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248 lines
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<title>mq_receive</title>
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<basefont size="3"> <a name="mq_receive"></a> <a name="tag_03_386"></a><!-- mq_receive -->
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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_386_01"></a>NAME</h4>
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<blockquote>mq_receive, mq_timedreceive - receive a message from a message queue (<b>REALTIME</b>)</blockquote>
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<h4><a name="tag_03_386_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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ssize_t mq_receive(mqd_t</tt> <i>mqdes</i><tt>, char *</tt><i>msg_ptr</i><tt>, size_t</tt> <i>msg_len</i><tt>,<br>
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unsigned *</tt><i>msg_prio</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]"
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border="0"></tt></code></div>
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<tt><br>
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<br>
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</tt>
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('MSG TMO')">MSG TMO</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"><br>
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#include <<a href="../basedefs/mqueue.h.html">mqueue.h</a>><br>
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#include <<a href="../basedefs/time.h.html">time.h</a>><br>
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ssize_t mq_timedreceive(mqd_t</tt> <i>mqdes</i><tt>, char *restrict</tt> <i>msg_ptr</i><tt>,<br>
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size_t</tt> <i>msg_len</i><tt>, unsigned *restrict</tt> <i>msg_prio</i><tt>,<br>
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const struct timespec *restrict</tt> <i>abs_timeout</i><tt>); <img src=
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"../images/opt-end.gif" alt="[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_386_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>mq_receive</i>() function shall receive the oldest of the highest priority message(s) from the message queue specified by
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<i>mqdes</i>. If the size of the buffer in bytes, specified by the <i>msg_len</i> argument, is less than the <i>mq_msgsize</i>
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attribute of the message queue, the function shall fail and return an error. Otherwise, the selected message shall be removed from
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the queue and copied to the buffer pointed to by the <i>msg_ptr</i> argument.</p>
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<p>If the value of <i>msg_len</i> is greater than {SSIZE_MAX}, the result is implementation-defined.</p>
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<p>If the argument <i>msg_prio</i> is not NULL, the priority of the selected message shall be stored in the location referenced by
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<i>msg_prio</i>.</p>
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<p>If the specified message queue is empty and O_NONBLOCK is not set in the message queue description associated with <i>mqdes</i>,
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<i>mq_receive</i>() shall block until a message is enqueued on the message queue or until <i>mq_receive</i>() is interrupted by a
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signal. If more than one thread is waiting to receive a message when a message arrives at an empty queue and the Priority
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Scheduling option is supported, then the thread of highest priority that has been waiting the longest shall be selected to receive
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the message. Otherwise, it is unspecified which waiting thread receives the message. If the specified message queue is empty and
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O_NONBLOCK is set in the message queue description associated with <i>mqdes</i>, no message shall be removed from the queue, and
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<i>mq_receive</i>() shall return an error.</p>
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<p><sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The <i>mq_timedreceive</i>() function shall receive the oldest of the highest priority messages from the message queue specified by
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<i>mqdes</i> as described for the <i>mq_receive</i>() function. However, if O_NONBLOCK was not specified when the message queue was
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opened via the <a href="../functions/mq_open.html"><i>mq_open</i>()</a> function, and no message exists on the queue to satisfy the
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receive, the wait for such a message shall be terminated when the specified timeout expires. If O_NONBLOCK is set, this function is
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equivalent to <i>mq_receive</i>().</p>
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<p>The timeout expires when the absolute time specified by <i>abs_timeout</i> passes, as measured by the clock on which timeouts
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are based (that is, when the value of that clock equals or exceeds <i>abs_timeout</i>), or if the absolute time specified by
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<i>abs_timeout</i> has already been passed at the time of the call. <img src="../images/opt-end.gif" alt="[Option End]" border=
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"0"></p>
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<p><sup>[<a href="javascript:open_code('TMO TMR')">TMO TMR</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"
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border="0"> If the Timers option is supported, the timeout shall be based on the CLOCK_REALTIME clock; if the Timers option is not
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supported, the timeout shall be based on the system clock as returned by the <a href="../functions/time.html"><i>time</i>()</a>
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function. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p><sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The resolution of the timeout shall be the resolution of the clock on which it is based. The <i>timespec</i> argument is defined in
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the <a href="../basedefs/time.h.html"><i><time.h></i></a> header.</p>
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<p>Under no circumstance shall the operation fail with a timeout if a message can be removed from the message queue immediately.
