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<title>dup2</title>
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<h2 align=center>dup2</h2>
<h4 align=center>OS/161 Reference Manual</h4>

<h3>Name</h3>
dup2 - clone file handles

<h3>Library</h3>
Standard C Library (libc, -lc)

<h3>Synopsis</h3>
#include &lt;unistd.h&gt;<br>
<br>
int<br>
dup2(int <em>oldfd</em>, int <em>newfd</em>);

<h3>Description</h3>

dup2 clones the file handle <em>oldfd</em> onto the file handle
<em>newfd</em>. If <em>newfd</em> names an open file, that file is
closed.
<p>

The two handles refer to the same "open" of the file - that is,
they are references to the same object and share the same seek
pointer. Note that this is different from opening the same file
twice. 
<p>

dup2 is most commonly used to relocate opened files onto
STDIN_FILENO, STDOUT_FILENO, and/or STDERR_FILENO.
<p>

Both filehandles must be non-negative.
<p>

Using dup2 to clone a file handle onto itself has no effect.
<p>

(The "2" in "dup2" arises from the existence of an older and less
powerful Unix system call "dup".)

<h3>Return Values</h3>
dup2 returns <em>newfd</em>. On error, -1 is returned, and
<A HREF=errno.html>errno</A> is set according to the error
encountered.

<h3>Errors</h3>

The following error codes should be returned under the conditions
given. Other error codes may be returned for other errors not
mentioned here.

<blockquote><table width=90%>
<tr><td width=10%>&nbsp;</td><td>&nbsp;</td></tr>
<tr><td>EBADF</td>		<td><em>oldfd</em> is not a valid file handle,
				or <em>newfd</em> is a value that
				cannot be a valid file handle.</td></tr>
<tr><td>EMFILE</td>		<td>The process's file table was full, or a
				process-specific limit on open files
				was reached.</td></tr>
</table></blockquote>

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