316 lines
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316 lines
14 KiB
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<title>readdir</title>
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<basefont size="3"> <a name="readdir"></a> <a name="tag_03_595"></a><!-- readdir -->
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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_595_01"></a>NAME</h4>
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<blockquote>readdir, readdir_r - read a directory</blockquote>
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<h4><a name="tag_03_595_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/dirent.h.html">dirent.h</a>><br>
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<br>
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struct dirent *readdir(DIR *</tt><i>dirp</i><tt>);<br>
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</tt></code></p>
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('TSF')">TSF</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> int readdir_r(DIR *restrict</tt> <i>dirp</i><tt>, struct dirent *restrict</tt> <i>entry</i><tt>,<br>
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struct dirent **restrict</tt> <i>result</i><tt>); <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_595_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The type <b>DIR</b>, which is defined in the <a href="../basedefs/dirent.h.html"><i><dirent.h></i></a> header, represents
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a <i>directory stream</i>, which is an ordered sequence of all the directory entries in a particular directory. Directory entries
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represent files; files may be removed from a directory or added to a directory asynchronously to the operation of
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<i>readdir</i>().</p>
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<p>The <i>readdir</i>() function shall return a pointer to a structure representing the directory entry at the current position in
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the directory stream specified by the argument <i>dirp</i>, and position the directory stream at the next entry. It shall return a
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null pointer upon reaching the end of the directory stream. The structure <b>dirent</b> defined in the <a href=
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"../basedefs/dirent.h.html"><i><dirent.h></i></a> header describes a directory entry.</p>
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<p>The <i>readdir</i>() function shall not return directory entries containing empty names. If entries for dot or dot-dot exist,
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one entry shall be returned for dot and one entry shall be returned for dot-dot; otherwise, they shall not be returned.</p>
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<p>The pointer returned by <i>readdir</i>() points to data which may be overwritten by another call to <i>readdir</i>() on the same
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directory stream. This data is not overwritten by another call to <i>readdir</i>() on a different directory stream.</p>
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<p>If a file is removed from or added to the directory after the most recent call to <a href=
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"../functions/opendir.html"><i>opendir</i>()</a> or <a href="../functions/rewinddir.html"><i>rewinddir</i>()</a>, whether a
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subsequent call to <i>readdir</i>() returns an entry for that file is unspecified.</p>
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<p>The <i>readdir</i>() function may buffer several directory entries per actual read operation; <i>readdir</i>() shall mark for
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update the <i>st_atime</i> field of the directory each time the directory is actually read.</p>
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<p>After a call to <a href="../functions/fork.html"><i>fork</i>()</a>, either the parent or child (but not both) may continue
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processing the directory stream using <i>readdir</i>(), <a href="../functions/rewinddir.html"><i>rewinddir</i>()</a>, <sup>[<a
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href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> or <a
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href="../functions/seekdir.html"><i>seekdir</i>()</a>. <img src="../images/opt-end.gif" alt="[Option End]" border="0"> If both the
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parent and child processes use these functions, the result is undefined.</p>
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<p>If the entry names a symbolic link, the value of the <i>d_ino</i> member is unspecified.</p>
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<p>The <i>readdir</i>() function need not be reentrant. A function that is not required to be reentrant is not required to be
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thread-safe.</p>
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<p><sup>[<a href="javascript:open_code('TSF')">TSF</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The <i>readdir_r</i>() function shall initialize the <b>dirent</b> structure referenced by <i>entry</i> to represent the directory
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entry at the current position in the directory stream referred to by <i>dirp</i>, store a pointer to this structure at the location
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referenced by <i>result</i>, and position the directory stream at the next entry.</p>
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<p>The storage pointed to by <i>entry</i> shall be large enough for a <b>dirent</b> with an array of <b>char</b> <i>d_name</i>
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members containing at least {NAME_MAX}+1 elements.</p>
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<p>Upon successful return, the pointer returned at *<i>result</i> shall have the same value as the argument <i>entry</i>. Upon
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reaching the end of the directory stream, this pointer shall have the value NULL.</p>
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<p>The <i>readdir_r</i>() function shall not return directory entries containing empty names.</p>
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<p>If a file is removed from or added to the directory after the most recent call to <a href=
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"../functions/opendir.html"><i>opendir</i>()</a> or <a href="../functions/rewinddir.html"><i>rewinddir</i>()</a>, whether a
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subsequent call to <i>readdir_r</i>() returns an entry for that file is unspecified.</p>
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<p>The <i>readdir_r</i>() function may buffer several directory entries per actual read operation; the <i>readdir_r</i>() function
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shall mark for update the <i>st_atime</i> field of the directory each time the directory is actually read. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p>Applications wishing to check for error situations should set <i>errno</i> to 0 before calling <i>readdir</i>(). If <i>errno</i>
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is set to non-zero on return, an error occurred.</p>
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</blockquote>
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<h4><a name="tag_03_595_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, <i>readdir</i>() shall return a pointer to an object of type <b>struct dirent</b>. When an error is
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encountered, a null pointer shall be returned and <i>errno</i> shall be set to indicate the error. When the end of the directory is
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encountered, a null pointer shall be returned and <i>errno</i> is not changed.</p>
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<p><sup>[<a href="javascript:open_code('TSF')">TSF</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If successful, the <i>readdir_r</i>() function shall return zero; otherwise, an error number shall be returned to indicate the
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error. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_595_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>readdir</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EOVERFLOW]</dt>
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<dd>One of the values in the structure to be returned cannot be represented correctly.</dd>
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</dl>
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<p>The <i>readdir</i>() function may fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The <i>dirp</i> argument does not refer to an open directory stream.</dd>
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<dt>[ENOENT]</dt>
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<dd>The current position of the directory stream is invalid.</dd>
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</dl>
