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| <h3 class="section">13.11 Control Operations on Files</h3> |
| |
| <p><a name="index-control-operations-on-files-1311"></a><a name="index-g_t_0040code_007bfcntl_007d-function-1312"></a>This section describes how you can perform various other operations on |
| file descriptors, such as inquiring about or setting flags describing |
| the status of the file descriptor, manipulating record locks, and the |
| like. All of these operations are performed by the function <code>fcntl</code>. |
| |
| <p>The second argument to the <code>fcntl</code> function is a command that |
| specifies which operation to perform. The function and macros that name |
| various flags that are used with it are declared in the header file |
| <samp><span class="file">fcntl.h</span></samp>. Many of these flags are also used by the <code>open</code> |
| function; see <a href="Opening-and-Closing-Files.html#Opening-and-Closing-Files">Opening and Closing Files</a>. |
| <a name="index-fcntl_002eh-1313"></a> |
| <!-- fcntl.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Function: int <b>fcntl</b> (<var>int filedes, int command, <small class="dots">...</small></var>)<var><a name="index-fcntl-1314"></a></var><br> |
| <blockquote><p>The <code>fcntl</code> function performs the operation specified by |
| <var>command</var> on the file descriptor <var>filedes</var>. Some commands |
| require additional arguments to be supplied. These additional arguments |
| and the return value and error conditions are given in the detailed |
| descriptions of the individual commands. |
| |
| <p>Briefly, here is a list of what the various commands are. |
| |
| <dl> |
| <dt><code>F_DUPFD</code><dd>Duplicate the file descriptor (return another file descriptor pointing |
| to the same open file). See <a href="Duplicating-Descriptors.html#Duplicating-Descriptors">Duplicating Descriptors</a>. |
| |
| <br><dt><code>F_GETFD</code><dd>Get flags associated with the file descriptor. See <a href="Descriptor-Flags.html#Descriptor-Flags">Descriptor Flags</a>. |
| |
| <br><dt><code>F_SETFD</code><dd>Set flags associated with the file descriptor. See <a href="Descriptor-Flags.html#Descriptor-Flags">Descriptor Flags</a>. |
| |
| <br><dt><code>F_GETFL</code><dd>Get flags associated with the open file. See <a href="File-Status-Flags.html#File-Status-Flags">File Status Flags</a>. |
| |
| <br><dt><code>F_SETFL</code><dd>Set flags associated with the open file. See <a href="File-Status-Flags.html#File-Status-Flags">File Status Flags</a>. |
| |
| <br><dt><code>F_GETLK</code><dd>Get a file lock. See <a href="File-Locks.html#File-Locks">File Locks</a>. |
| |
| <br><dt><code>F_SETLK</code><dd>Set or clear a file lock. See <a href="File-Locks.html#File-Locks">File Locks</a>. |
| |
| <br><dt><code>F_SETLKW</code><dd>Like <code>F_SETLK</code>, but wait for completion. See <a href="File-Locks.html#File-Locks">File Locks</a>. |
| |
| <br><dt><code>F_GETOWN</code><dd>Get process or process group ID to receive <code>SIGIO</code> signals. |
| See <a href="Interrupt-Input.html#Interrupt-Input">Interrupt Input</a>. |
| |
| <br><dt><code>F_SETOWN</code><dd>Set process or process group ID to receive <code>SIGIO</code> signals. |
| See <a href="Interrupt-Input.html#Interrupt-Input">Interrupt Input</a>. |
| </dl> |
| |
| <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>fcntl</code> is |
| called. If the thread gets canceled these resources stay allocated |
| until the program ends. To avoid this calls to <code>fcntl</code> should be |
| protected using cancellation handlers. |
| <!-- ref pthread_cleanup_push / pthread_cleanup_pop --> |
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