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251 lines
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<title>mq_open</title>
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<basefont size="3"> <a name="mq_open"></a> <a name="tag_03_385"></a><!-- mq_open -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_385_01"></a>NAME</h4>
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<blockquote>mq_open - open a message queue (<b>REALTIME</b>)</blockquote>
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<h4><a name="tag_03_385_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('MSG')">MSG</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/mqueue.h.html">mqueue.h</a>><br>
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<br>
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mqd_t mq_open(const char *</tt><i>name</i><tt>, int</tt> <i>oflag</i><tt>, ...); <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_385_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>mq_open</i>() function shall establish the connection between a process and a message queue with a message queue
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descriptor. It shall create an open message queue description that refers to the message queue, and a message queue descriptor that
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refers to that open message queue description. The message queue descriptor is used by other functions to refer to that message
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queue. The <i>name</i> argument points to a string naming a message queue. It is unspecified whether the name appears in the file
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system and is visible to other functions that take pathnames as arguments. The <i>name</i> argument shall conform to the
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construction rules for a pathname. If <i>name</i> begins with the slash character, then processes calling <i>mq_open</i>() with the
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same value of <i>name</i> shall refer to the same message queue object, as long as that name has not been removed. If <i>name</i>
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does not begin with the slash character, the effect is implementation-defined. The interpretation of slash characters other than
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the leading slash character in <i>name</i> is implementation-defined. If the <i>name</i> argument is not the name of an existing
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message queue and creation is not requested, <i>mq_open</i>() shall fail and return an error.</p>
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<p>A message queue descriptor may be implemented using a file descriptor, in which case applications can open up to at least
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{OPEN_MAX} file and message queues.</p>
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<p>The <i>oflag</i> argument requests the desired receive and/or send access to the message queue. The requested access permission
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to receive messages or send messages shall be granted if the calling process would be granted read or write access, respectively,
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to an equivalently protected file.</p>
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<p>The value of <i>oflag</i> is the bitwise-inclusive OR of values from the following list. Applications shall specify exactly one
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of the first three values (access modes) below in the value of <i>oflag</i>:</p>
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<dl compact>
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<dt>O_RDONLY</dt>
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<dd>Open the message queue for receiving messages. The process can use the returned message queue descriptor with <a href=
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"../functions/mq_receive.html"><i>mq_receive</i>()</a>, but not <a href="../functions/mq_send.html"><i>mq_send</i>()</a>. A message
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queue may be open multiple times in the same or different processes for receiving messages.</dd>
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<dt>O_WRONLY</dt>
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<dd>Open the queue for sending messages. The process can use the returned message queue descriptor with <a href=
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"../functions/mq_send.html"><i>mq_send</i>()</a> but not <a href="../functions/mq_receive.html"><i>mq_receive</i>()</a>. A message
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queue may be open multiple times in the same or different processes for sending messages.</dd>
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<dt>O_RDWR</dt>
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<dd>Open the queue for both receiving and sending messages. The process can use any of the functions allowed for O_RDONLY and
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O_WRONLY. A message queue may be open multiple times in the same or different processes for sending messages.</dd>
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</dl>
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<p>Any combination of the remaining flags may be specified in the value of <i>oflag</i>:</p>
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<dl compact>
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<dt>O_CREAT</dt>
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<dd>Create a message queue. It requires two additional arguments: <i>mode</i>, which shall be of type <b>mode_t</b>, and
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<i>attr</i>, which shall be a pointer to an <b>mq_attr</b> structure. If the pathname <i>name</i> has already been used to create a
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message queue that still exists, then this flag shall have no effect, except as noted under O_EXCL. Otherwise, a message queue
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shall be created without any messages in it. The user ID of the message queue shall be set to the effective user ID of the process,
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and the group ID of the message queue shall be set to the effective group ID of the process. The file permission bits shall be set
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to the value of <i>mode</i>. When bits in <i>mode</i> other than file permission bits are set, the effect is
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implementation-defined. If <i>attr</i> is NULL, the message queue shall be created with implementation-defined default message
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queue attributes. If <i>attr</i> is non-NULL and the calling process has the appropriate privilege on <i>name</i>, the message
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queue <i>mq_maxmsg</i> and <i>mq_msgsize</i> attributes shall be set to the values of the corresponding members in the
