136 lines
4.3 KiB
HTML
136 lines
4.3 KiB
HTML
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The President and Fellows of Harvard College.
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-->
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<html>
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<head>
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<title>lseek</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>lseek</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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lseek - change current position in file
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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>off_t</tt><br>
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<tt>lseek(int </tt><em>fd</em><tt>, off_t </tt><em>pos</em><tt>,
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int </tt><em>whence</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>lseek</tt> alters the current seek position of the file handle
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<em>filehandle</em>, seeking to a new position based on <em>pos</em>
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and <em>whence</em>.
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</p>
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<p>
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If <em>whence</em> is
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<ul>
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<li> SEEK_SET, the new position is <em>pos</em>.
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<li> SEEK_CUR, the new position is the current position plus <em>pos</em>.
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<li> SEEK_END, the new position is the position of end-of-file
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plus <em>pos</em>.
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<li> anything else, lseek fails.
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</ul>
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Note that <em>pos</em> is a signed quantity.
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</p>
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<p>
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It is not meaningful to seek on certain objects, such as the console
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device. All seeks on these objects fail.
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</p>
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<p>
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Seek positions less than zero are invalid. Seek positions beyond EOF
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are legal, at least on regular files.
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</p>
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<p>
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As discussed under <A HREF=getdirentry.html>getdirentry</A>, seek
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positions on directories are defined by the file system and should not
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be interpreted.
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</p>
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<p>
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Note that each distinct open of a file should have an independent seek
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pointer.
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</p>
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<p>
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<tt>lseek</tt> (like all system calls) should be atomic. In this case
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this means that multiple threads or processes sharing the same seek
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pointer should be able to update it without seeing or generating
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invalid intermediate states. There is no provision for making pairs of
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<tt>lseek</tt> and <tt>read</tt> or <tt>write</tt> calls atomic. The
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<tt>pread</tt> and <tt>pwrite</tt> calls in Unix were invented to
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address this issue. (These are not in OS/161 by default but are easy
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to implement.)
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</p>
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<h3>Return Values</h3>
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<p>
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On success, <tt>lseek</tt> returns the new position. On error, -1 is
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returned, and <A HREF=errno.html>errno</A> is set according to the
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error 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
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mentioned here.
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<table width=90%>
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<tr><td width=5% rowspan=4> </td>
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<td width=10% valign=top>EBADF</td>
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<td><em>fd</em> is not a valid file
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handle.</td></tr>
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<tr><td valign=top>ESPIPE</td> <td><em>fd</em> refers to an object
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which does not support seeking.</td></tr>
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<tr><td valign=top>EINVAL</td> <td><em>whence</em> is invalid.</td></tr>
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<tr><td valign=top>EINVAL</td> <td>The resulting seek position would
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be negative.</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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