135 lines
4.1 KiB
HTML
135 lines
4.1 KiB
HTML
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<html>
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<head>
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<title>dup2</title>
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<link rel="stylesheet" type="text/css" media="all" href="../man.css">
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</head>
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<body bgcolor=#ffffff>
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<h2 align=center>dup2</h2>
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<h4 align=center>OS/161 Reference Manual</h4>
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<h3>Name</h3>
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<p>
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dup2 - clone file handles
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</p>
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<h3>Library</h3>
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<p>
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Standard C Library (libc, -lc)
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</p>
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<h3>Synopsis</h3>
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<p>
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<tt>#include <unistd.h></tt><br>
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<br>
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<tt>int</tt><br>
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<tt>dup2(int </tt><em>oldfd</em><tt>, int </tt><em>newfd</em><tt>);</tt>
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</p>
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<h3>Description</h3>
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<p>
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<tt>dup2</tt> clones the file handle <em>oldfd</em> onto the file
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handle <em>newfd</em>. If <em>newfd</em> names an already-open file,
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that file is closed.
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</p>
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<p>
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The two handles refer to the same "open" of the file -- that is,
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they are references to the same object and share the same seek
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pointer. Note that this is different from opening the same file
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twice.
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</p>
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<p>
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dup2 is most commonly used to relocate opened files onto
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<tt>STDIN_FILENO</tt>, <tt>STDOUT_FILENO</tt>, and/or
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<tt>STDERR_FILENO</tt>.
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</p>
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<p>
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Both filehandles must be non-negative, and, if applicable, smaller
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than the maximum allowed file handle number.
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</p>
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<p>
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The call (like all system calls) should be atomic; for single-threaded
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processes this is trivial.
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Multithreaded processes should never e.g. see an intermediate state
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where <em>newfd</em> has been closed but <em>oldfd</em> has not yet
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been cloned onto it.
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Similarly, if two threads attempt e.g. <tt>dup2(3, 4)</tt> and
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<tt>dup2(4, 3)</tt> simultaneously, the results must be equivalent to
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one of the calls completing before the other starts.
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</p>
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<p>
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Using dup2 to clone a file handle onto itself has no effect.
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</p>
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<p>
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(The "2" in "dup2" arises from the existence of an older and less
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powerful Unix system call "dup".)
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</p>
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<h3>Return Values</h3>
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<p>
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<tt>dup2</tt> returns <em>newfd</em>. On error, -1 is returned, and
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<A HREF=errno.html>errno</A> is set according to the error
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encountered.
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</p>
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<h3>Errors</h3>
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<p>
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The following error codes should be returned under the conditions
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given. Other error codes may be returned for other cases not mentioned
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here.
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<table width=90%>
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<tr><td width=5% rowspan=3> </td>
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<td width=10% valign=top>EBADF</td>
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<td><em>oldfd</em> is not a valid file
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handle, or <em>newfd</em> is a value
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that cannot be a valid file
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handle.</td></tr>
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<tr><td valign=top>EMFILE</td> <td>The process's file table was full, or a
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process-specific limit on open files
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was reached.</td></tr>
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<tr><td valign=top>ENFILE</td> <td>The system's file table was full,
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if such a thing is possible, or a
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global limit on open files was
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reached.</td></tr>
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</table>
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</p>
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</body>
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</html>
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