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217 lines
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<title>pthread_cond_broadcast</title>
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<basefont size="3"> <a name="pthread_cond_broadcast"></a> <a name="tag_03_515"></a><!-- pthread_cond_broadcast -->
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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_515_01"></a>NAME</h4>
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<blockquote>pthread_cond_broadcast, pthread_cond_signal - broadcast or signal a condition</blockquote>
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<h4><a name="tag_03_515_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('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/pthread.h.html">pthread.h</a>><br>
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<br>
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int pthread_cond_broadcast(pthread_cond_t *</tt><i>cond</i><tt>);<br>
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int pthread_cond_signal(pthread_cond_t *</tt><i>cond</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border=
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"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_515_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>These functions shall unblock threads blocked on a condition variable.</p>
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<p>The <i>pthread_cond_broadcast</i>() function shall unblock all threads currently blocked on the specified condition variable
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<i>cond</i>.</p>
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<p>The <i>pthread_cond_signal</i>() function shall unblock at least one of the threads that are blocked on the specified condition
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variable <i>cond</i> (if any threads are blocked on <i>cond</i>).</p>
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<p>If more than one thread is blocked on a condition variable, the scheduling policy shall determine the order in which threads are
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unblocked. When each thread unblocked as a result of a <i>pthread_cond_broadcast</i>() or <i>pthread_cond_signal</i>() returns from
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its call to <a href="../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> or <a href=
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"../functions/pthread_cond_timedwait.html"><i>pthread_cond_timedwait</i>()</a>, the thread shall own the mutex with which it called
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<a href="../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> or <a href=
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"../functions/pthread_cond_timedwait.html"><i>pthread_cond_timedwait</i>()</a>. The thread(s) that are unblocked shall contend for
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the mutex according to the scheduling policy (if applicable), and as if each had called <a href=
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"../functions/pthread_mutex_lock.html"><i>pthread_mutex_lock</i>()</a>.</p>
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<p>The <i>pthread_cond_broadcast</i>() or <i>pthread_cond_signal</i>() functions may be called by a thread whether or not it
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currently owns the mutex that threads calling <a href="../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> or <a
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href="../functions/pthread_cond_timedwait.html"><i>pthread_cond_timedwait</i>()</a> have associated with the condition variable
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during their waits; however, if predictable scheduling behavior is required, then that mutex shall be locked by the thread calling
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<i>pthread_cond_broadcast</i>() or <i>pthread_cond_signal</i>().</p>
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<p>The <i>pthread_cond_broadcast</i>() and <i>pthread_cond_signal</i>() functions shall have no effect if there are no threads
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currently blocked on <i>cond</i>.</p>
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</blockquote>
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<h4><a name="tag_03_515_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>If successful, the <i>pthread_cond_broadcast</i>() and <i>pthread_cond_signal</i>() functions shall return zero; otherwise, an
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error number shall be returned to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_515_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>pthread_cond_broadcast</i>() and <i>pthread_cond_signal</i>() function may fail if:</p>
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<dl compact>
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<dt>[EINVAL]</dt>
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<dd>The value <i>cond</i> does not refer to an initialized condition variable.</dd>
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</dl>
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<p>These functions shall not return an error code of [EINTR].</p>
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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_515_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_515_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <i>pthread_cond_broadcast</i>() function is used whenever the shared-variable state has been changed in a way that more than
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one thread can proceed with its task. Consider a single producer/multiple consumer problem, where the producer can insert multiple
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items on a list that is accessed one item at a time by the consumers. By calling the <i>pthread_cond_broadcast</i>() function, the
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producer would notify all consumers that might be waiting, and thereby the application would receive more throughput on a
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multi-processor. In addition, <i>pthread_cond_broadcast</i>() makes it easier to implement a read-write lock. The
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<i>pthread_cond_broadcast</i>() function is needed in order to wake up all waiting readers when a writer releases its lock.