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The validity of the <i>abs_timeout</i> parameter need not be checked if a message can be removed from the message queue
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immediately. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_386_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, the <i>mq_receive</i>() <sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src=
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"../images/opt-start.gif" alt="[Option Start]" border="0"> and <i>mq_timedreceive</i>() <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"> functions shall return the length of the selected message in bytes and the message shall be removed from
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the queue. Otherwise, no message shall be removed from the queue, the functions shall return a value of -1, and set <i>errno</i> to
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indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_386_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>mq_receive</i>() <sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> and <i>mq_timedreceive</i>() <img src="../images/opt-end.gif" alt="[Option End]" border="0">
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functions shall fail if:</p>
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<dl compact>
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<dt>[EAGAIN]</dt>
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<dd>O_NONBLOCK was set in the message description associated with <i>mqdes</i>, and the specified message queue is empty.</dd>
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<dt>[EBADF]</dt>
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<dd>The <i>mqdes</i> argument is not a valid message queue descriptor open for reading.</dd>
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<dt>[EMSGSIZE]</dt>
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<dd>The specified message buffer size, <i>msg_len</i>, is less than the message size attribute of the message queue.</dd>
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<dt>[EINTR]</dt>
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<dd>The <i>mq_receive</i>() <sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> or <i>mq_timedreceive</i>() <img src="../images/opt-end.gif" alt="[Option End]" border="0">
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operation was interrupted by a signal.</dd>
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<dt>[EINVAL]</dt>
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<dd><sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The process or thread would have blocked, and the <i>abs_timeout</i> parameter specified a nanoseconds field value less than zero
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or greater than or equal to 1000 million. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
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<dt>[ETIMEDOUT]</dt>
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<dd><sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The O_NONBLOCK flag was not set when the message queue was opened, but no message arrived on the queue before the specified timeout
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expired. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
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</dl>
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<p>The <i>mq_receive</i>() <sup>[<a href="javascript:open_code('TMO')">TMO</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> and <i>mq_timedreceive</i>() <img src="../images/opt-end.gif" alt="[Option End]" border="0">
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functions may fail if:</p>
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<dl compact>
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<dt>[EBADMSG]</dt>
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<dd>The implementation has detected a data corruption problem with the message.</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_386_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_386_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_386_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_386_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_386_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="mq_open.html"><i>mq_open</i>()</a> , <a href="mq_send.html"><i>mq_send</i>()</a> , <a href=
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"mq_timedsend.html"><i>mq_timedsend</i>()</a> , <a href="msgctl.html"><i>msgctl</i>()</a> , <a href=
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"msgget.html"><i>msgget</i>()</a> , <a href="msgrcv.html"><i>msgrcv</i>()</a> , <a href="msgsnd.html"><i>msgsnd</i>()</a> , <a
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href="time.html"><i>time</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>, <a href="../basedefs/time.h.html"><i><time.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_386_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_386_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>mq_receive</i>() function is marked as part of the Message Passing option.</p>
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<p>The Open Group Corrigendum U021/4 is applied. The DESCRIPTION is changed to refer to <i>msg_len</i> rather than
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<i>maxsize</i>.</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 following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>
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<ul>
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<li>
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<p>In this function it is possible for the return value to exceed the range of the type <b>ssize_t</b> (since <b>size_t</b> has a
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larger range of positive values than <b>ssize_t</b>). A sentence restricting the size of the <b>size_t</b> object is added to the
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description to resolve this conflict.</p>
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</li>
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</ul>
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<p>The <i>mq_timedreceive</i>() function is added for alignment with IEEE Std 1003.1d-1999.</p>
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<p>The <b>restrict</b> keyword is added to the <i>mq_timedreceive</i>() prototype for alignment with the ISO/IEC 9899:1999
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standard.</p>
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<p>IEEE PASC Interpretation 1003.1 #109 is applied, correcting the return type for <i>mq_timedreceive</i>() from <b>int</b> to
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<b>ssize_t</b>.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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