor or the argument <i>fildes2</i> is negative or greater than or
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equal to {OPEN_MAX}.</dd>
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<dt>[EINTR]</dt>
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<dd>The <i>dup2</i>() function was interrupted by a signal.</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_114_06"></a>EXAMPLES</h4>
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<blockquote>
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<h5><a name="tag_03_114_06_01"></a>Redirecting Standard Output to a File</h5>
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<p>The following example closes standard output for the current processes, re-assigns standard output to go to the file referenced
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by <i>pfd</i>, and closes the original file descriptor to clean up.</p>
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<pre>
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<tt>#include <unistd.h>
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...
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int pfd;
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...
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close(1);
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dup(pfd);
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close(pfd);
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...
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</tt>
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</pre>
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<h5><a name="tag_03_114_06_02"></a>Redirecting Error Messages</h5>
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<p>The following example redirects messages from <i>stderr</i> to <i>stdout</i>.</p>
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<pre>
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<tt>#include <unistd.h>
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...
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dup2(1, 2);
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...
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_03_114_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_114_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The <i>dup</i>() and <i>dup2</i>() functions are redundant. Their services are also provided by the <a href=
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"../functions/fcntl.html"><i>fcntl</i>()</a> function. They have been included in this volume of IEEE Std 1003.1-2001
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primarily for historical reasons, since many existing applications use them.</p>
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<p>While the brief code segment shown is very similar in behavior to <i>dup2</i>(), a conforming implementation based on other
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functions defined in this volume of IEEE Std 1003.1-2001 is significantly more complex. Least obvious is the possible
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effect of a signal-catching function that could be invoked between steps and allocate or deallocate file descriptors. This could be
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avoided by blocking signals.</p>
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<p>The <i>dup2</i>() function is not marked obsolescent because it presents a type-safe version of functionality provided in a
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type-unsafe version by <a href="../functions/fcntl.html"><i>fcntl</i>()</a>. It is used in the POSIX Ada binding.</p>
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<p>The <i>dup2</i>() function is not intended for use in critical regions as a synchronization mechanism.</p>
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<p>In the description of [EBADF], the case of <i>fildes</i> being out of range is covered by the given case of <i>fildes</i> not
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being valid. The descriptions for <i>fildes</i> and <i>fildes2</i> are different because the only kind of invalidity that is
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relevant for <i>fildes2</i> is whether it is out of range; that is, it does not matter whether <i>fildes2</i> refers to an open
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file when the <i>dup2</i>() call is made.</p>
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</blockquote>
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<h4><a name="tag_03_114_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_114_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="close.html"><i>close</i>()</a> , <a href="fcntl.html"><i>fcntl</i>()</a> , <a href="open.html"><i>open</i>()</a> , the
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Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_114_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 1. Derived from Issue 1 of the SVID.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<hr size="2" noshade>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
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