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Finally, the two-phase commit algorithm can use this broadcast function to notify all clients of an impending transaction
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commit.</p>
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<p>It is not safe to use the <i>pthread_cond_signal</i>() function in a signal handler that is invoked asynchronously. Even if it
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were safe, there would still be a race between the test of the Boolean <a href=
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"../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> that could not be efficiently eliminated.</p>
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<p>Mutexes and condition variables are thus not suitable for releasing a waiting thread by signaling from code running in a signal
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handler.</p>
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</blockquote>
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<h4><a name="tag_03_515_08"></a>RATIONALE</h4>
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<blockquote>
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<h5><a name="tag_03_515_08_01"></a>Multiple Awakenings by Condition Signal</h5>
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<p>On a multi-processor, it may be impossible for an implementation of <i>pthread_cond_signal</i>() to avoid the unblocking of more
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than one thread blocked on a condition variable. For example, consider the following partial implementation of <a href=
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"../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> and <i>pthread_cond_signal</i>(), executed by two threads in
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the order given. One thread is trying to wait on the condition variable, another is concurrently executing
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<i>pthread_cond_signal</i>(), while a third thread is already waiting.</p>
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<pre>
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<tt>pthread_cond_wait(mutex, cond):
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value = cond->value; /* 1 */
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pthread_mutex_unlock(mutex); /* 2 */
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pthread_mutex_lock(cond->mutex); /* 10 */
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if (value == cond->value) { /* 11 */
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me->next_cond = cond->waiter;
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cond->waiter = me;
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pthread_mutex_unlock(cond->mutex);
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unable_to_run(me);
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} else
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pthread_mutex_unlock(cond->mutex); /* 12 */
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pthread_mutex_lock(mutex); /* 13 */
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<br>
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pthread_cond_signal(cond):
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pthread_mutex_lock(cond->mutex); /* 3 */
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cond->value++; /* 4 */
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if (cond->waiter) { /* 5 */
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sleeper = cond->waiter; /* 6 */
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cond->waiter = sleeper->next_cond; /* 7 */
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able_to_run(sleeper); /* 8 */
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}
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pthread_mutex_unlock(cond->mutex); /* 9 */
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</tt>
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</pre>
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<p>The effect is that more than one thread can return from its call to <a href=
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"../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> or <a href=
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"../functions/pthread_cond_timedwait.html"><i>pthread_cond_timedwait</i>()</a> as a result of one call to
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<i>pthread_cond_signal</i>(). This effect is called "spurious wakeup". Note that the situation is self-correcting in that the
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number of threads that are so awakened is finite; for example, the next thread to call <a href=
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"../functions/pthread_cond_wait.html"><i>pthread_cond_wait</i>()</a> after the sequence of events above blocks.</p>
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<p>While this problem could be resolved, the loss of efficiency for a fringe condition that occurs only rarely is unacceptable,
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especially given that one has to check the predicate associated with a condition variable anyway. Correcting this problem would
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unnecessarily reduce the degree of concurrency in this basic building block for all higher-level synchronization operations.</p>
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<p>An added benefit of allowing spurious wakeups is that applications are forced to code a predicate-testing-loop around the
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condition wait. This also makes the application tolerate superfluous condition broadcasts or signals on the same condition variable
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that may be coded in some other part of the application. The resulting applications are thus more robust. Therefore,
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IEEE Std 1003.1-2001 explicitly documents that spurious wakeups may occur.</p>
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</blockquote>
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<h4><a name="tag_03_515_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_515_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="pthread_cond_destroy.html"><i>pthread_cond_destroy</i>()</a> , <a href=
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"pthread_cond_timedwait.html"><i>pthread_cond_timedwait</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001,
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<a href="../basedefs/pthread.h.html"><i><pthread.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_515_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 Threads Extension.</p>
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</blockquote>
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<h4><a name="tag_03_515_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>pthread_cond_broadcast</i>() and <i>pthread_cond_signal</i>() functions are marked as part of the Threads option.</p>
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<p>The APPLICATION USAGE section is added.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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