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| <h4 class="subsection">13.14.2 Open-time Flags</h4> |
| |
| <p>The open-time flags specify options affecting how <code>open</code> will behave. |
| These options are not preserved once the file is open. The exception to |
| this is <code>O_NONBLOCK</code>, which is also an I/O operating mode and so it |
| <em>is</em> saved. See <a href="Opening-and-Closing-Files.html#Opening-and-Closing-Files">Opening and Closing Files</a>, for how to call |
| <code>open</code>. |
| |
| <p>There are two sorts of options specified by open-time flags. |
| |
| <ul> |
| <li><dfn>File name translation flags</dfn> affect how <code>open</code> looks up the |
| file name to locate the file, and whether the file can be created. |
| <a name="index-file-name-translation-flags-1337"></a><a name="index-flags_002c-file-name-translation-1338"></a> |
| <li><dfn>Open-time action flags</dfn> specify extra operations that <code>open</code> will |
| perform on the file once it is open. |
| <a name="index-open_002dtime-action-flags-1339"></a><a name="index-flags_002c-open_002dtime-action-1340"></a></ul> |
| |
| <p>Here are the file name translation flags. |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_CREAT</b><var><a name="index-O_005fCREAT-1341"></a></var><br> |
| <blockquote><p>If set, the file will be created if it doesn't already exist. |
| <!-- !!! mode arg, umask --> |
| <a name="index-create-on-open-_0028file-status-flag_0029-1342"></a></p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_EXCL</b><var><a name="index-O_005fEXCL-1343"></a></var><br> |
| <blockquote><p>If both <code>O_CREAT</code> and <code>O_EXCL</code> are set, then <code>open</code> fails |
| if the specified file already exists. This is guaranteed to never |
| clobber an existing file. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_NONBLOCK</b><var><a name="index-O_005fNONBLOCK-1344"></a></var><br> |
| <blockquote><p><a name="index-non_002dblocking-open-1345"></a>This prevents <code>open</code> from blocking for a “long time” to open the |
| file. This is only meaningful for some kinds of files, usually devices |
| such as serial ports; when it is not meaningful, it is harmless and |
| ignored. Often opening a port to a modem blocks until the modem reports |
| carrier detection; if <code>O_NONBLOCK</code> is specified, <code>open</code> will |
| return immediately without a carrier. |
| |
| <p>Note that the <code>O_NONBLOCK</code> flag is overloaded as both an I/O operating |
| mode and a file name translation flag. This means that specifying |
| <code>O_NONBLOCK</code> in <code>open</code> also sets nonblocking I/O mode; |
| see <a href="Operating-Modes.html#Operating-Modes">Operating Modes</a>. To open the file without blocking but do normal |
| I/O that blocks, you must call <code>open</code> with <code>O_NONBLOCK</code> set and |
| then call <code>fcntl</code> to turn the bit off. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_NOCTTY</b><var><a name="index-O_005fNOCTTY-1346"></a></var><br> |
| <blockquote><p>If the named file is a terminal device, don't make it the controlling |
| terminal for the process. See <a href="Job-Control.html#Job-Control">Job Control</a>, for information about |
| what it means to be the controlling terminal. |
| |
| <p>In the GNU system and 4.4 BSD, opening a file never makes it the |
| controlling terminal and <code>O_NOCTTY</code> is zero. However, other |
| systems may use a nonzero value for <code>O_NOCTTY</code> and set the |
| controlling terminal when you open a file that is a terminal device; so |
| to be portable, use <code>O_NOCTTY</code> when it is important to avoid this. |
| <a name="index-controlling-terminal_002c-setting-1347"></a></p></blockquote></div> |
| |
| <p>The following three file name translation flags exist only in the GNU system. |
| |
| <!-- fcntl.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: int <b>O_IGNORE_CTTY</b><var><a name="index-O_005fIGNORE_005fCTTY-1348"></a></var><br> |
| <blockquote><p>Do not recognize the named file as the controlling terminal, even if it |
| refers to the process's existing controlling terminal device. Operations |
| on the new file descriptor will never induce job control signals. |
| See <a href="Job-Control.html#Job-Control">Job Control</a>. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: int <b>O_NOLINK</b><var><a name="index-O_005fNOLINK-1349"></a></var><br> |
| <blockquote><p>If the named file is a symbolic link, open the link itself instead of |
| the file it refers to. (<code>fstat</code> on the new file descriptor will |
| return the information returned by <code>lstat</code> on the link's name.) |
| <a name="index-symbolic-link_002c-opening-1350"></a></p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: int <b>O_NOTRANS</b><var><a name="index-O_005fNOTRANS-1351"></a></var><br> |
| <blockquote><p>If the named file is specially translated, do not invoke the translator. |
| Open the bare file the translator itself sees. |
| </p></blockquote></div> |
| |
| <p>The open-time action flags tell <code>open</code> to do additional operations |
| which are not really related to opening the file. The reason to do them |
| as part of <code>open</code> instead of in separate calls is that <code>open</code> |
| can do them <i>atomically</i>. |
| |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>O_TRUNC</b><var><a name="index-O_005fTRUNC-1352"></a></var><br> |
| <blockquote><p>Truncate the file to zero length. This option is only useful for |
| regular files, not special files such as directories or FIFOs. POSIX.1 |
| requires that you open the file for writing to use <code>O_TRUNC</code>. In |
| BSD and GNU you must have permission to write the file to truncate it, |
| but you need not open for write access. |
| |
| <p>This is the only open-time action flag specified by POSIX.1. There is |
| no good reason for truncation to be done by <code>open</code>, instead of by |
| calling <code>ftruncate</code> afterwards. The <code>O_TRUNC</code> flag existed in |
| Unix before <code>ftruncate</code> was invented, and is retained for backward |
| compatibility. |
| </p></blockquote></div> |
| |
| <p>The remaining operating modes are BSD 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_SHLOCK</b><var><a name="index-O_005fSHLOCK-1353"></a></var><br> |
| <blockquote><p>Acquire a shared lock on the file, as with <code>flock</code>. |
| See <a href="File-Locks.html#File-Locks">File Locks</a>. |
| |
| <p>If <code>O_CREAT</code> is specified, the locking is done atomically when |
| creating the file. You are guaranteed that no other process will get |
| the lock on the new file first. |
| </p></blockquote></div> |
| |
| <!-- fcntl.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Macro: int <b>O_EXLOCK</b><var><a name="index-O_005fEXLOCK-1354"></a></var><br> |
| <blockquote><p>Acquire an exclusive lock on the file, as with <code>flock</code>. |
| See <a href="File-Locks.html#File-Locks">File Locks</a>. This is atomic like <code>O_SHLOCK</code>. |
| </p></blockquote></div> |
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