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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>access</title></head><body bgcolor="white"><basefont size="3"> <a name="access"></a> <a name="tag_03_10"></a><!-- access --> <!--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_10_01"></a>NAME</h4><blockquote>access - determine accessibility of a file</blockquote><h4><a name="tag_03_10_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/unistd.h.html">unistd.h</a>><br><br> int access(const char *</tt><i>path</i><tt>, int</tt> <i>amode</i><tt>);<br></tt></code></p></blockquote><h4><a name="tag_03_10_03"></a>DESCRIPTION</h4><blockquote><p>The <i>access</i>() function shall check the file named by the pathname pointed to by the <i>path</i> argument for accessibilityaccording to the bit pattern contained in <i>amode</i>, using the real user ID in place of the effective user ID and the real groupID in place of the effective group ID.</p><p>The value of <i>amode</i> is either the bitwise-inclusive OR of the access permissions to be checked (R_OK, W_OK, X_OK) or theexistence test (F_OK).</p><p>If any access permissions are checked, each shall be checked individually, as described in the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap03.html">Chapter 3, Definitions</a>. If the process has appropriateprivileges, an implementation may indicate success for X_OK even if none of the execute file permission bits are set.</p></blockquote><h4><a name="tag_03_10_04"></a>RETURN VALUE</h4><blockquote><p>If the requested access is permitted, <i>access</i>() succeeds and shall return 0; otherwise, -1 shall be returned and<i>errno</i> shall be set to indicate the error.</p></blockquote><h4><a name="tag_03_10_05"></a>ERRORS</h4><blockquote><p>The <i>access</i>() function shall fail if:</p><dl compact><dt>[EACCES]</dt><dd>Permission bits of the file mode do not permit the requested access, or search permission is denied on a component of the pathprefix.</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><dt>[EROFS]</dt><dd>Write access is requested for a file on a read-only file system.</dd></dl><p>The <i>access</i>() function may fail if:</p><dl compact><dt>[EINVAL]</dt><dd>The value of the <i>amode</i> argument is invalid.</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><dt>[ETXTBSY]</dt><dd>Write access is requested for a pure procedure (shared text) file that is being executed.</dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_10_06"></a>EXAMPLES</h4><blockquote><h5><a name="tag_03_10_06_01"></a>Testing for the Existence of a File</h5><p>The following example tests whether a file named <b>myfile</b> exists in the <b>/tmp</b> directory.</p><pre><tt>#include <unistd.h>...int result;const char *filename = "/tmp/myfile";<br>result = access (filename, F_OK);</tt></pre></blockquote><h4><a name="tag_03_10_07"></a>APPLICATION USAGE</h4><blockquote><p>Additional values of <i>amode</i> other than the set defined in the description may be valid; for example, if a system hasextended access controls.</p></blockquote><h4><a name="tag_03_10_08"></a>RATIONALE</h4><blockquote><p>In early proposals, some inadequacies in the <i>access</i>() function led to the creation of an <i>eaccess</i>() functionbecause:</p><ol><li><p>Historical implementations of <i>access</i>() do not test file access correctly when the process' real user ID is superuser. Inparticular, they always return zero when testing execute permissions without regard to whether the file is executable.</p></li><li><p>The superuser has complete access to all files on a system. As a consequence, programs started by the superuser and switched tothe effective user ID with lesser privileges cannot use <i>access</i>() to test their file access permissions.</p></li></ol><p>However, the historical model of <i>eaccess</i>() does not resolve problem (1), so this volume of IEEE Std 1003.1-2001now allows <i>access</i>() to behave in the desired way because several implementations have corrected the problem. It was alsoargued that problem (2) is more easily solved by using <a href="../functions/open.html"><i>open</i>()</a>, <a href="../functions/chdir.html"><i>chdir</i>()</a>, or one of the <i><a href="../functions/exec.html">exec</a></i> functions asappropriate and responding to the error, rather than creating a new function that would not be as reliable. Therefore,<i>eaccess</i>() is not included in this volume of IEEE Std 1003.1-2001.</p><p>The sentence concerning appropriate privileges and execute permission bits reflects the two possibilities implemented byhistorical implementations when checking superuser access for X_OK.</p><p>New implementations are discouraged from returning X_OK unless at least one execution permission bit is set.</p></blockquote><h4><a name="tag_03_10_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_10_10"></a>SEE ALSO</h4><blockquote><p><a href="chmod.html"><i>chmod</i>()</a>, <a href="stat.html"><i>stat</i>()</a>, the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a></p></blockquote><h4><a name="tag_03_10_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 1. Derived from Issue 1 of the SVID.</p></blockquote><h4><a name="tag_03_10_12"></a>Issue 6</h4><blockquote><p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p><ul><li><p>The [ELOOP] mandatory error condition is added.</p></li><li><p>A second [ENAMETOOLONG] is added as an optional error condition.</p></li><li><p>The [ETXTBSY] optional error condition is added.</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></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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