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| <h4 class="subsection">13.14.3 I/O Operating Modes</h4> |
| |
| <p>The operating modes affect how input and output operations using a file |
| descriptor work. These flags are set by <code>open</code> and can be fetched |
| and changed with <code>fcntl</code>. |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_APPEND</b><var><a name="index-O_005fAPPEND-1355"></a></var><br> |
| <blockquote><p>The bit that enables append mode for the file. If set, then all |
| <code>write</code> operations write the data at the end of the file, extending |
| it, regardless of the current file position. This is the only reliable |
| way to append to a file. In append mode, you are guaranteed that the |
| data you write will always go to the current end of the file, regardless |
| of other processes writing to the file. Conversely, if you simply set |
| the file position to the end of file and write, then another process can |
| extend the file after you set the file position but before you write, |
| resulting in your data appearing someplace before the real end of file. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_NONBLOCK</b><var><a name="index-O_005fNONBLOCK-1356"></a></var><br> |
| <blockquote><p>The bit that enables nonblocking mode for the file. If this bit is set, |
| <code>read</code> requests on the file can return immediately with a failure |
| status if there is no input immediately available, instead of blocking. |
| Likewise, <code>write</code> requests can also return immediately with a |
| failure status if the output can't be written immediately. |
| |
| <p>Note that the <code>O_NONBLOCK</code> flag is overloaded as both an I/O |
| operating mode and a file name translation flag; see <a href="Open_002dtime-Flags.html#Open_002dtime-Flags">Open-time Flags</a>. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>O_NDELAY</b><var><a name="index-O_005fNDELAY-1357"></a></var><br> |
| <blockquote><p>This is an obsolete name for <code>O_NONBLOCK</code>, provided for |
| compatibility with BSD. It is not defined by the POSIX.1 standard. |
| </p></blockquote></div> |
| |
| <p>The remaining operating modes are BSD and GNU extensions. They exist only |
| on some systems. On other systems, these macros are not defined. |
| |
| <!-- fcntl.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>O_ASYNC</b><var><a name="index-O_005fASYNC-1358"></a></var><br> |
| <blockquote><p>The bit that enables asynchronous input mode. If set, then <code>SIGIO</code> |
| signals will be generated when input is available. See <a href="Interrupt-Input.html#Interrupt-Input">Interrupt Input</a>. |
| |
| <p>Asynchronous input mode is a BSD feature. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>O_FSYNC</b><var><a name="index-O_005fFSYNC-1359"></a></var><br> |
| <blockquote><p>The bit that enables synchronous writing for the file. If set, each |
| <code>write</code> call will make sure the data is reliably stored on disk before |
| returning. <!-- !!! xref fsync --> |
| |
| <p>Synchronous writing is a BSD feature. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>O_SYNC</b><var><a name="index-O_005fSYNC-1360"></a></var><br> |
| <blockquote><p>This is another name for <code>O_FSYNC</code>. They have the same value. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: int <b>O_NOATIME</b><var><a name="index-O_005fNOATIME-1361"></a></var><br> |
| <blockquote><p>If this bit is set, <code>read</code> will not update the access time of the |
| file. See <a href="File-Times.html#File-Times">File Times</a>. This is used by programs that do backups, so |
| that backing a file up does not count as reading it. |
| Only the owner of the file or the superuser may use this bit. |
| |
| <p>This is a GNU extension. |
| </p></blockquote></div> |
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