382 lines
9.4 KiB
HTML
382 lines
9.4 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>dup()--Duplicate Open File Descriptor</title>
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<body>
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>dup()--Duplicate Open File Descriptor</h2>
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<div class="box" style="width: 70%;">
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<br>
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Syntax<br>
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<pre>
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#include <unistd.h>
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int dup(int <em>fildes</em>);
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</pre>
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Service Program Name: QP0LLIB1<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: Yes<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>The <strong>dup()</strong> function returns a new open file descriptor. The
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new descriptor refers to the same open file as <em>fildes</em> and shares any
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locks.</p>
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<p>If the original file descriptor was opened in text mode, data conversion is
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also done on the duplicated file descriptor.</p>
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<p>The FD_CLOEXEC flag that is associated with the new file descriptor is
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cleared. Refer to <a href="fcntl.htm">fcntl()</a>--Perform File Control Command
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for additional information about the FD_CLOEXEC flag.</p>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong><em>fildes</em></strong></dt>
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<dd>(Input) A descriptor to be duplicated.</dd>
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</dl>
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<p>The following operations are equivalent:</p>
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<pre>
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fd = dup(fildes);
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fd = fcntl(fildes,F_DUPFD,0);
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</pre>
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<p>For further information, see <a href="fcntl.htm">fcntl()--Perform File
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Control Command</a>.</p>
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<br>
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<h3>Authorities</h3>
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<p>No authorization is required.</p>
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<br>
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<h3>Return Value</h3>
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<dl compact>
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<dt><em>value</em></dt>
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<dd><strong>dup()</strong> was successful. The value returned is the new
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descriptor.</dd>
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<dt><em>-1</em></dt>
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<dd><strong>dup()</strong> was not successful. The <em>errno</em> global
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variable is set to indicate the error.</dd>
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</dl>
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<br>
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<h3>Error Conditions</h3>
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<p>If <strong>dup()</strong> is not successful, <em>errno</em> usually
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indicates one of the following errors. Under some conditions, <em>errno</em>
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could indicate an error other than those listed here.</p>
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<table cellpadding="5">
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<!-- cols="25 75" -->
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<tr>
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<th align="left" valign="bottom">Error condition</th>
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<th align="left" valign="bottom">Additional information</th>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EACCES">EACCES</a>]</em></td>
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<td align="left" valign="top">
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<p>If you are accessing a remote file through the Network File System, update
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operations to file permissions at the server are not reflected at the client
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until updates to data that is stored locally by the Network File System take
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place. (Several options on the Add Mounted File System (ADDMFS) command
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determine the time between refresh operations of local data.) Access to a
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remote file may also fail due to different mappings of user IDs (UID) or group
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IDs (GID) on the local and remote systems.</p>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EBADF">EBADF</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EBADFID">EBADFID</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EBUSY">EBUSY</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ECANCEL">ECANCEL</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EINVAL">EINVAL</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EIO">EIO</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ENOSYS">ENOSYS</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ENOTAVAIL">ENOTAVAIL</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ESTALE">ESTALE</a>]</em></td>
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<td align="left" valign="top">
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<p>If you are accessing a remote file through the Network File System, the file
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may have been deleted at the server.</p>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EUNKNOWN">EUNKNOWN</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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</table>
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<br>
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<br>
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<h3>Error Messages</h3>
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<p>The following messages may be sent from this function:</p>
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<table width="100%" cellpadding="5">
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<!-- cols="15 85" -->
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<tr>
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<th align="left" valign="top">Message ID</th>
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<th align="left" valign="top">Error Message Text</th>
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</tr>
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<tr>
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<td width="15%" valign="top">CPE3418 E</td>
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<td width="85%" valign="top">Possible APAR condition or hardware failure.</td>
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</tr>
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<tr>
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<td valign="top">CPFA0D4 E</td>
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<td valign="top">File system error occurred. Error number &1.</td>
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</tr>
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<tr>
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<td valign="top">CPF3CF2 E</td>
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<td valign="top">Error(s) occurred during running of &1 API.</td>
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</tr>
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<tr>
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<td valign="top">CPF9872 E</td>
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<td valign="top">Program or service program &1 in library &2 ended.
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Reason code &3.</td>
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</tr>
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</table>
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<br>
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<br>
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<h3><a name="USAGE_NOTES">Usage Notes</a></h3>
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<!-- It is deliberate that this API does not have thread conditional -->
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<!-- usage note. dup won't go into any file system code and -->
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<!-- since LFS is threadsafe, dup is always threadsafe. -->
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<ol>
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<li>This function will fail with
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error code [EBADF] when <em>fildes</em> is a scan descriptor that was passed to
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one of the scan-related exit programs. See <a href="ifsopenexit.htm">Integrated
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File System Scan on Open Exit Programs</a> and <a href="ifscloseexit.htm">
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Integrated File System Scan on Close Exit Programs</a> for more information.
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<br>
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<br>
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</li>
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</ol>
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<br>
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<h3>Related Information</h3>
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<ul>
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<li>The <<strong>unistd.h</strong>> file (see <a href="unix13.htm">Header
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Files for UNIX-Type Functions</a>)<br>
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</li>
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<li><a href="close.htm">close()</a>--Close File or Socket Descriptor<br>
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</li>
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<li><a href="creat.htm">creat()</a>--Create or Rewrite File<br>
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</li>
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<li><a href="dup2.htm">dup2()</a>--Duplicate Open File Descriptor to Another
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Descriptor<br>
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</li>
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<li><a href="fcntl.htm">fcntl()</a>--Perform File Control Command<br>
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</li>
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<li><a href="open.htm">open()</a>--Open File</li>
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</ul>
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<br>
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<h3>Example</h3>
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<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
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for information pertaining to code examples.</p>
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<p>The following example duplicates an open descriptor:</p>
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<pre>
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#include <fcntl.h>
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#include <unistd.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <stdio.h>
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#include <errno.h>
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void print_file_id(int file_descriptor) {
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struct stat info;
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if (fstat(file_descriptor, &info) != 0)
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fprintf(stderr, "stat() error for file_descriptor %d: %s\n",
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strerror(errno));
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else
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printf("The file id of file_descriptor %d is %d\n",
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file_descriptor,(int) info.st_ino);
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}
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main() {
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int file_descriptor, file_descriptor2;
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char fn[]="original.file";
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/* create original file */
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if((file_descriptor = creat(fn,S_IRUSR | S_IWUSR)) < 0)
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perror("creat() error");
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/* generate a duplicate file descriptor of file_descriptor */
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else {
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if ((file_descriptor2 = dup(file_descriptor)) < 0)
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perror("dup() error");
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/* print resulting information */
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else {
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print_file_id(file_descriptor);
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print_file_id(file_descriptor2);
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puts("The file descriptors are different but");
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puts("they point to the same file.");
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close(file_descriptor);
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close(file_descriptor2);
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}
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unlink(fn);
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}
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}
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</pre>
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<p><strong>Output:</strong></p>
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<pre>
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The file id of file_descriptor 0 is 30
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The file id of file_descriptor 3 is 30
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The file descriptors are different but
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they point to the same file.
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</pre>
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<br>
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<hr>
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API introduced: V3R1
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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</html>
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