<!-- Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009, 2013 The President and Fellows of Harvard College. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. --> <html> <head> <title>lseek</title> <body bgcolor=#ffffff> <h2 align=center>lseek</h2> <h4 align=center>OS/161 Reference Manual</h4> <h3>Name</h3> <p> lseek - change current position in file </p> <h3>Library</h3> <p> Standard C Library (libc, -lc) </p> <h3>Synopsis</h3> <p> <tt>#include <unistd.h></tt><br> <br> <tt>off_t</tt><br> <tt>lseek(int </tt><em>fd</em><tt>, off_t </tt><em>pos</em><tt>, int </tt><em>whence</em><tt>);</tt> </p> <h3>Description</h3> <p> <tt>lseek</tt> alters the current seek position of the file handle <em>filehandle</em>, seeking to a new position based on <em>pos</em> and <em>whence</em>. </p> <p> If <em>whence</em> is <ul> <li> SEEK_SET, the new position is <em>pos</em>. <li> SEEK_CUR, the new position is the current position plus <em>pos</em>. <li> SEEK_END, the new position is the position of end-of-file plus <em>pos</em>. <li> anything else, lseek fails. </ul> Note that <em>pos</em> is a signed quantity. </p> <p> It is not meaningful to seek on certain objects, such as the console device. All seeks on these objects fail. </p> <p> Seek positions less than zero are invalid. Seek positions beyond EOF are legal, at least on regular files. </p> <p> As discussed under <A HREF=getdirentry.html>getdirentry</A>, seek positions on directories are defined by the file system and should not be interpreted. </p> <p> Note that each distinct open of a file should have an independent seek pointer. </p> <p> <tt>lseek</tt> (like all system calls) should be atomic. In this case this means that multiple threads or processes sharing the same seek pointer should be able to update it without seeing or generating invalid intermediate states. There is no provision for making pairs of <tt>lseek</tt> and <tt>read</tt> or <tt>write</tt> calls atomic. The <tt>pread</tt> and <tt>pwrite</tt> calls in Unix were invented to address this issue. (These are not in OS/161 by default but are easy to implement.) </p> <h3>Return Values</h3> <p> On success, <tt>lseek</tt> returns the new position. On error, -1 is returned, and <A HREF=errno.html>errno</A> is set according to the error encountered. </p> <h3>Errors</h3> <p> The following error codes should be returned under the conditions given. Other error codes may be returned for other cases not mentioned here. <table width=90%> <tr><td width=5% rowspan=4> </td> <td width=10% valign=top>EBADF</td> <td><em>fd</em> is not a valid file handle.</td></tr> <tr><td valign=top>ESPIPE</td> <td><em>fd</em> refers to an object which does not support seeking.</td></tr> <tr><td valign=top>EINVAL</td> <td><em>whence</em> is invalid.</td></tr> <tr><td valign=top>EINVAL</td> <td>The resulting seek position would be negative.</td></tr> </table> </p> </body> </html>