216 lines
8.5 KiB
HTML
216 lines
8.5 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 -->
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>lseek</title>
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</head>
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<body bgcolor="white">
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<script type="text/javascript" language="JavaScript" src="../jscript/codes.js">
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<basefont size="3"> <a name="lseek"></a> <a name="tag_03_354"></a><!-- lseek -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_354_01"></a>NAME</h4>
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<blockquote>lseek - move the read/write file offset</blockquote>
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<h4><a name="tag_03_354_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/unistd.h.html">unistd.h</a>><br>
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<br>
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off_t lseek(int</tt> <i>fildes</i><tt>, off_t</tt> <i>offset</i><tt>, int</tt> <i>whence</i><tt>);<br>
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</tt></code></p>
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</blockquote>
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<h4><a name="tag_03_354_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>lseek</i>() function shall set the file offset for the open file description associated with the file descriptor
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<i>fildes,</i> as follows:</p>
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<ul>
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<li>
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<p>If <i>whence</i> is SEEK_SET, the file offset shall be set to <i>offset</i> bytes.</p>
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</li>
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<li>
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<p>If <i>whence</i> is SEEK_CUR, the file offset shall be set to its current location plus <i>offset</i>.</p>
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</li>
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<li>
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<p>If <i>whence</i> is SEEK_END, the file offset shall be set to the size of the file plus <i>offset</i>.</p>
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</li>
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</ul>
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<p>The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in <a href=
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"../basedefs/unistd.h.html"><i><unistd.h></i></a>.</p>
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<p>The behavior of <i>lseek</i>() on devices which are incapable of seeking is implementation-defined. The value of the file offset
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associated with such a device is undefined.</p>
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<p>The <i>lseek</i>() function shall allow the file offset to be set beyond the end of the existing data in the file. If data is
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later written at this point, subsequent reads of data in the gap shall return bytes with the value 0 until data is actually written
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into the gap.</p>
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<p>The <i>lseek</i>() function shall not, by itself, extend the size of a file.</p>
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<p><sup>[<a href="javascript:open_code('SHM')">SHM</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If <i>fildes</i> refers to a shared memory object, the result of the <i>lseek</i>() function is unspecified. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p><sup>[<a href="javascript:open_code('TYM')">TYM</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If <i>fildes</i> refers to a typed memory object, the result of the <i>lseek</i>() function is unspecified. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_354_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, the resulting offset, as measured in bytes from the beginning of the file, shall be returned.
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Otherwise, (<b>off_t</b>)-1 shall be returned, <i>errno</i> shall be set to indicate the error, and the file offset shall remain
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unchanged.</p>
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</blockquote>
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<h4><a name="tag_03_354_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>lseek</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The <i>fildes</i> argument is not an open file descriptor.</dd>
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<dt>[EINVAL]</dt>
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<dd>The <i>whence</i> argument is not a proper value, or the resulting file offset would be negative for a regular file, block
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special file, or directory.</dd>
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<dt>[EOVERFLOW]</dt>
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<dd>The resulting file offset would be a value which cannot be represented correctly in an object of type <b>off_t</b>.</dd>
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<dt>[ESPIPE]</dt>
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<dd>The <i>fildes</i> argument is associated with a pipe, FIFO, or socket.</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_354_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_354_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_354_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The ISO C standard includes the functions <a href="../functions/fgetpos.html"><i>fgetpos</i>()</a> and <a href=
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"../functions/fsetpos.html"><i>fsetpos</i>()</a>, which work on very large files by use of a special positioning type.</p>
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<p>Although <i>lseek</i>() may position the file offset beyond the end of the file, this function does not itself extend the size
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of the file. While the only function in IEEE Std 1003.1-2001 that may directly extend the size of the file is <a href=
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"../functions/write.html"><i>write</i>()</a>, <a href="../functions/truncate.html"><i>truncate</i>()</a>, and <a href=
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"../functions/ftruncate.html"><i>ftruncate</i>()</a>, several functions originally derived from the ISO C standard, such as <a
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href="../functions/fwrite.html"><i>fwrite</i>()</a>, <a href="../functions/fprintf.html"><i>fprintf</i>()</a>, and so on, may do so
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(by causing calls on <a href="../functions/write.html"><i>write</i>()</a>).</p>
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<p>An invalid file offset that would cause [EINVAL] to be returned may be both implementation-defined and device-dependent (for
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example, memory may have few invalid values). A negative file offset may be valid for some devices in some implementations.</p>
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<p>The POSIX.1-1990 standard did not specifically prohibit <i>lseek</i>() from returning a negative offset. Therefore, an
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application was required to clear <i>errno</i> prior to the call and check <i>errno</i> upon return to determine whether a return
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value of ( <b>off_t</b>)-1 is a negative offset or an indication of an error condition. The standard developers did not wish to
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require this action on the part of a conforming application, and chose to require that <i>errno</i> be set to [EINVAL] when the
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resulting file offset would be negative for a regular file, block special file, or directory.</p>
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</blockquote>
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<h4><a name="tag_03_354_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_354_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="open.html"><i>open</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/sys/types.h.html"><i><sys/types.h></i></a>, <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_354_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 1. Derived from Issue 1 of the SVID.</p>
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</blockquote>
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<h4><a name="tag_03_354_12"></a>Issue 5</h4>
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<blockquote>
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<p>The DESCRIPTION is updated for alignment with the POSIX Realtime Extension.</p>
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<p>Large File Summit extensions are added.</p>
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</blockquote>
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<h4><a name="tag_03_354_13"></a>Issue 6</h4>
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<blockquote>
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<p>In the SYNOPSIS, the optional include of the <a href="../basedefs/sys/types.h.html"><i><sys/types.h></i></a> header is
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removed.</p>
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<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>
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<ul>
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<li>
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<p>The requirement to include <a href="../basedefs/sys/types.h.html"><i><sys/types.h></i></a> has been removed. Although <a
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href="../basedefs/sys/types.h.html"><i><sys/types.h></i></a> was required for conforming implementations of previous POSIX
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specifications, it was not required for UNIX applications.</p>
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</li>
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<li>
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<p>The [EOVERFLOW] error condition is added. This change is to support large files.</p>
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</li>
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</ul>
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<p>An additional [ESPIPE] error condition is added for sockets.</p>
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<p>The DESCRIPTION is updated for alignment with IEEE Std 1003.1j-2000 by specifying that <i>lseek</i>() results are
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unspecified for typed memory objects.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<hr size="2" noshade>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
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