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<b>mq_attr</b> structure referred to by <i>attr</i>. If <i>attr</i> is non-NULL, but the calling process does not have the
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appropriate privilege on <i>name</i>, the <i>mq_open</i>() function shall fail and return an error without creating the message
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queue.</dd>
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<dt>O_EXCL</dt>
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<dd>If O_EXCL and O_CREAT are set, <i>mq_open</i>() shall fail if the message queue <i>name</i> exists. The check for the existence
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of the message queue and the creation of the message queue if it does not exist shall be atomic with respect to other threads
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executing <i>mq_open</i>() naming the same <i>name</i> with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the
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result is undefined.</dd>
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<dt>O_NONBLOCK</dt>
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<dd>Determines whether an <a href="../functions/mq_send.html"><i>mq_send</i>()</a> or <a href=
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"../functions/mq_receive.html"><i>mq_receive</i>()</a> waits for resources or messages that are not currently available, or fails
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with <i>errno</i> set to [EAGAIN]; see <a href="mq_send.html"><i>mq_send</i>()</a> and <a href=
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"mq_receive.html"><i>mq_receive</i>()</a> for details.</dd>
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</dl>
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<p>The <i>mq_open</i>() function does not add or remove messages from the queue.</p>
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</blockquote>
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<h4><a name="tag_03_385_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, the function shall return a message queue descriptor; otherwise, the function shall return
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(<b>mqd_t</b>)-1 and set <i>errno</i> to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_385_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>mq_open</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EACCES]</dt>
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<dd>The message queue exists and the permissions specified by <i>oflag</i> are denied, or the message queue does not exist and
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permission to create the message queue is denied.</dd>
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<dt>[EEXIST]</dt>
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<dd>O_CREAT and O_EXCL are set and the named message queue already exists.</dd>
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<dt>[EINTR]</dt>
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<dd>The <i>mq_open</i>() function was interrupted by a signal.</dd>
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<dt>[EINVAL]</dt>
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<dd>The <i>mq_open</i>() function is not supported for the given name.</dd>
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<dt>[EINVAL]</dt>
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<dd>O_CREAT was specified in <i>oflag</i>, the value of <i>attr</i> is not NULL, and either <i>mq_maxmsg</i> or <i>mq_msgsize</i>
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was less than or equal to zero.</dd>
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<dt>[EMFILE]</dt>
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<dd>Too many message queue descriptors or file descriptors are currently in use by this process.</dd>
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<dt>[ENAMETOOLONG]</dt>
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<dd>
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The length of the <i>name</i> argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.</dd>
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<dt>[ENFILE]</dt>
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<dd>Too many message queues are currently open in the system.</dd>
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<dt>[ENOENT]</dt>
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<dd>O_CREAT is not set and the named message queue does not exist.</dd>
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<dt>[ENOSPC]</dt>
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<dd>There is insufficient space for the creation of the new message queue.</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_385_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_385_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_385_08"></a>RATIONALE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_385_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_385_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="mq_close.html"><i>mq_close</i>()</a> , <a href="mq_getattr.html"><i>mq_getattr</i>()</a> , <a href=
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"mq_receive.html"><i>mq_receive</i>()</a> , <a href="mq_send.html"><i>mq_send</i>()</a> , <a href=
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"mq_setattr.html"><i>mq_setattr</i>()</a> , <a href="mq_timedreceive.html"><i>mq_timedreceive</i>()</a> , <a href=
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"mq_timedsend.html"><i>mq_timedsend</i>()</a> , <a href="mq_unlink.html"><i>mq_unlink</i>()</a> , <a href=
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"msgctl.html"><i>msgctl</i>()</a> , <a href="msgget.html"><i>msgget</i>()</a> , <a href="msgrcv.html"><i>msgrcv</i>()</a> , <a
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href="msgsnd.html"><i>msgsnd</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/mqueue.h.html"><i><mqueue.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_385_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Realtime Extension.</p>
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</blockquote>
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<h4><a name="tag_03_385_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>mq_open</i>() function is marked as part of the Message Passing option.</p>
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<p>The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Message
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Passing option.</p>
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<p>The <a href="../functions/mq_timedreceive.html"><i>mq_timedreceive</i>()</a> and <a href=
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"../functions/mq_timedsend.html"><i>mq_timedsend</i>()</a> functions are added to the SEE ALSO section for alignment with
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IEEE Std 1003.1d-1999.</p>
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<p>The DESCRIPTION of O_EXCL is updated in response to IEEE PASC Interpretation 1003.1c #48.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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