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<p>The <i>readdir_r</i>() function may fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The <i>dirp</i> argument does not refer to an open directory stream.</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_595_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>The following sample code searches the current directory for the entry <i>name</i>.</p>
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<pre>
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<tt>dirp = opendir(".");
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<br>
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while (dirp) {
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errno = 0;
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if ((dp = readdir(dirp)) != NULL) {
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if (strcmp(dp->d_name, name) == 0) {
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closedir(dirp);
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return FOUND;
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}
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} else {
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if (errno == 0) {
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closedir(dirp);
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return NOT_FOUND;
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}
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closedir(dirp);
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return READ_ERROR;
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}
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}
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<br>
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return OPEN_ERROR;
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_03_595_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <i>readdir</i>() function should be used in conjunction with <a href="../functions/opendir.html"><i>opendir</i>()</a>, <a
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href="../functions/closedir.html"><i>closedir</i>()</a>, and <a href="../functions/rewinddir.html"><i>rewinddir</i>()</a> to
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examine the contents of the directory.</p>
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<p>The <i>readdir_r</i>() function is thread-safe and shall return values in a user-supplied buffer instead of possibly using a
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static data area that may be overwritten by each call.</p>
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</blockquote>
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<h4><a name="tag_03_595_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The returned value of <i>readdir</i>() merely <i>represents</i> a directory entry. No equivalence should be inferred.</p>
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<p>Historical implementations of <i>readdir</i>() obtain multiple directory entries on a single read operation, which permits
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subsequent <i>readdir</i>() operations to operate from the buffered information. Any wording that required each successful
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<i>readdir</i>() operation to mark the directory <i>st_atime</i> field for update would disallow such historical
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performance-oriented implementations.</p>
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<p>Since <i>readdir</i>() returns NULL when it detects an error and when the end of the directory is encountered, an application
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that needs to tell the difference must set <i>errno</i> to zero before the call and check it if NULL is returned. Since the
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function must not change <i>errno</i> in the second case and must set it to a non-zero value in the first case, a zero <i>errno</i>
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after a call returning NULL indicates end-of-directory; otherwise, an error.</p>
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<p>Routines to deal with this problem more directly were proposed:</p>
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<pre>
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<tt>int derror (</tt><i>dirp</i><tt>)
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DIR *</tt><i>dirp</i><tt>;
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<br>
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void clearderr (</tt><i>dirp</i><tt>)
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DIR *</tt><i>dirp</i><tt>;
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</tt>
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</pre>
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<p>The first would indicate whether an error had occurred, and the second would clear the error indication. The simpler method
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involving <i>errno</i> was adopted instead by requiring that <i>readdir</i>() not change <i>errno</i> when end-of-directory is
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encountered.</p>
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<p>An error or signal indicating that a directory has changed while open was considered but rejected.</p>
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<p>The thread-safe version of the directory reading function returns values in a user-supplied buffer instead of possibly using a
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static data area that may be overwritten by each call. Either the {NAME_MAX} compile-time constant or the corresponding <a href=
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"../functions/pathconf.html"><i>pathconf</i>()</a> option can be used to determine the maximum sizes of returned pathnames.</p>
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</blockquote>
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<h4><a name="tag_03_595_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_595_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="closedir.html"><i>closedir</i>()</a> , <a href="lstat.html"><i>lstat</i>()</a> , <a href=
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"opendir.html"><i>opendir</i>()</a> , <a href="rewinddir.html"><i>rewinddir</i>()</a> , <a href="symlink.html"><i>symlink</i>()</a>
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, the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/dirent.h.html"><i><dirent.h></i></a>,
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<a href="../basedefs/sys/types.h.html"><i><sys/types.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_595_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 2.</p>
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</blockquote>
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<h4><a name="tag_03_595_12"></a>Issue 5</h4>
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<blockquote>
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<p>Large File Summit extensions are added.</p>
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<p>The <i>readdir_r</i>() function is included for alignment with the POSIX Threads Extension.</p>
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<p>A note indicating that the <i>readdir</i>() function need not be reentrant is added to the DESCRIPTION.</p>
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</blockquote>
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<h4><a name="tag_03_595_13"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>readdir_r</i>() function is marked as part of the Thread-Safe Functions option.</p>
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<p>The Open Group Corrigendum U026/7 is applied, correcting the prototype for <i>readdir_r</i>().</p>
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<p>The Open Group Corrigendum U026/8 is applied, clarifying the wording of the successful return for the <i>readdir_r</i>()
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function.</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>A statement is added to the DESCRIPTION indicating the disposition of certain fields in <b>struct dirent</b> when an entry
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refers to a symbolic link.</p>
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</li>
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<li>
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<p>The [EOVERFLOW] mandatory error condition is added. This change is to support large files.</p>
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</li>
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<li>
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<p>The [ENOENT] optional error condition is added.</p>
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</li>
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</ul>
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<p>The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a
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static data area.</p>
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<p>The <b>restrict</b> keyword is added to the <i>readdir_r</i>() prototype for alignment with the ISO/IEC 9899:1999
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standard.</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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