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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>readlink</title></head><body bgcolor="white"><basefont size="3"> <a name="readlink"></a> <a name="tag_03_596"></a><!-- readlink --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_596_01"></a>NAME</h4><blockquote>readlink - read the contents of a symbolic link</blockquote><h4><a name="tag_03_596_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/unistd.h.html">unistd.h</a>><br><br> ssize_t readlink(const char *restrict</tt> <i>path</i><tt>, char *restrict</tt> <i>buf</i><tt>,<br> size_t</tt> <i>bufsize</i><tt>);<br></tt></code></p></blockquote><h4><a name="tag_03_596_03"></a>DESCRIPTION</h4><blockquote><p>The <i>readlink</i>() function shall place the contents of the symbolic link referred to by <i>path</i> in the buffer <i>buf</i>which has size <i>bufsize</i>. If the number of bytes in the symbolic link is less than <i>bufsize</i>, the contents of theremainder of <i>buf</i> are unspecified. If the <i>buf</i> argument is not large enough to contain the link content, the first<i>bufsize</i> bytes shall be placed in <i>buf</i>.</p><p>If the value of <i>bufsize</i> is greater than {SSIZE_MAX}, the result is implementation-defined.</p></blockquote><h4><a name="tag_03_596_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, <i>readlink</i>() shall return the count of bytes placed in the buffer. Otherwise, it shall return avalue of -1, leave the buffer unchanged, and set <i>errno</i> to indicate the error.</p></blockquote><h4><a name="tag_03_596_05"></a>ERRORS</h4><blockquote><p>The <i>readlink</i>() function shall fail if:</p><dl compact><dt>[EACCES]</dt><dd>Search permission is denied for a component of the path prefix of <i>path</i>.</dd><dt>[EINVAL]</dt><dd>The <i>path</i> argument names a file that is not a symbolic link.</dd><dt>[EIO]</dt><dd>An I/O error occurred while reading from the file system.</dd><dt>[ELOOP]</dt><dd>A loop exists in symbolic links encountered during resolution of the <i>path</i> argument.</dd><dt>[ENAMETOOLONG]</dt><dd>The length of the <i>path</i> argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.</dd><dt>[ENOENT]</dt><dd>A component of <i>path</i> does not name an existing file or <i>path</i> is an empty string.</dd><dt>[ENOTDIR]</dt><dd>A component of the path prefix is not a directory.</dd></dl><p>The <i>readlink</i>() function may fail if:</p><dl compact><dt>[EACCES]</dt><dd>Read permission is denied for the directory.</dd><dt>[ELOOP]</dt><dd>More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the <i>path</i> argument.</dd><dt>[ENAMETOOLONG]</dt><dd>As a result of encountering a symbolic link in resolution of the <i>path</i> argument, the length of the substituted pathnamestring exceeded {PATH_MAX}.</dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_596_06"></a>EXAMPLES</h4><blockquote><h5><a name="tag_03_596_06_01"></a>Reading the Name of a Symbolic Link</h5><p>The following example shows how to read the name of a symbolic link named <b>/modules/pass1</b>.</p><pre><tt>#include <unistd.h><br>char buf[1024];ssizet_t len;...if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1) buf[len] = '\0';</tt></pre></blockquote><h4><a name="tag_03_596_07"></a>APPLICATION USAGE</h4><blockquote><p>Conforming applications should not assume that the returned contents of the symbolic link are null-terminated.</p></blockquote><h4><a name="tag_03_596_08"></a>RATIONALE</h4><blockquote><p>Since IEEE Std 1003.1-2001 does not require any association of file times with symbolic links, there is no requirementthat file times be updated by <i>readlink</i>(). The type associated with <i>bufsiz</i> is a <b>size_t</b> in order to beconsistent with both the ISO C standard and the definition of <a href="../functions/read.html"><i>read</i>()</a>. The behaviorspecified for <i>readlink</i>() when <i>bufsiz</i> is zero represents historical practice. For this case, the standard developersconsidered a change whereby <i>readlink</i>() would return the number of non-null bytes contained in the symbolic link with thebuffer <i>buf</i> remaining unchanged; however, since the <b>stat</b> structure member <i>st_size</i> value can be used todetermine the size of buffer necessary to contain the contents of the symbolic link as returned by <i>readlink</i>(), this proposalwas rejected, and the historical practice retained.</p></blockquote><h4><a name="tag_03_596_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_596_10"></a>SEE ALSO</h4><blockquote><p><a href="lstat.html"><i>lstat</i>()</a>, <a href="stat.html"><i>stat</i>()</a>, <a href="symlink.html"><i>symlink</i>()</a>,the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a></p></blockquote><h4><a name="tag_03_596_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 4, Version 2.</p></blockquote><h4><a name="tag_03_596_12"></a>Issue 5</h4><blockquote><p>Moved from X/OPEN UNIX extension to BASE.</p></blockquote><h4><a name="tag_03_596_13"></a>Issue 6</h4><blockquote><p>The return type is changed to <b>ssize_t</b>, to align with the IEEE P1003.1a draft standard.</p><p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p><ul><li><p>This function is made mandatory.</p></li><li><p>In this function it is possible for the return value to exceed the range of the type <b>ssize_t</b> (since <b>size_t</b> has alarger range of positive values than <b>ssize_t</b>). A sentence restricting the size of the <b>size_t</b> object is added to thedescription to resolve this conflict.</p></li></ul><p>The following changes are made for alignment with the ISO POSIX-1:1996 standard:</p><ul><li><p>The FUTURE DIRECTIONS section is changed to None.</p></li></ul><p>The following changes were made to align with the IEEE P1003.1a draft standard:</p><ul><li><p>The [ELOOP] optional error condition is added.</p></li></ul><p>The <b>restrict</b> keyword is added to the <i>readlink</i>() prototype for alignment with the ISO/IEC 9899:1999standard.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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