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| <h3 class="section">13.1 Opening and Closing Files</h3> |
| |
| <p><a name="index-opening-a-file-descriptor-1197"></a><a name="index-closing-a-file-descriptor-1198"></a>This section describes the primitives for opening and closing files |
| using file descriptors. The <code>open</code> and <code>creat</code> functions are |
| declared in the header file <samp><span class="file">fcntl.h</span></samp>, while <code>close</code> is |
| declared in <samp><span class="file">unistd.h</span></samp>. |
| <a name="index-unistd_002eh-1199"></a><a name="index-fcntl_002eh-1200"></a> |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Function: int <b>open</b> (<var>const char *filename, int flags</var>[<var>, mode_t mode</var>])<var><a name="index-open-1201"></a></var><br> |
| <blockquote><p>The <code>open</code> function creates and returns a new file descriptor |
| for the file named by <var>filename</var>. Initially, the file position |
| indicator for the file is at the beginning of the file. The argument |
| <var>mode</var> is used only when a file is created, but it doesn't hurt |
| to supply the argument in any case. |
| |
| <p>The <var>flags</var> argument controls how the file is to be opened. This is |
| a bit mask; you create the value by the bitwise OR of the appropriate |
| parameters (using the ‘<samp><span class="samp">|</span></samp>’ operator in C). |
| See <a href="File-Status-Flags.html#File-Status-Flags">File Status Flags</a>, for the parameters available. |
| |
| <p>The normal return value from <code>open</code> is a non-negative integer file |
| descriptor. In the case of an error, a value of -1 is returned |
| instead. 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>The file exists but is not readable/writable as requested by the <var>flags</var> |
| argument, the file does not exist and the directory is unwritable so |
| it cannot be created. |
| |
| <br><dt><code>EEXIST</code><dd>Both <code>O_CREAT</code> and <code>O_EXCL</code> are set, and the named file already |
| exists. |
| |
| <br><dt><code>EINTR</code><dd>The <code>open</code> operation was interrupted by a signal. |
| See <a href="Interrupted-Primitives.html#Interrupted-Primitives">Interrupted Primitives</a>. |
| |
| <br><dt><code>EISDIR</code><dd>The <var>flags</var> argument specified write access, and the file is a directory. |
| |
| <br><dt><code>EMFILE</code><dd>The process has too many files open. |
| The maximum number of file descriptors is controlled by the |
| <code>RLIMIT_NOFILE</code> resource limit; see <a href="Limits-on-Resources.html#Limits-on-Resources">Limits on Resources</a>. |
| |
| <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>ENOENT</code><dd>The named file does not exist, and <code>O_CREAT</code> is not specified. |
| |
| <br><dt><code>ENOSPC</code><dd>The directory or file system that would contain the new file cannot be |
| extended, because there is no disk space left. |
| |
| <br><dt><code>ENXIO</code><dd><code>O_NONBLOCK</code> and <code>O_WRONLY</code> are both set in the <var>flags</var> |
| argument, the file named by <var>filename</var> is a FIFO (see <a href="Pipes-and-FIFOs.html#Pipes-and-FIFOs">Pipes and FIFOs</a>), and no process has the file open for reading. |
| |
| <br><dt><code>EROFS</code><dd>The file resides on a read-only file system and any of <code>O_WRONLY</code><!-- /@w -->, |
| <code>O_RDWR</code>, and <code>O_TRUNC</code> are set in the <var>flags</var> argument, |
| or <code>O_CREAT</code> is set and the file does not already exist. |
| </dl> |
| |
| <!-- !!! umask --> |
| <p>If on a 32 bit machine the sources are translated with |
| <code>_FILE_OFFSET_BITS == 64</code> the function <code>open</code> returns a file |
| descriptor opened in the large file mode which enables the file handling |
| functions to use files up to 2^63 bytes in size and offset from |
| -2^63 to 2^63. This happens transparently for the user |
| since all of the lowlevel file handling functions are equally replaced. |
| |
| <p>This function is a cancellation point in multi-threaded programs. This |
| is a problem if the thread allocates some resources (like memory, file |
| descriptors, semaphores or whatever) at the time <code>open</code> is |
| called. If the thread gets canceled these resources stay allocated |
| until the program ends. To avoid this calls to <code>open</code> should be |
| protected using cancellation handlers. |
| <!-- ref pthread_cleanup_push / pthread_cleanup_pop --> |
| |
| <p>The <code>open</code> function is the underlying primitive for the <code>fopen</code> |
| and <code>freopen</code> functions, that create streams. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Function: int <b>open64</b> (<var>const char *filename, int flags</var>[<var>, mode_t mode</var>])<var><a name="index-open64-1202"></a></var><br> |
| <blockquote><p>This function is similar to <code>open</code>. It returns a file descriptor |
| which can be used to access the file named by <var>filename</var>. The only |
