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| <h4 class="subsection">14.2.2 Opening a Directory Stream</h4> |
| |
| <p><a name="index-dirent_002eh-1411"></a>This section describes how to open a directory stream. All the symbols |
| are declared in the header file <samp><span class="file">dirent.h</span></samp>. |
| |
| <!-- dirent.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Data Type: <b>DIR</b><var><a name="index-DIR-1412"></a></var><br> |
| <blockquote><p>The <code>DIR</code> data type represents a directory stream. |
| </p></blockquote></div> |
| |
| <p>You shouldn't ever allocate objects of the <code>struct dirent</code> or |
| <code>DIR</code> data types, since the directory access functions do that for |
| you. Instead, you refer to these objects using the pointers returned by |
| the following functions. |
| |
| <!-- dirent.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: DIR * <b>opendir</b> (<var>const char *dirname</var>)<var><a name="index-opendir-1413"></a></var><br> |
| <blockquote><p>The <code>opendir</code> function opens and returns a directory stream for |
| reading the directory whose file name is <var>dirname</var>. The stream has |
| type <code>DIR *</code>. |
| |
| <p>If unsuccessful, <code>opendir</code> returns a null pointer. In addition to |
| the usual file name errors (see <a href="File-Name-Errors.html#File-Name-Errors">File Name Errors</a>), the |
| following <code>errno</code> error conditions are defined for this function: |
| |
| <dl> |
| <dt><code>EACCES</code><dd>Read permission is denied for the directory named by <code>dirname</code>. |
| |
| <br><dt><code>EMFILE</code><dd>The process has too many files open. |
| |
| <br><dt><code>ENFILE</code><dd>The entire system, or perhaps the file system which contains the |
| directory, cannot support any additional open files at the moment. |
| (This problem cannot happen on the GNU system.) |
| |
| <br><dt><code>ENOMEM</code><dd>Not enough memory available. |
| </dl> |
| |
| <p>The <code>DIR</code> type is typically implemented using a file descriptor, |
| and the <code>opendir</code> function in terms of the <code>open</code> function. |
| See <a href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO">Low-Level I/O</a>. Directory streams and the underlying |
| file descriptors are closed on <code>exec</code> (see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>). |
| </p></blockquote></div> |
| |
| <p>The directory which is opened for reading by <code>opendir</code> is |
| identified by the name. In some situations this is not sufficient. |
| Or the way <code>opendir</code> implicitly creates a file descriptor for the |
| directory is not the way a program might want it. In these cases an |
| alternative interface can be used. |
| |
| <!-- dirent.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: DIR * <b>fdopendir</b> (<var>int fd</var>)<var><a name="index-fdopendir-1414"></a></var><br> |
| <blockquote><p>The <code>fdopendir</code> function works just like <code>opendir</code> but |
| instead of taking a file name and opening a file descriptor for the |
| directory the caller is required to provide a file descriptor. This |
| file descriptor is then used in subsequent uses of the returned |
| directory stream object. |
| |
| <p>The caller must make sure the file descriptor is associated with a |
| directory and it allows reading. |
| |
| <p>If the <code>fdopendir</code> call returns successfully the file descriptor |
| is now under the control of the system. It can be used in the same |
| way the descriptor implicitly created by <code>opendir</code> can be used |
| but the program must not close the descriptor. |
| |
| <p>In case the function is unsuccessful it returns a null pointer and the |
| file descriptor remains to be usable by the program. The following |
| <code>errno</code> error conditions are defined for this function: |
| |
| <dl> |
| <dt><code>EBADF</code><dd>The file descriptor is not valid. |
| |
| <br><dt><code>ENOTDIR</code><dd>The file descriptor is not associated with a directory. |
| |
| <br><dt><code>EINVAL</code><dd>The descriptor does not allow reading the directory content. |
| |
| <br><dt><code>ENOMEM</code><dd>Not enough memory available. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>In some situations it can be desirable to get hold of the file |
| descriptor which is created by the <code>opendir</code> call. For instance, |
| to switch the current working directory to the directory just read the |
| <code>fchdir</code> function could be used. Historically the <code>DIR</code> type |
| was exposed and programs could access the fields. This does not happen |
| in the GNU C library. Instead a separate function is provided to allow |
| access. |
| |
| <!-- dirent.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>dirfd</b> (<var>DIR *dirstream</var>)<var><a name="index-dirfd-1415"></a></var><br> |
| <blockquote><p>The function <code>dirfd</code> returns the file descriptor associated with |
| the directory stream <var>dirstream</var>. This descriptor can be used until |
| the directory is closed with <code>closedir</code>. If the directory stream |
| implementation is not using file descriptors the return value is |
| <code>-1</code>. |
| </p></blockquote></div> |
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