| difference is that on 32 bit systems the file is opened in the |
| large file mode. I.e., file length and file offsets can exceed 31 bits. |
| |
| <p>When the sources are translated with <code>_FILE_OFFSET_BITS == 64</code> this |
| function is actually available under the name <code>open</code>. I.e., the |
| new, extended API using 64 bit file sizes and offsets transparently |
| replaces the old API. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Obsolete function: int <b>creat</b> (<var>const char *filename, mode_t mode</var>)<var><a name="index-creat-1203"></a></var><br> |
| <blockquote><p>This function is obsolete. The call: |
| |
| <pre class="smallexample"> creat (<var>filename</var>, <var>mode</var>) |
| </pre> |
| <p class="noindent">is equivalent to: |
| |
| <pre class="smallexample"> open (<var>filename</var>, O_WRONLY | O_CREAT | O_TRUNC, <var>mode</var>) |
| </pre> |
| <p>If on a 32 bit machine the sources are translated with |
| <code>_FILE_OFFSET_BITS == 64</code> the function <code>creat</code> returns a file |
| descriptor opened in the large file mode which enables the file handling |
| functions to use files up to 2^63 in size and offset from |
| -2^63 to 2^63. This happens transparently for the user |
| since all of the lowlevel file handling functions are equally replaced. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Obsolete function: int <b>creat64</b> (<var>const char *filename, mode_t mode</var>)<var><a name="index-creat64-1204"></a></var><br> |
| <blockquote><p>This function is similar to <code>creat</code>. It returns a file descriptor |
| which can be used to access the file named by <var>filename</var>. The only |
| the difference is that on 32 bit systems the file is opened in the |
| large file mode. I.e., file length and file offsets can exceed 31 bits. |
| |
| <p>To use this file descriptor one must not use the normal operations but |
| instead the counterparts named <code>*64</code>, e.g., <code>read64</code>. |
| |
| <p>When the sources are translated with <code>_FILE_OFFSET_BITS == 64</code> this |
| function is actually available under the name <code>open</code>. I.e., the |
| new, extended API using 64 bit file sizes and offsets transparently |
| replaces the old API. |
| </p></blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>close</b> (<var>int filedes</var>)<var><a name="index-close-1205"></a></var><br> |
| <blockquote><p>The function <code>close</code> closes the file descriptor <var>filedes</var>. |
| Closing a file has the following consequences: |
| |
| <ul> |
| <li>The file descriptor is deallocated. |
| |
| <li>Any record locks owned by the process on the file are unlocked. |
| |
| <li>When all file descriptors associated with a pipe or FIFO have been closed, |
| any unread data is discarded. |
| </ul> |
| |
| <p>This function is a cancellation point in multi-threaded programs. This |
| is a problem if the thread allocates some resources (like memory, file |
| descriptors, semaphores or whatever) at the time <code>close</code> is |
| called. If the thread gets canceled these resources stay allocated |
| until the program ends. To avoid this, calls to <code>close</code> should be |
| protected using cancellation handlers. |
| <!-- ref pthread_cleanup_push / pthread_cleanup_pop --> |
| |
| <p>The normal return value from <code>close</code> is 0; a value of -1 |
| is returned in case of failure. The following <code>errno</code> error |
| conditions are defined for this function: |
| |
| <dl> |
| <dt><code>EBADF</code><dd>The <var>filedes</var> argument is not a valid file descriptor. |
| |
| <br><dt><code>EINTR</code><dd>The <code>close</code> call was interrupted by a signal. |
| See <a href="Interrupted-Primitives.html#Interrupted-Primitives">Interrupted Primitives</a>. |
| Here is an example of how to handle <code>EINTR</code> properly: |
| |
| <pre class="smallexample"> TEMP_FAILURE_RETRY (close (desc)); |
| </pre> |
| <br><dt><code>ENOSPC</code><dt><code>EIO</code><dt><code>EDQUOT</code><dd>When the file is accessed by NFS, these errors from <code>write</code> can sometimes |
| not be detected until <code>close</code>. See <a href="I_002fO-Primitives.html#I_002fO-Primitives">I/O Primitives</a>, for details |
| on their meaning. |
| </dl> |
| |
| <p>Please note that there is <em>no</em> separate <code>close64</code> function. |
| This is not necessary since this function does not determine nor depend |
| on the mode of the file. The kernel which performs the <code>close</code> |
| operation knows which mode the descriptor is used for and can handle |
| this situation. |
| </p></blockquote></div> |
| |
| <p>To close a stream, call <code>fclose</code> (see <a href="Closing-Streams.html#Closing-Streams">Closing Streams</a>) instead |
| of trying to close its underlying file descriptor with <code>close</code>. |
| This flushes any buffered output and updates the stream object to |
| indicate that it is closed. |
| |
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